International Journal of
Molecular Sciences
Article
Towards Understanding Excited-State Properties of Organic
Molecules Using Time-Resolved Soft X-ray
Absorption Spectroscopy
Holger Stiel 1,2,*, Julia Braenzel 1,2, Adrian Jonas 1,3, Richard Gnewkow 1,3,4, Lisa Theresa Glöggler 1,3,†,
Denny Sommer 2, Thomas Krist 5, Alexei Erko 6, Johannes Tümmler 1,2 and Ioanna Mantouvalou 1,3,4
Citation: Stiel, H.; Braenzel, J.; Jonas,
A.; Gnewkow, R.; Glöggler, L.T.;
Sommer, D.; Krist, T.; Erko, A.;
Tümmler, J.; Mantouvalou, I. Towards
Understanding Excited-State
Properties of Organic Molecules
Using Time-Resolved Soft X-ray
Absorption Spectroscopy. Int. J. Mol.
Sci. 2021,22, 13463. https://doi.org/
10.3390/ijms222413463
Academic Editors: Dieter Leupold
and Hugo Scheer
Received: 5 November 2021
Accepted: 8 December 2021
Published: 15 December 2021
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Attribution (CC BY) license (https://
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4.0/).
1Berlin Laboratory for Innovative X-ray Technologies (BLiX), D-10623 Berlin, Germany;
[email protected] (I.M.)
2Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, D-12489 Berlin, Germany;
Denny[email protected]
3Analytical X-ray Physics, TU Berlin, D-10623 Berlin, Germany
4Helmholtz Zentrum Berlin, D-12489 Berlin, Germany
5NOB Nano Optics Berlin GmbH, D-10627 Berlin, Germany; [email protected]
6IAP eV, D-12489 Berlin, Germany; [email protected]
*Correspondence: [email protected]
† Current address: Physics Department, CERN, 1211 Geneva, Switzerland.
Abstract:
The extension of the pump-probe approach known from UV/VIS spectroscopy to very short
wavelengths together with advanced simulation techniques allows a detailed analysis of excited-state
dynamics in organic molecules or biomolecular structures on a nanosecond to femtosecond time level.
Optical pump soft X-ray probe spectroscopy is a relatively new approach to detect and characterize
optically dark states in organic molecules, exciton dynamics or transient ligand-to-metal charge
transfer states. In this paper, we describe two experimental setups for transient soft X-ray absorption
spectroscopy based on an LPP emitting picosecond and sub-nanosecond soft X-ray pulses in the
photon energy range between 50 and 1500 eV. We apply these setups for near-edge X-ray absorption
fine structure (NEXAFS) investigations of thin films of a metal-free porphyrin, an aggregate forming
carbocyanine and a nickel oxide molecule. NEXAFS investigations have been carried out at the
carbon, nitrogen and oxygen K-edge as well as on the Ni L-edge. From time-resolved NEXAFS
carbon, K-edge measurements of the metal-free porphyrin first insights into a long-lived trap state
are gained. Our findings are discussed and compared with density functional theory calculations.
Keywords: NEXAFS; pump-probe; porphyrin; ultrafast X-ray absorption; pseudoisocyanine; TD-DFT
1. Introduction
Sir George Porter stated in his Nobel prize lecture [
1
] that “
. . .
since each molecule has
only one ground state, but several excited states, it is clear that this field of investigation is,
in principle, a bigger subject than the whole of conventional chemistry
. . .
” This statement
was based on his work on flash photolysis [
2
] using flash lamps as well as first available
lasers with pulse durations in the nanosecond range. Now, more than 50 years later,
femtochemistry [
3
] is a well-established technology to prepare and control excited-state
species on a very fast time scale using ultrashort femtosecond laser pulses. In this regard,
pump-probe spectroscopy using a strong optical pump pulse for the preparation of the
excited state and a weaker pulse for probing this state is the main experimental approach [
4
].
This approach has been successfully applied to detect transient states in organic molecules,
such as carbocyanines [
5
–
8
], porphyrins [
9
–
14
] and polyenes [
15
–
18
]. Ultrafast optical
transient spectroscopy plays an important role in elucidating charge and energy transfer
Int. J. Mol. Sci. 2021,22, 13463. https://doi.org/10.3390/ijms222413463 https://www.mdpi.com/journal/ijms
Int. J. Mol. Sci. 2021,22, 13463 2 of 18
processes in bacterial [
18
–
22
] and plant photosynthesis [
23
–
26
] as well as ligand–metal
interactions [10,11] in catalysis.
The extension of the pump-probe approach to very long or very short wavelengths [
27
]
together with advanced simulation techniques [
28
–
30
] allow a detailed analysis of excited-
state dynamics in organic molecules or biomolecular structures on a ns-to-fs time level.
Optical pump soft X-ray probe spectroscopy is a relatively new approach to detect and
characterize optically dark states in organic molecules [
31
–
34
] exciton dynamics [
35
,
36
] or
transient ligand-to-metal charge transfer states in metalloporphyrins [37].
Optical pump soft X-ray probe spectroscopy can also contribute to understanding
structure–function relationships in natural [
38
] or artificial “molecular machines” [
39
].
Time-resolved spectroscopic data taken at optical frequencies are not directly related to
large molecular structures at an atomic level, whereas time-resolved soft X-ray absorption
spectroscopy (tr-XAS) is capable of probing transient structures on an atomic level
[39–41]
.
A detailed knowledge of the molecular structure at an atomic level is indispensable,
as was shown, e.g., by X-ray diffraction investigations [
24
,
38
] for crystallized parts of the
photosynthetic apparatus.
Recent developments on soft X-ray sources, such as high harmonics generation
(HHG) [
37
,
42
,
43
] or laser-produced plasma sources (LPP) [
34
,
44
,
45
], open up new op-
portunities for studying excited-state dynamics in organic molecules on a laboratory scale.
In parallel, the above-mentioned improvements in simulation techniques, together with the
tremendous increase of computing power, allow understanding the excited-state behavior
even of very complex organic molecules in more detail [28].
In this paper, we describe two experimental setups for tr-NEXAFS experiments based
on an LPP emitting picosecond and sub-nanosecond soft X-ray pulses in the photon
energy range between 50 and 1500 eV. As an example, we apply these setups for NEXAFS
investigations of thin films of two organic molecules at the carbon and nitrogen K-edge and
compare the results with DFT calculations. In addition, we will show that the method is
also capable of elucidating the electronic structure of transition metal compounds playing
a role, e.g., in metalloporphyrins.
2. Results and Discussion
The calculated HOMO and LUMO iso-surfaces of the wave functions of TAP and
PIC have been rendered using the software Avogadro with an isosurface value of 0.01.
The result is shown in Figure 1. It can be seen that the wave functions (blue = negative,
red = positive) are mainly located at the site of the carbon rings and less at the alkyl
substituent, which is common for such organic compounds with conjugated systems.
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 2 of 18
experimental approach [4]. This approach has been successfully applied to detect transient
states in organic molecules, such as carbocyanines [5–8], porphyrins [9–14] and polyenes
[15–18]. Ultrafast optical transient spectroscopy plays an important role in elucidating
charge and energy transfer processes in bacterial [18–22] and plant photosynthesis [23–
26] as well as ligand–metal interactions [10,11] in catalysis.
The extension of the pump-probe approach to very long or very short wavelengths
[27] together with advanced simulation techniques [28–30] allow a detailed analysis of
excited-state dynamics in organic molecules or biomolecular structures on a ns-to-fs time
level. Optical pump soft X-ray probe spectroscopy is a relatively new approach to detect
and characterize optically dark states in organic molecules [31–34] exciton dynamics
[35,36] or transient ligand-to-metal charge transfer states in metalloporphyrins [37].
Optical pump soft X-ray probe spectroscopy can also contribute to understanding
structure–function relationships in natural [38] or artificial “molecular machines” [39].
Time-resolved spectroscopic data taken at optical frequencies are not directly related to
large molecular structures at an atomic level, whereas time-resolved soft X-ray absorption
spectroscopy (tr-XAS) is capable of probing transient structures on an atomic level [39–
41]. A detailed knowledge of the molecular structure at an atomic level is indispensable,
as was shown, e.g., by X-ray diffraction investigations [24,38] for crystallized parts of the
photosynthetic apparatus.
Recent developments on soft X-ray sources, such as high harmonics generation
(HHG) [37,42,43] or laser-produced plasma sources (LPP) [34,44,45], open up new
opportunities for studying excited-state dynamics in organic molecules on a laboratory
scale. In parallel, the above-mentioned improvements in simulation techniques, together
with the tremendous increase of computing power, allow understanding the excited-state
behavior even of very complex organic molecules in more detail [28].
In this paper, we describe two experimental setups for tr-NEXAFS experiments based
on an LPP emitting picosecond and sub-nanosecond soft X-ray pulses in the photon
energy range between 50 and 1500 eV. As an example, we apply these setups for NEXAFS
investigations of thin films of two organic molecules at the carbon and nitrogen K-edge
and compare the results with DFT calculations. In addition, we will show that the method
is also capable of elucidating the electronic structure of transition metal compounds
playing a role, e.g., in metalloporphyrins.
2. Results and Discussion
The calculated HOMO and LUMO iso-surfaces of the wave functions of TAP and PIC
have been rendered using the software Avogadro with an isosurface value of 0.01. The
result is shown in Figure 1. It can be seen that the wave functions (blue = negative, red =
positive) are mainly located at the site of the carbon rings and less at the alkyl substituent,
which is common for such organic compounds with conjugated systems.
.
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 3 of 18
Figure 1. Electronic structure of the HOMO und LUMO of TAP (top) and PIC (bottom). Carbon
atoms are depicted in black nitrogen in blue and hydrogen in white. For both molecules, the HOMO
and LUMO extends over the whole conjugated system.
Ground-state NEXAFS measurements of TAP and PIC samples have been performed
at the carbon and nitrogen K-edges using the sub-ns LPP source. The measured spectra
can be seen in Figure 2. Additionally, the calculated spectra are depicted in grey. The
curves of measured and calculated spectra agree well for all edges. Differences in the
relative heights of the spectra are partly due to unknown line widths and to the finite
number of calculated transitions. The energetic distances of the transitions agree well,
which allows an assignment of isolated transitions to the individual atomic groups via the
DFT calculation. From the calculation, the individual contributions of the nitrogen atoms
to the C and N K-edge NEXAFS spectrum can be derived.
Figure 2. Carbon and nitrogen K-edge NEXAFS spectra of thin films of PIC and TAP in comparison with DFT calculations.
2.1. TAP
The assignment of the features in the carbon K-edge spectrum of TAP is summarized
in Table 1. It follows the explanations given in [34]. According to our TD-DFT calculations,
the two features A and C located at 283.9 and 285.4 eV respectively belong to the extended
πelectron system of the ring. The first peak is separated by 1.5 eV from the second one,
which is very similar to the energetic difference between the SORET- and the Q-band in
the UV/Vis spectrum (compare. Figure 11). Features B and E originate from carbon atoms
Figure 1.
Electronic structure of the HOMO und LUMO of TAP (
top
) and PIC (
bottom
). Carbon
atoms are depicted in black nitrogen in blue and hydrogen in white. For both molecules, the HOMO
and LUMO extends over the whole conjugated system.
Int. J. Mol. Sci. 2021,22, 13463 3 of 18
Ground-state NEXAFS measurements of TAP and PIC samples have been performed
at the carbon and nitrogen K-edges using the sub-ns LPP source. The measured spectra can
be seen in Figure 2. Additionally, the calculated spectra are depicted in grey. The curves
of measured and calculated spectra agree well for all edges. Differences in the relative
heights of the spectra are partly due to unknown line widths and to the finite number of
calculated transitions. The energetic distances of the transitions agree well, which allows an
assignment of isolated transitions to the individual atomic groups via the DFT calculation.
From the calculation, the individual contributions of the nitrogen atoms to the C and N
K-edge NEXAFS spectrum can be derived.
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 3 of 18
Figure 1. Electronic structure of the HOMO und LUMO of TAP (top) and PIC (bottom). Carbon
atoms are depicted in black nitrogen in blue and hydrogen in white. For both molecules, the HOMO
and LUMO extends over the whole conjugated system.
Ground-state NEXAFS measurements of TAP and PIC samples have been performed
at the carbon and nitrogen K-edges using the sub-ns LPP source. The measured spectra
can be seen in Figure 2. Additionally, the calculated spectra are depicted in grey. The
curves of measured and calculated spectra agree well for all edges. Differences in the
relative heights of the spectra are partly due to unknown line widths and to the finite
number of calculated transitions. The energetic distances of the transitions agree well,
which allows an assignment of isolated transitions to the individual atomic groups via the
DFT calculation. From the calculation, the individual contributions of the nitrogen atoms
to the C and N K-edge NEXAFS spectrum can be derived.
Figure 2. Carbon and nitrogen K-edge NEXAFS spectra of thin films of PIC and TAP in comparison with DFT calculations.
2.1. TAP
The assignment of the features in the carbon K-edge spectrum of TAP is summarized
in Table 1. It follows the explanations given in [34]. According to our TD-DFT calculations,
the two features A and C located at 283.9 and 285.4 eV respectively belong to the extended
πelectron system of the ring. The first peak is separated by 1.5 eV from the second one,
which is very similar to the energetic difference between the SORET- and the Q-band in
the UV/Vis spectrum (compare. Figure 11). Features B and E originate from carbon atoms
Figure 2.
Carbon and nitrogen K-edge NEXAFS spectra of thin films of PIC and TAP in comparison with DFT calculations.
2.1. TAP
The assignment of the features in the carbon K-edge spectrum of TAP is summarized
in Table 1. It follows the explanations given in [
34
]. According to our TD-DFT calculations,
the two features A and C located at 283.9 and 285.4 eV respectively belong to the extended
π
electron system of the ring. The first peak is separated by 1.5 eV from the second one,
which is very similar to the energetic difference between the SORET- and the Q-band
in the UV/Vis spectrum (compare. Figure 11). Features B and E originate from carbon
atoms that are only bound to other carbon atoms within the pyrrole ring. Feature D arises
solely from the butyl substitution. In the nitrogen K-edge spectrum of TAP, the most
prominent feature A
0
is the strong peak at 398.3 eV. This peak can be assigned to the 1s ->
π
* LUMO transition on the porphyrin macrocycle. Feature B
0
and D
0
at 401.5 eV and 403.4
eV, respectively, are mostly generated by the nitrogen atoms within the pyrrole, which are
bound to two carbon atoms and one hydrogen atom. As for the carbon K-edge spectrum,
the separation between the peaks A
0
and B
0
fits very well with the energetic difference
between SORET- and Q-band. Feature C
0
originates from nitrogen atoms inside of the
pyrrole ring that show a double bond with a carbon atom.
Int. J. Mol. Sci. 2021,22, 13463 4 of 18
Table 1.
Assignment of the measured peak positions for the carbon and nitrogen K-edge NEXAFS spectra of TAP and PIC.
Carbon K-Edge Nitrogen K-Edge
Feature Measured (eV) Assignment Feature MEASURED (eV) Assignment
TAP A 283.9 ±0.2 C bound to C (pyrrole) A0398.4 ±0.2 N 1s -> π* LUMO
B 285.4 ±0.2 C bound to N (pyrrole) B0400.1 ±0.2 N 1s -> π* LUMO+1
C 287.4 ±0.2 C bound to C (pyrrole) C0401.5 ±0.2
N bound to H (pyrrole)
D 288.2 ±0.2 C bound to C (butyl) D0403.4 ±0.2
E 289.0 ±0.2 C bound to N (pyrrole)
PIC F 285.7 ±0.2 C bound to C (ring) F0398.1 ±0.2 N 1s -> π* (LUMO)
G 286.7 ±0.2 C bound to N G0399.8 ±0.2 N 1s -> LUMO +1
H 289.3 ±0.2 C (ethyl) H0400.5 ±0.2 N 1s -> LUMO +3
For the carbon K-edge of TAP, the transient absorption spectra were measured using
the sub-nanosecond tr-NEXAFS setup, as already presented in [
34
]. The sample was
excited at the SORET-band with the third harmonic (343 nm) of the laser with a pulse
energy of 1 mJ/cm
2
and a pulse duration of 0.5 ns. The results are shown in Figure 3.
The tr-NEXAFS spectrum was taken at several time delays between 0.2 and 43 ns after
the excitation. It has been found that these changes remained constant with time delay.
Figure 3
shows in blue the averaged differences of all tr-NEXAFS spectra. In order to
validate the results and to obtain a reliable value for the uncertainty of the measured
absorption difference
∆
A, the difference spectrum with negative time delay (purple curve)
is shown. Both curves (blue and purple) have been smoothed into 1 eV-bins using a
15-pixel box filter. The estimated uncertainty of the absorption difference is
±
5
×
10
−4
.
The biggest light-induced change (d2) can be seen between features A and B at feature
B (d3) and between feature B and C (d4). While the transient absorption is reduced for
the carbon atoms that are bound to nitrogen, the absorption increased at d1, d2 and d4.
Because the energy gap between d3 and d1 matches the energy of the LUMO-HOMO
transition, it can be assumed that the density of unoccupied states in the d3 region is
transferred to d1. The feature d1 could also possibly arise from an X-ray optical double
resonance meaning the 1s electron is transferred to a previously occupied state that is
partially depleted due to the laser pulse (
cp. Figure 8
). The features d2 and d4 cannot easily
be isolated due to many possible contributions. However, it can be presumed that the
butyl groups do not have an effect on the tr-NEXAFS. The observed slow decay channel
(
lifetime > 43 ns
) responsible for feature d3 could be assigned to a long-living trap state.
A similar behavior was recently observed by N K-edge spectroscopy in nonaggregated
units in a thin Zn-porphyrin film [
46
]. To understand the nature of this electronic state,
which is unobservable in UV/Vis spectroscopy, in more detail, tr-NEXAFS investigations
at the nitrogen K-edge of TAP are planned in the next future.
Int. J. Mol. Sci. 2021,22, 13463 5 of 18
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 5 of 18
Figure 3. tr-NEXAFS at the carbon K-edge of TAP. The ground state spectrum cf. Figure 2 is shown
in black. Die difference spectra with negative and positive time delays between excitation and
probing are shown in purple and blue.
2.2. PIC
The carbon K-edge spectrum of PIC differs significantly from that of TAP. With many
chemical similar carbon atoms, the contributions of the different transitions from the C K-
edge cannot be easily isolated. Still, it can be seen that the lowest energetic transition
(feature F) mainly arises from the carbon atoms that are only bonded to other carbon
atoms. Feature G originates from the region around the nitrogen atom, while feature H
emerges from the two ethyl groups. Whereas the peak at 283.9 eV is missing, the NEXAFS
spectra show two peaks at 285.7 eV and 286.7 eV, separated by only about 1 eV. These
structures can be assigned to 1s -> π* transitions, whereas both the bandwidth and
position of these features are fingerprints of the aggregation state of the dye [36,47]. In our
case, the PIC molecules are only weakly coupled, and they are arranged as H-aggregates
in the film (cp. Figure 12). To elucidate the influence of the aggregation state on the
NEXAFS spectrum of PIC in more detail, alternative thin film preparation techniques are
under investigation.
PIC has two nitrogen atoms that have the same chemical environment. Therefore,
both nitrogen atoms contribute equally to the NEXAFS. The three features F′, G′ and H′
can be assigned to transitions of the N 1s electron into different LUMO states. As for the
TAP molecule, the energetic difference between the F’ and G’ feature is close to the
difference of the peak position of the monomeric first (2.3 eV) and second (3.8 eV) excited-
state absorption (cp. Figure 12).
2.3. NiO
Figure 4a shows the NiO oxygen K-edge NEXAFS spectrum recorded with the sub-
ns LPP together with a spectrum obtained at the synchrotron beamline of PTB (BESSY II-
HZB). As can be seen from Figure 4a, the NEXAFS spectrum obtained using the LPP
agrees with the related synchrotron data very well, demonstrating the potential of our
lab-based NEXAFS. The O K-edge NEXAFS reflects the unoccupied orbital states of the
Ni cation that hybridize with the oxygen 2p orbital [48]. There are five main features (A–
E) in the NEXAFS spectrum that could be assigned to different electronic states of the
molecule (cp. Table 2).
Figure 3.
tr-NEXAFS at the carbon K-edge of TAP. The ground state spectrum cf. Figure 2 is shown in
black. Die difference spectra with negative and positive time delays between excitation and probing
are shown in purple and blue.
2.2. PIC
The carbon K-edge spectrum of PIC differs significantly from that of TAP. With many
chemical similar carbon atoms, the contributions of the different transitions from the C
K-edge cannot be easily isolated. Still, it can be seen that the lowest energetic transition
(feature F) mainly arises from the carbon atoms that are only bonded to other carbon atoms.
Feature G originates from the region around the nitrogen atom, while feature H emerges
from the two ethyl groups. Whereas the peak at 283.9 eV is missing, the NEXAFS spectra
show two peaks at 285.7 eV and 286.7 eV, separated by only about 1 eV. These structures
can be assigned to 1s ->
π
* transitions, whereas both the bandwidth and position of these
features are fingerprints of the aggregation state of the dye [
36
,
47
]. In our case, the PIC
molecules are only weakly coupled, and they are arranged as H-aggregates in the film (cp.
Figure 12). To elucidate the influence of the aggregation state on the NEXAFS spectrum of
PIC in more detail, alternative thin film preparation techniques are under investigation.
PIC has two nitrogen atoms that have the same chemical environment. Therefore,
both nitrogen atoms contribute equally to the NEXAFS. The three features F
0
, G
0
and H
0
can
be assigned to transitions of the N 1s electron into different LUMO states. As for the TAP
molecule, the energetic difference between the F’ and G’ feature is close to the difference
of the peak position of the monomeric first (2.3 eV) and second (3.8 eV) excited-state
absorption (cp. Figure 12).
2.3. NiO
Figure 4a shows the NiO oxygen K-edge NEXAFS spectrum recorded with the sub-ns
LPP together with a spectrum obtained at the synchrotron beamline of PTB (BESSY II-HZB).
As can be seen from Figure 4a, the NEXAFS spectrum obtained using the LPP agrees
with the related synchrotron data very well, demonstrating the potential of our lab-based
NEXAFS. The O K-edge NEXAFS reflects the unoccupied orbital states of the Ni cation
that hybridize with the oxygen 2p orbital [
48
]. There are five main features (A–E) in the
NEXAFS spectrum that could be assigned to different electronic states of the molecule
(cp. Table 2).
Int. J. Mol. Sci. 2021,22, 13463 6 of 18
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 6 of 18
Figure 4. (a) Oxygen K-edge NEXAFS spectra of a NiO thin film measured with the sub-ns laser plasma source (green)
and at the PTB/BESSY II synchrotron beamline (blue). (b) Nickel L-edge NEXAFS spectra of a NiO thin film measured
with the sub-ns laser plasma source and an RZP on plane substrate optimally aligned for the L2-edge (black) and L3-edge
(red), respectively. For comparison, the spectrum (green) recorded with the ps-LPP and the RZP A9 on a bent substrate is
also shown.
The lowest-energy feature A at 531.6 eV can be attributed to a Ni 3d8 state while the
features B and C at 537.1 eV and 540.0 eV could be assigned to the Ni 4sp state [48]. The
features D–F at higher photon energies are mainly due to multiple-scattering effects of the
p photoelectron.
L-edge NEXAFS of Ni as a 3d transition metal mainly corresponds to electric dipole
transitions from Ni 2p core levels to 3d orbitals. Due to the strong correlation between the
3d electrons, multiplet structures in the NEXAFS spectrum are predicted by theoretical
models [49] using the multi-electron approach. In order to compare these predictions with
experimental data in more detail, our measured nickel L-edge NEXAFS spectra were
evaluated (Figure4b). Due to the required high spectral resolution, two different
spectrometer settings using the planar RZP in the ns-LPP setup were applied. In order to
resolve the L3-edge fine structure (features A′ and B′ in Figure 4b), the RZP on the plane
substrate was optimally aligned for the 853 eV region according to a procedure described
in [50]. The features C′ and D′ (“multiplet splitting”) could be optimally resolved by
alignment of the RZP at 870 eV (L2-edge). For comparison, the spectrum recorded with
our proto-type RZP (A9, see above) on a bent substrate is also shown. For this
measurement, only 100 images each accumulating 2 laser shots of the ps-LPP were
processed. The comparable lower statistical amount is close to resolving the line splitting
of Ni L2 and feature A′. The assignments of the features following theoretical predictions
and experimental data [49,51] are summarized in Table 2.
Figure 4.
(
a
) Oxygen K-edge NEXAFS spectra of a NiO thin film measured with the sub-ns laser plasma source (green)
and at the PTB/BESSY II synchrotron beamline (blue). (
b
) Nickel L-edge NEXAFS spectra of a NiO thin film measured
with the sub-ns laser plasma source and an RZP on plane substrate optimally aligned for the L2-edge (black) and L3-edge
(red), respectively. For comparison, the spectrum (green) recorded with the ps-LPP and the RZP A9 on a bent substrate is
also shown.
The lowest-energy feature A at 531.6 eV can be attributed to a Ni 3d
8
state while
the features B and C at 537.1 eV and 540.0 eV could be assigned to the Ni 4sp state [
48
].
The features D–F at higher photon energies are mainly due to multiple-scattering effects of
the p photoelectron.
L-edge NEXAFS of Ni as a 3d transition metal mainly corresponds to electric dipole
transitions from Ni 2p core levels to 3d orbitals. Due to the strong correlation between the
3d electrons, multiplet structures in the NEXAFS spectrum are predicted by theoretical
models [
49
] using the multi-electron approach. In order to compare these predictions with
experimental data in more detail, our measured nickel L-edge NEXAFS spectra were evalu-
ated (Figure 4b). Due to the required high spectral resolution, two different spectrometer
settings using the planar RZP in the ns-LPP setup were applied. In order to resolve the
L3-edge fine structure (features A
0
and B
0
in Figure 4b), the RZP on the plane substrate
was optimally aligned for the 853 eV region according to a procedure described in [
50
].
The features C
0
and D
0
(“multiplet splitting”) could be optimally resolved by alignment of
the RZP at 870 eV (L2-edge). For comparison, the spectrum recorded with our proto-type
RZP (A9, see above) on a bent substrate is also shown. For this measurement, only 100 im-
ages each accumulating 2 laser shots of the ps-LPP were processed. The comparable lower
statistical amount is close to resolving the line splitting of Ni L2 and feature A
0
. The assign-
ments of the features following theoretical predictions and experimental data [
49
,
51
] are
summarized in Table 2.
Int. J. Mol. Sci. 2021,22, 13463 7 of 18
Table 2. Assignment of the measured peak positions for the oxygen K-edge and nickel L-edge NEXAFS spectra of NiO.
Oxygen K-Edge Nickel L-Edge
Feature Measured (eV) Assignment Feature Measured (eV) Assignment
A 531.6 ±0.2 Ni 3d—O 2p
mixing A0852.6 ±0.2 2p3/2—3d
B 537.1 ±0.2 Ni 4sp B0854.3 ±0.2 2p3/2—3d
C 540.0 ±0.2 Ni 4sp C0865.9 ±0.2 2p–4sp [51]
D 546.0 ±0.2
Multiple scattering
D0869.8 ±0.2
870.6 ±0.2
2p1/2-3d (f2g eg)
Multiplet splitting
E 555.4 ±0.2
Multiple scattering
E0885.4 ±0.2
Multiple scattering
F 561.2 ±0.2
Multiple scattering
F0902.4±0.2
Multiple scattering
3. Materials and Methods
3.1. Experimental
The two NEXAFS setups are transmission instruments, where polychromatic soft
X-rays from a laser-produced plasma source (LPP) are first transmitted through a thin
homogeneous sample and successively dispersed with the help of reflection zone plates.
The absorption spectra are then collected with a soft X-ray CCD camera as a detector, see,
for example, Figure 5. The individual components are described and discussed in the
following chapters.
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 7 of 18
Table 2. Assignment of the measured peak positions for the oxygen K-edge and nickel L-edge NEXAFS spectra of NiO.
Oxygen K-Edge Nickel L-Edge
Feature Measured (eV) Assignment Feature Measured (eV) Assignment
A 531.6 ± 0.2 Ni 3d—O 2p mixing A′ 852.6 ± 0.2 2p
3/2
—3d
B 537.1 ± 0.2 Ni 4sp B′ 854.3 ± 0.2 2p
3/2
—3d
C 540.0 ± 0.2 Ni 4sp C′ 865.9 ± 0.2 2p–4sp [51]
D 546.0 ± 0.2 Multiple scattering D′ 869.8 ± 0.2
870.6 ± 0.2
2p
1/2
-3d (f
2g
e
g
)
Multiplet splitting
E 555.4 ± 0.2 Multiple scattering E′ 885.4 ± 0.2 Multiple scattering
F 561.2 ± 0.2 Multiple scattering F′ 902.4± 0.2 Multiple scattering
3. Materials and Methods
3.1. Experimental
The two NEXAFS setups are transmission instruments, where polychromatic soft X-
rays from a laser-produced plasma source (LPP) are first transmitted through a thin
homogeneous sample and successively dispersed with the help of reflection zone plates.
The absorption spectra are then collected with a soft X-ray CCD camera as a detector, see,
for example, Figure 5. The individual components are described and discussed in the
following chapters.
Figure 5. Scheme of the tr-NEXAFS setup using a picosecond driver laser and a reflection zone plate on a bent substrate.
3.1.1.
Laser Produced Plasma Sources
In order to generate soft X-ray radiation in the laboratory, we rely on LPP sources.
An intense short laser pulse hits a solid target in vacuum and creates a plasma that emits
soft X-ray radiation (SXR). In order to meet different requirements concerning achievable
Figure 5. Scheme of the tr-NEXAFS setup using a picosecond driver laser and a reflection zone plate on a bent substrate.
Int. J. Mol. Sci. 2021,22, 13463 8 of 18
3.1.1. Laser Produced Plasma Sources
In order to generate soft X-ray radiation in the laboratory, we rely on LPP sources.
An intense short laser pulse hits a solid target in vacuum and creates a plasma that emits
soft X-ray radiation (SXR). In order to meet different requirements concerning achievable
time and spectral structure of the SXR radiation, we have developed two LPP sources
enabling state-of-the-art NEXAFS spectroscopy in the laboratory.
The first LPP source we want to present is pumped by a chirped pulse amplification
(CPA) thin-disk laser system. The driving laser operates at 1030 nm with a pulse energy
of 120 mJ, a 1.2 ps pulse duration and 100 Hz repetition rate. The plasma is created by
focusing the laser pulse on a rotating metal cylinder target. The spot size amounts to 17
µ
m
(FWHM) in diameter, delivering an intensity of the target in about 10
16
W/cm
2
. This high
intensity on the metal target creates a plasma that emits an X-ray spectrum ranging from
50 eV to 1500 eV. Depending on the used target material, the spectrum consists either of
characteristic line emission (e.g., Cu, Fe, Sn) or, for some target elements (e.g., W, Au) with
high density of multiplet emission, a quasi-broadband spectrum. The LPP source delivers
incoherent soft X-ray (SXR) emission with high photon numbers: >10
12
photons/s*sr @
0.1% bandwidth in the range 50–500 eV and 10
11
photons/s*sr @ 0.1% bandwidth in the
range 500–1500 eV [
52
]. The pulse duration of the source depends on the pump laser pulse
duration as well as on the target material. For a tungsten target and a 1.2 ps pump laser
duration, we have estimated an SXR pulse duration at 700 eV of about 10 ps [53].
Both the infrared and the SXR optical path is debris-screened against the ablation
of the target material. A 100
µ
m glass plate is used for screening the vacuum window
in the pump laser path, and a 900 nm mylar or parylene foil shields the SXR beamline.
Having passed the debris foil, the SXR pulse transmits the sample and is collected by a bent
reflection zone plate (RZP A9, see below). The whole setup, including the spectrometer,
is depicted in Figure 5.
The second LPP setup with a sub-ns-pulse duration has already been described
in detail in previous work [
34
,
45
]. In brief, this setup uses an Yb:YAG thin-disk laser
system with 200 mJ maximum single-pulse energy, 100 Hz repetition rate, 500 ps pulse
duration and a solid target for the plasma formation [
54
]. For the Cu XX line at 1.1594 nm,
a brilliance of >10
10
ph/(mm
2
mrad
2
s line) is reached. Using the other metal targets values
of
>3 ×1011 ph/(mm2mrad2s
), a line for the water window range, as well as for the EUV,
can be achieved [55].
This source utilizes stabilization mechanisms in order to compensate small rotational
and translational movements and local variations in the diameter of the target material.
The stabilization results in a reduction in source movement from 60
µ
m to 13
µ
m standard
deviation with a photon-energy-dependent source diameter between 40 µm and 70 µm.
Both sources cover the whole photon energy region relevant for NEXAFS investigation
on organo-metallic compounds, starting from the carbon K-edge up to Mg and Al K-edges,
the L-edges of transition metals, as well as M-edges of lanthanides.
For transient experiments, small portions of the laser beam can be used as pump,
which are perfectly synchronized to the probe pulse. Both setups offer optical delay lines
with variable pump-probe separation (ps setup: 1 ps—1.5 ns, sub-ns setup: 0.5–43 ns) and
wavelength using non-linear crystals and dye lasers. Due to the complementary temporal
pulse structures of the LPPs, transient NEXAFS spectra covering the temporal range for
the pump-probe delay from few picoseconds to tens of ns can be recorded.
3.1.2. Reflection Zone Plate Optics
The dispersive element in tr-NEXAFS using LPP sources is a critical component.
Due to the isotropic nature of the emitted radiation, it should collect the largest emission
angle possible. In order to resolve very small features in the tr-NEXAFS spectrum, a re-
solving power E/
∆
E up to 1000 is required. Finally, a high efficiency (throughput) of
the optics is crucial for reasonable data acquisition times. Off-axis reflection zone plates
(RZPs) are two-dimensional laminar grating structures, where the grating lines follow an
Int. J. Mol. Sci. 2021,22, 13463 9 of 18
elliptical shape. They focus and disperse broadband radiation from a point source onto
the image plane with high efficiency [
56
,
57
]. The design wavelength is concentrated in the
focus as the image of the source, while energies smaller and larger than the focus energy
are displayed as slightly curved lines in an X-shaped pattern on the detector. The sub-ns
NEXAFS setup is equipped with two sets of planar off-axis reflection zone plates (NOB,
Nano Optics Berlin GmbH) [45].
RZPs on planar substrates suffer from a relatively narrow energy range with high
spectral resolution around the design wavelength. To overcome this limitation, a special
“misalignment” technique has been proposed tuning the focused wavelength of a planar
RZP over a broader spectral range [
50
,
57
]. Another option to obtain a high spectral
resolution over a wide photon energy range is to fabricate RZP structures on a spherical
substrate [
58
]. In contrast to an RZP on a planar substrate, the new design allows a high
spectral resolution of up to
∆
E/E = 1000 on the detector for a wide spectral range (design-
energy
±
50%) without an energy-dependent spatial and spectral limited focusing. This
enables recording a spectrum with retaining one dimension for spatial imaging, similar
to commonly used Varied Line-Spaced (VLS) grating. In contrast to a VLS, an RZP on
a curved substrate offers a high efficiency up to 25% in a large spectral range covering
photon energies of up to 1300 eV [58].
We tested a prototype (called RZP A9) of this new generation of RZPs that is written
on a curved substrate using electron beam lithography. The RZP (NOB Nano Optics Berlin
GmbH) consists of three different structures, each 10 mm in height designed for energy
of 250 eV, 500 eV, and 780 eV, respectively. The radius of curvature of the RZP substrate
amounts to 54.879 m, and the angle of incidence is 2.13
◦
with a reflection angle of 3.52
◦
.
The distances between source and RZP and detector and RZP accounted for 1500 mm and
2500 mm, respectively. On the top and bottom of each structure, the RZP has an additional
alignment structure for the design-energy of 966.485 eV. The alignment structure represents
a miniaturized planar RZP structure. It serves the general alignment of the device as well
as for referencing the design energy of the main structure. The RZP A9 was designed to
illuminate about 80% of the area of a 1” CCD chip. As an example, Figure 6shows two
detector images, as seen on the CCD, if one RZP structure has been illuminated by soft
X-rays from our ps-LPP source.
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 9 of 18
displayed as slightly curved lines in an X-shaped pattern on the detector. The sub-ns
NEXAFS setup is equipped with two sets of planar off-axis reflection zone plates (NOB,
Nano Optics Berlin GmbH) [45].
RZPs on planar substrates suffer from a relatively narrow energy range with high
spectral resolution around the design wavelength. To overcome this limitation, a special
“misalignment” technique has been proposed tuning the focused wavelength of a planar
RZP over a broader spectral range [50,57]. Another option to obtain a high spectral
resolution over a wide photon energy range is to fabricate RZP structures on a spherical
substrate [58]. In contrast to an RZP on a planar substrate, the new design allows a high
spectral resolution of up to ΔE/E = 1000 on the detector for a wide spectral range (design-
energy ± 50%) without an energy-dependent spatial and spectral limited focusing. This
enables recording a spectrum with retaining one dimension for spatial imaging, similar to
commonly used Varied Line-Spaced (VLS) grating. In contrast to a VLS, an RZP on a
curved substrate offers a high efficiency up to 25% in a large spectral range covering
photon energies of up to 1300 eV [58].
We tested a prototype (called RZP A9) of this new generation of RZPs that is written
on a curved substrate using electron beam lithography. The RZP (NOB Nano Optics Berlin
GmbH) consists of three different structures, each 10 mm in height designed for energy of
250 eV, 500 eV, and 780 eV, respectively. The radius of curvature of the RZP substrate
amounts to 54.879 m, and the angle of incidence is 2.13° with a reflection angle of 3.52°.
The distances between source and RZP and detector and RZP accounted for 1500 mm and
2500 mm, respectively. On the top and bottom of each structure, the RZP has an additional
alignment structure for the design-energy of 966.485 eV. The alignment structure
represents a miniaturized planar RZP structure. It serves the general alignment of the
device as well as for referencing the design energy of the main structure. The RZP A9 was
designed to illuminate about 80% of the area of a 1” CCD chip. As an example, Figure 6
shows two detector images, as seen on the CCD, if one RZP structure has been illuminated
by soft X-rays from our ps-LPP source.
Figure 6. Detector image of a tungsten spectrum (Left) taken with the so-called S2 structure (design energy 450 eV) of the
RZP A9. In the reference structure on the top and bottom, the oxygen K-edge absorption from the mylar foil is visible.
(Right): Detector image of an iron spectrum taken with the so-called S3 structure (design energy 780 eV). Design energy
of the reference structure: 966.45 eV. The integration time was 945 ms at a laser repetition rate of 100 Hz. Detector: Back-
illuminated soft X-ray CCD camera (Greateyes GmbH).
Figure 6.
Detector image of a tungsten spectrum (
Left
) taken with the so-called S2 structure (design energy 450 eV)
of the RZP A9. In the reference structure on the top and bottom, the oxygen K-edge absorption from the mylar foil is
visible. (
Right
): Detector image of an iron spectrum taken with the so-called S3 structure (design energy 780 eV). Design
energy of the reference structure: 966.45 eV. The integration time was 945 ms at a laser repetition rate of 100 Hz. Detector:
Back-illuminated soft X-ray CCD camera (Greateyes GmbH).
Int. J. Mol. Sci. 2021,22, 13463 10 of 18
3.1.3. General Considerations for Optical Pump X-ray Probe Experiments on
Organic Molecules
In optical pump X-ray probe experiments, molecules are excited with a light pulse.
Afterwards, the X-ray spectrum of the excited molecule is detected. By varying the time de-
lay between the two short pulses, the temporal evolution of the system can be investigated.
The achievable time resolution is given by the longer pulse duration, which is, in most
cases, the X-ray pulse durations. Figure 7illustrates the pump-probe scheme for a typical
organic molecule. The optical pump pulse tuned to a transition between the ground singlet
state S
0
and an excited state of the molecule (S
1
, S
2
,
. . .
) creates a vacancy in the highest
occupied molecular orbital (HOMO), which is detected by an X-ray pulse tuned to the K-
or L- absorption edge of the atom of interest.
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 10 of 18
3.1.3. General Considerations for Optical Pump X-ray Probe Experiments on Organic
Molecules
In optical pump X-ray probe experiments, molecules are excited with a light pulse.
Afterwards, the X-ray spectrum of the excited molecule is detected. By varying the time
delay between the two short pulses, the temporal evolution of the system can be
investigated. The achievable time resolution is given by the longer pulse duration, which
is, in most cases, the X-ray pulse durations. Figure 7 illustrates the pump-probe scheme
for a typical organic molecule. The optical pump pulse tuned to a transition between the
ground singlet state S0 and an excited state of the molecule (S1, S2 …) creates a vacancy in
the highest occupied molecular orbital (HOMO), which is detected by an X-ray pulse
tuned to the K- or L- absorption edge of the atom of interest.
For successful tr-NEXAFS, it is necessary to excite a sufficient number of molecules
depending on the achievable signal-to-noise ratio (SNR) of the setup and the strength of
the transient signal. Compared to measurements in liquids, where rapid sample
replenishment is possible, the excitation percentage of molecular thin films is often limited
to single digits due to radiation-induced damage and sample evaporation. Typical
damage thresholds for thin films of organic molecules on Si3N4 or SiC membranes are in
the range of 1–5 mJ/cm². If evaporation due to sample heating is the dominating factor,
the maximum power density is almost independent of the pulse length.
Since the absorption coefficient of the optical pump is usually much higher compared
to the X-ray probe pulse the maximum sample thickness is determined by the optical
pulse, which leads to optically thin samples for the X-ray pulse. The consequence of the
low excitation ratio and optically thin samples for X-rays is a small absorption difference
in the range of 10−2 to 10−4. The SNR needed to detect this transient change can only be
achieved by taking multiple measurements. This requires a stable setup and will be
discussed in the detectors and data acquisition section.
To determine the damage threshold and excitation ratio before the tr-XAS
experiment, non-linear absorption (NLA) measurements can be conducted. In an NLA
setup, the absorption of a laser through the sample is detected for increasing laser
intensities. When part of the sample is excited during the laser pulse, the transmission
becomes non-linear due to ground state bleaching. This allows the estimation of the
excited-state fraction and modeling of the energy level scheme using a rate equation
system for the population densities and a photon transport equation for the radiation
transport through the sample [31].
Figure 7. Term scheme illustrating the principle of an optical pump (multi-color arrow, hνA) and X-
ray probe experiment for a typical organic molecule. The optical pump pulse tuned to a transition
Figure 7.
Term scheme illustrating the principle of an optical pump (multi-color arrow, h
νA
) and
X-ray probe experiment for a typical organic molecule. The optical pump pulse tuned to a transition
between S
0
and an excited state of the molecule (S
1
, S
2
,
. . .
) creates a vacancy in the highest occupied
molecular orbital (HOMO), which is detected by an X-ray pulse tuned to the K- or L-absorption edge
of the atom of interest. LUMO denotes the lowest unoccupied molecular orbital related to an excited
singlet state of the molecule. h
νF
: fluorescence, h
νPh
: phosphorescence, IC: internal conversion, VR:
vibrational relaxation. ISC: intersystem crossing.
For successful tr-NEXAFS, it is necessary to excite a sufficient number of molecules
depending on the achievable signal-to-noise ratio (SNR) of the setup and the strength of the
transient signal. Compared to measurements in liquids, where rapid sample replenishment
is possible, the excitation percentage of molecular thin films is often limited to single digits
due to radiation-induced damage and sample evaporation. Typical damage thresholds for
thin films of organic molecules on Si
3
N
4
or SiC membranes are in the range of 1–5 mJ/cm
2
.
If evaporation due to sample heating is the dominating factor, the maximum power density
is almost independent of the pulse length.
Since the absorption coefficient of the optical pump is usually much higher compared
to the X-ray probe pulse the maximum sample thickness is determined by the optical pulse,
which leads to optically thin samples for the X-ray pulse. The consequence of the low
excitation ratio and optically thin samples for X-rays is a small absorption difference in the
range of 10
−2
to 10
−4
. The SNR needed to detect this transient change can only be achieved
by taking multiple measurements. This requires a stable setup and will be discussed in the
detectors and data acquisition section.
To determine the damage threshold and excitation ratio before the tr-XAS experiment,
non-linear absorption (NLA) measurements can be conducted. In an NLA setup, the ab-
sorption of a laser through the sample is detected for increasing laser intensities. When part
Int. J. Mol. Sci. 2021,22, 13463 11 of 18
of the sample is excited during the laser pulse, the transmission becomes non-linear due to
ground state bleaching. This allows the estimation of the excited-state fraction and model-
ing of the energy level scheme using a rate equation system for the population densities
and a photon transport equation for the radiation transport through the sample [31].
3.2. Materials
In order to evaluate the potential of our lab-based tr-NEXAFS setups, we choose three
sample systems, see Figure 8: (i) a metal-free porphyrin, (ii) an aggregate forming carbocya-
nine and (iii) a nickel oxide sample. The metal-free tetra(tert-butyl)-porphyrazine (TAP) is
a typical molecule belonging to the class of tetrapyrroles, which exhibits great application
potential in optoelectronics and photovoltaics, as well as pigments in natural or artificial
photosynthesis. Pseudoisocyanine (1,1
0
-Diethyl-2,2
0
-cyanine iodide, PIC) is a J-aggregate
forming carbocyanine. J-aggregates of PIC could be regarded as supramolecular polymers
that show exceptional photophysical properties, such as giant dipole transition moments
and strong exciton-exciton annihilation. NiO as a typical large bandgap semiconductor is
applied in photovoltaic solar cells and as an anode material in lithium battery technology.
Ni-porphyrin molecules are promising candidates for organic solar cells. The knowledge
of its electronic properties is crucial for optimizing the efficiency of these devices.
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 11 of 18
between S0 and an excited state of the molecule (S1, S2 …) creates a vacancy in the highest occupied
molecular orbital (HOMO), which is detected by an X-ray pulse tuned to the K- or L-absorption
edge of the atom of interest. LUMO denotes the lowest unoccupied molecular orbital related to an
excited singlet state of the molecule. hνF: fluorescence, hνPh: phosphorescence, IC: internal
conversion, VR: vibrational relaxation. ISC: intersystem crossing.
3.2. Materials
In order to evaluate the potential of our lab-based tr-NEXAFS setups, we choose three
sample systems, see Figure 8: (i) a metal-free porphyrin, (ii) an aggregate forming
carbocyanine and (iii) a nickel oxide sample. The metal-free tetra(tert-butyl)-porphyrazine
(TAP) is a typical molecule belonging to the class of tetrapyrroles, which exhibits great
application potential in optoelectronics and photovoltaics, as well as pigments in natural
or artificial photosynthesis. Pseudoisocyanine (1,1′-Diethyl-2,2′-cyanine iodide, PIC) is a
J-aggregate forming carbocyanine. J-aggregates of PIC could be regarded as
supramolecular polymers that show exceptional photophysical properties, such as giant
dipole transition moments and strong exciton-exciton annihilation. NiO as a typical large
bandgap semiconductor is applied in photovoltaic solar cells and as an anode material in
lithium battery technology. Ni-porphyrin molecules are promising candidates for organic
solar cells. The knowledge of its electronic properties is crucial for optimizing the
efficiency of these devices.
Both tr-NEXAFS setups rely on the detection of transmitted radiation through a
sample, where the thickness of the sample determines the contrast at the absorption edge
and the number of detected photons. Therefore, large, homogeneous thin samples are
required. Typically, organic molecule samples are deposited on 150 nm thin Si3N4
membranes with a window size of 1 mm × 2 mm or 2 mm × 2 mm via spin coating or
evaporation using an effusion cell. The choice of preparation is dependent on the chemical
properties of the molecule, such as solvability or evaporation temperature (see
Supplemental Information for details).
After preparation, the organic samples are also pre-characterized to NLA
measurements using UV/Vis spectroscopy, an EUV spectrometer for thickness
determination and an atomic force microscope (AFM).
Details concerning instrumentation and measurement parameters can be found in
the Supplementary Material (Table S1).
Figure 8. Structure of the three samples under investigation: TAP = tetra(tert-butyl)-porphyrazine),
PIC = 1,1′-diethyl-2,2′-cyanine iodide and NiO = nickel oxide.
3.3. NiO
The NiO samples were prepared on 200 nm thick Si3N4 windows (3 × 3 mm) by
depositing Ni using a reactive electron beam evaporation in an oxygen environment at
room temperature. The thickness we monitored with a quartz crystal and the
stoichiometry with EDX measurements delivered an O:Ni ratio of about 51:48. (see
Figures S1 and S2) Figure 9 shows the UV/Vis spectrum of a 300 nm thick NiO sample
with the bandgap energy of NiO at 3.6 eV [59]. This value is very close to the photon
energy of the third harmonic of the pump laser. The optical density (OD = −log(T)) at 3.6
Figure 8.
Structure of the three samples under investigation: TAP = tetra(tert-butyl)-porphyrazine),
PIC = 1,10-diethyl-2,20-cyanine iodide and NiO = nickel oxide.
Both tr-NEXAFS setups rely on the detection of transmitted radiation through a
sample, where the thickness of the sample determines the contrast at the absorption
edge and the number of detected photons. Therefore, large, homogeneous thin samples
are required. Typically, organic molecule samples are deposited on 150 nm thin Si
3
N
4
membranes with a window size of 1 mm
×
2 mm or 2 mm
×
2 mm via spin coating
or evaporation using an effusion cell. The choice of preparation is dependent on the
chemical properties of the molecule, such as solvability or evaporation temperature (see
Supplemental Information for details).
After preparation, the organic samples are also pre-characterized to NLA measure-
ments using UV/Vis spectroscopy, an EUV spectrometer for thickness determination and
an atomic force microscope (AFM).
Details concerning instrumentation and measurement parameters can be found in the
Supplementary Material (Table S1).
3.3. NiO
The NiO samples were prepared on 200 nm thick Si
3
N
4
windows (3
×
3 mm) by
depositing Ni using a reactive electron beam evaporation in an oxygen environment at
room temperature. The thickness we monitored with a quartz crystal and the stoichiometry
with EDX measurements delivered an O:Ni ratio of about 51:48 (see Figures S1 and S2).
Figure 9shows the UV/Vis spectrum of a 300 nm thick NiO sample with the bandgap
energy of NiO at 3.6 eV [
59
]. This value is very close to the photon energy of the third
harmonic of the pump laser. The optical density (OD =
−
log(T)) at 3.6 eV accounts for 0.54.
In comparison, the related value in the Ni L-edge region (850–870 eV) amounts to 1.1.
Int. J. Mol. Sci. 2021,22, 13463 12 of 18
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 12 of 18
eV accounts for 0.54. In comparison, the related value in the Ni L-edge region (850–870
eV) amounts to 1.1.
Figure 9. UV/Vis spectrum of the NiO sample (thickness 300 nm) on Si3N4 corrected for substrate
transmission.
3.4. TAP
TAP thin films were prepared by evaporation on 1 × 1 mm² Si3N4 membranes. The
UV/Vis spectrum of such a 120 nm thick film in comparison to TAP solved in ethanol (c =
3.6 × 10−4 mol/L, 1 mm quartz cuvette) is shown in Figure 11. In porphyrins and its
derivatives, the S1 state splits into two states due to molecular symmetry, which are named
Qx and Qy. Transitions to the S2 state led to the formation of the Soret band or B band [60].
Figure 10 (inset) shows the thickness distribution using the EUV spectrometer of a 2
mm × 2 mm TAP sample (thickness: 250 nm) after a pump-probe NEXAFS measurement.
The pixel values of the CCD images were converted into TAP film thickness using the
formalisms explained in [61]. Inhomogeneities in the order of 20 nm to 30 nm can be seen
with an average thickness of 250 nm. The relative error of the EUV transmission
measurement is estimated to be 20%, and the lateral resolution was 25 μm. The dark dots
are small debris particles from the LPP as the sample was not protected by a debris foil in
this measurement.
Figure 9.
UV/Vis spectrum of the NiO sample (thickness 300 nm) on Si
3
N
4
corrected for substrate
transmission.
3.4. TAP
TAP thin films were prepared by evaporation on 1
×
1 mm
2
Si
3
N
4
membranes.
The UV/Vis spectrum of such a 120 nm thick film in comparison to TAP solved in ethanol
(c = 3.6
×
10
−4
mol/L, 1 mm quartz cuvette) is shown in Figure 11. In porphyrins and its
derivatives, the S
1
state splits into two states due to molecular symmetry, which are named
Q
x
and Q
y
. Transitions to the S
2
state led to the formation of the Soret band or B band [
60
].
Figure 10 (inset) shows the thickness distribution using the EUV spectrometer of a
2 mm ×2 mm
TAP sample (thickness: 250 nm) after a pump-probe NEXAFS measurement.
The pixel values of the CCD images were converted into TAP film thickness using the
formalisms explained in [
61
]. Inhomogeneities in the order of 20 nm to 30 nm can be
seen with an average thickness of 250 nm. The relative error of the EUV transmission
measurement is estimated to be 20%, and the lateral resolution was 25 µm. The dark dots
are small debris particles from the LPP as the sample was not protected by a debris foil in
this measurement.
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 12 of 18
eV accounts for 0.54. In comparison, the related value in the Ni L-edge region (850–870
eV) amounts to 1.1.
Figure 9. UV/Vis spectrum of the NiO sample (thickness 300 nm) on Si3N4 corrected for substrate
transmission.
3.4. TAP
TAP thin films were prepared by evaporation on 1 × 1 mm² Si3N4 membranes. The
UV/Vis spectrum of such a 120 nm thick film in comparison to TAP solved in ethanol (c =
3.6 × 10−4 mol/L, 1 mm quartz cuvette) is shown in Figure 11. In porphyrins and its
derivatives, the S1 state splits into two states due to molecular symmetry, which are named
Qx and Qy. Transitions to the S2 state led to the formation of the Soret band or B band [60].
Figure 10 (inset) shows the thickness distribution using the EUV spectrometer of a 2
mm × 2 mm TAP sample (thickness: 250 nm) after a pump-probe NEXAFS measurement.
The pixel values of the CCD images were converted into TAP film thickness using the
formalisms explained in [61]. Inhomogeneities in the order of 20 nm to 30 nm can be seen
with an average thickness of 250 nm. The relative error of the EUV transmission
measurement is estimated to be 20%, and the lateral resolution was 25 μm. The dark dots
are small debris particles from the LPP as the sample was not protected by a debris foil in
this measurement.
Figure 10.
UV/Vis spectrum of TAP in solution and prepared as a thin film (thickness of 120 nm).
Inset: EUV transmission image of a thin film TAP sample (thickness about 250 mn).
Int. J. Mol. Sci. 2021,22, 13463 13 of 18
3.5. PIC
PIC thin films were also prepared by evaporation. The UV/Vis spectrum of a 250 nm
thick PIC sample is depicted in Figure 11. The solution at concentrations below
10−4mol/L
in a 1 mm thick quartz cuvette shows absorption bands located at 527 nm, whereas an
increase in the concentration leads to the appearance of the characteristic J-band at 573 nm.
Our preparation technique results in the formation of a mixture of monomers and H-
aggregates in the film with a hypsochromic shift of the H-aggregate absorption band in
comparison with the monomer. This behavior is in contrast to other works using a spin
coating preparation technique, which results in the formation of J-aggregates [
62
]. Because
in our case the preparation starts with a dye powder rather than an aqueous solution
as in [
62
], the thin film contains mainly H-aggregates and monomers. Simulations of
the aggregation process during thin film formation suggest a strong dependence of PIC
aggregation on the initial conditions of the preparation process, indicating that, in most
cases, a mixture of different aggregates and monomers exits [63].
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 13 of 18
Figure 10 UV/Vis spectrum of TAP in solution and prepared as a thin film (thickness of 120 nm).
Inset: EUV transmission image of a thin film TAP sample (thickness about 250 mn).
3.5. PIC
PIC thin films were also prepared by evaporation. The UV/Vis spectrum of a 250 nm
thick PIC sample is depicted in Figure 11. The solution at concentrations below 10−4 mol/l
in a 1 mm thick quartz cuvette shows absorption bands located at 527 nm, whereas an
increase in the concentration leads to the appearance of the characteristic J-band at 573
nm. Our preparation technique results in the formation of a mixture of monomers and H-
aggregates in the film with a hypsochromic shift of the H-aggregate absorption band in
comparison with the monomer. This behavior is in contrast to other works using a spin
coating preparation technique, which results in the formation of J-aggregates [62]. Because
in our case the preparation starts with a dye powder rather than an aqueous solution as
in [62], the thin film contains mainly H-aggregates and monomers. Simulations of the
aggregation process during thin film formation suggest a strong dependence of PIC
aggregation on the initial conditions of the preparation process, indicating that, in most
cases, a mixture of different aggregates and monomers exits [63].
For the 270 nm thick PIC sample, an AFM image of a 5 μm × 5 μm and a line scan on
the edge were performed (cp. Inset in Figure 11). The deviation from the z = 0 position of
the AFM tip is shown in the color scale. On the nanometer scale, the film thickness only
varies about 1 nm to 2 nm. The line scan at the edge of the sample shows a film thickness
of 300 nm ± 10 nm, which agrees with EUV transmission measurements.
Figure 11. UV/Vis absorption of PIC. The thin film spectrum (blue) shows a blue shift in comparison
with the monomer (green curve) spectrum in solution, indicating an H-aggregation of the film. At
concentrations > 10−2 mol/L, the characteristic J-band appears (red curve) in aqueous solution. inset:
AFM analysis of the homogeneity of the prepared thin film (thickness 270 nm).
3.6. DFT Simulations
Calculations based on time-dependent density functional theory (TD-DFT) with the
aid of the freeware ORCA [64] were performed to better understand the carbon and
nitrogen K-edge absorption spectra. Information from these calculations includes the
shape of the HOMO and LUMO, the shape of the NEXAFS spectrum and the assignment
of the NEXAFS structures to the involved transitions of the corresponding atoms. For the
TD-DFT calculation, the B3LYP functional, together with the def2-TZVP, basis set was
used. The RIJCOSX approximation was employed using the auxiliary basis set def2/J. This
Figure 11.
UV/Vis absorption of PIC. The thin film spectrum (blue) shows a blue shift in comparison
with the monomer (green curve) spectrum in solution, indicating an H-aggregation of the film.
At concentrations > 10
−2
mol/L, the characteristic J-band appears (red curve) in aqueous solution.
inset: AFM analysis of the homogeneity of the prepared thin film (thickness 270 nm).
For the 270 nm thick PIC sample, an AFM image of a 5
µ
m
×
5
µ
m and a line scan on
the edge were performed (cp. Inset in Figure 11). The deviation from the z = 0 position of
the AFM tip is shown in the color scale. On the nanometer scale, the film thickness only
varies about 1 nm to 2 nm. The line scan at the edge of the sample shows a film thickness
of 300 nm ±10 nm, which agrees with EUV transmission measurements.
3.6. DFT Simulations
Calculations based on time-dependent density functional theory (TD-DFT) with the
aid of the freeware ORCA [
64
] were performed to better understand the carbon and
nitrogen K-edge absorption spectra. Information from these calculations includes the shape
of the HOMO and LUMO, the shape of the NEXAFS spectrum and the assignment of
the NEXAFS structures to the involved transitions of the corresponding atoms. For the
TD-DFT calculation, the B3LYP functional, together with the def2-TZVP, basis set was used.
The RIJCOSX approximation was employed using the auxiliary basis set def2/J. This level
of theory was also used to optimize the geometry of the PIC molecule. The geometry of the
TAP molecule was optimized by using the universal force field (UFF) method.
Int. J. Mol. Sci. 2021,22, 13463 14 of 18
TD-DFT calculations deliver oscillator strengths of the first 100 electronic transitions
at the respective energy as a delta function for each atom. The delta functions were
subsequently convolved with Gaussian functions with a full width at half maximum of
0.5 eV to better reflect the actual shape of the experimental NEXAFS spectrum. The whole
spectrum is then uniformly shifted by several eV.
3.7. Detectors and Data Acquisition
The standard detectors used are soft X-ray CCD cameras (Greateyes GE 2048 BI) with
read-out times of a few hundreds of ms–seconds depending on the number of pixels binned.
Alternatively, CMOS detectors offer the possibility for a faster acquisition [44].
For the sub-ns setup, the reference and sample spectra are focused with pairs of RZP
optics onto the large (27.6 mm
×
27.6 mm) sensor area, rendering single-shot NEXAFS
spectroscopy feasible. Additionally, a pinhole positioned in the direct path between the
source and the detector enables the imaging of the source positions and intensity. NEXAFS
measurements using this setup are described in detail in [
45
]. For the measurements at
the carbon K-edge and N K-edge of PIC and TAP samples between 200–20,000 images
have been collected. For the NiO O K-edge and NiO Ni L-edge measurements, around
4000 images were processed. For tr-NEXAFS measurements on TAP, the experimental
details are summarized in [34].
The ps- setup (cf. Figure 5) with the prototype RZP A9 used full-frame measurements
of the 1” CCD detector for either the reference or sample transmitted signal. Hence, the ps-
setup with the prototype RZP does not yet allow a coincident reference and measurement
signal (cf. Figure 6) nor a faster readout by binning. Concerning the source pointing
correction, this setup uses a post-evaluation correction applied on close-to-single laser
shot measurement series. Measurements with this setup on the Ni L-edge processed
measurement series of about 100 images.
Further details of the NEXAFS measurements and its evaluation methods are de-
scribed in [45] and in Table S1 in the Supplemental Information.
4. Conclusions and Outlook
We have developed an experimental approach for elucidating the structure of transient
electronic states using ultrafast laboratory-based soft X-ray sources. For this purpose,
highly brilliant soft X-ray sources were developed, as well as measurement schemes
adapted. In order to obtain the required high spectral resolution, advanced X-ray optics
with reflection zone plates as dispersive elements were applied. Here, the development of
structures on curved substrates enhances the spectral range of high resolving power by a
factor of more than 30, rendering efficient analysis of multiple edges feasible. Additionally,
EXAFS measurements become easily accessible.
We presented the first measurements with a prototype RZP on a bent substrate in the
photon energy range at 850 eV. We could show that this new optical element provides a
high spectral resolution for a wide spectral range close to the resolving power of optimized
methods for planar RZPs. With a higher number of recorded images, the statistics and by
this the resolving power could be improved, favorably with using an sCMOS detector for
a significantly shorter read-out time [
44
]. The current development of this experimental
setup focuses on the coincident measurement using two RZPs on curved substrates as on
the optimization of alignment routines and post-processing.
With these new laboratory instruments, we pave the way for state-of-the-art tr-
NEXAFS spectroscopy independent of large-scale facilities. Thereby, a variety of ap-
plication fields is opening up, which are by no means restricted to organo-molecular films.
The adaption of our approach to other ultrafast lab-based sources, such as harmonics gener-
ation [
65
], is under the way and could extend the time resolution for soft X-ray absorption
investigations to the attosecond range [43].
Our TD-DFT calculations carried out for the TAP and PIC films explain the measured
NEXAFS spectra well and assign the observed spectral features to the electronic structure
Int. J. Mol. Sci. 2021,22, 13463 15 of 18
of the molecule in its ground state. The simulation of the NEXAFS spectrum of an optically
excited state (as for tr-NEXAFS) requires a higher effort (for porphyrin molecules cp. [
29
]),
and it was not in the scope of this publication. However, such simulations are planned for
the future.
Besides the possibility to detect and characterize transient electronic states, our lab-
based tr-NEXAFS approach allows the investigation of thermally or non-thermally induced
phase transitions as well as relatively slow in situ reactions or slow dynamic processes
in catalysis [
66
]. The application of our setup for quick X-ray absorption fine structure
measurements on Ni/TiO2nanostructures is described in [44].
For successful tr-NEXAFS experiments, pre-characterization of samples is a pre-
requisite. Through the careful matching of optical pump pulse parameters using UV/Vis
spectroscopy and NLA measurements, radiation damage of the sample can be minimized
or prevented and transient contrasts enhanced. The design of the spectrometers addition-
ally minimizes radiation damage, as, on the one hand, both the pump and the probe pulse
have large footprints on the sample. On the other hand, the systems operate completely
background-free, i.e., no residual (probe) light reaches the samples in between pulses. Since
the complete spectrum is measured for each laser shot, energy scanning is not necessary,
which reduces the radiation dose significantly. Nevertheless, the optical pumping does lead
to heating of the samples, thereby possibly introducing temperature effects that complicate
the analysis of the transient signals. For this purpose, a sample holder for static temperature
measurements is designed for our setups. With this holder, first UV/Vis experiments were
conducted, which allow choosing an optimal wavelength for pumping by selecting the
wavelength with minimal change in the UV/Vis spectrum. In the future, for each new
sample system, a pre-characterization concerning temperature effects will be implemented
in the experimental methodology. Since heat loss by thermal radiation is high compared to
heat transfer by conduction in nanometer-thin films, the sample holder will use hot gas to
heat the sample evenly in the future. A very promising approach to prevent sample damage
due to heating effects is the usage of a liquid flat jet carrying the molecule of interest in
the solution [
67
]. However, this approach requires a relatively high volume of sample
molecules, not applicable, i.e., for specially prepared parts of photosynthetic apparatus.
To overcome this limitation, a liquid cell for transmission measurements on molecules in a
solution based on an existing cell for fluorescence experiments [
68
] is under development.
Supplementary Materials:
Supplementary materials can be found at https://www.mdpi.com/
article/10.3390/ijms222413463/s1.
Author Contributions:
Conceptualization: J.B., I.M., A.J. and H.S.; Methodology: J.T., D.S., R.G.,
L.T.G., A.E. and T.K. All authors have read and agreed to the published version of the manuscript.
Funding:
Part of this work was funded by Deutsche Forschungsgemeinschaft (# 313838950). HS and
JB are grateful for support from ProFit (MOSFER # 10168892). TK and AE are grateful for support
from ProFit (MOSFER # 10168775), AE is grateful for support from ProFit (MOSFER # 10168769).This
project has received funding from the European Union’s Horizon 2020 research and innovation
program under grant agreement no. 871124 Laserlab-Europe.
Institutional Review Board Statement: Not applicable to our research topic.
Informed Consent Statement: Not applicable to our research topic.
Data Availability Statement:
The authors confirm that the data supporting the findings of this study
are available within the article and its supplementary materials.
Acknowledgments:
The authors would like to thank B. Beckhoff (PTB) and R. Unterumsberger (PTB)
for providing the NiO NEXAFS measurements at the SX700 beamline at BESSYII Berlin.
Conflicts of Interest: The authors declare no conflict of interest.
Int. J. Mol. Sci. 2021,22, 13463 16 of 18
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