This is a sel -a chi ed e sion o an o iginal a icle. This e sion
may di e om he o iginal in pagina ion and ypog aphic de ails.
Au ho (s):
Ti le:
Yea :
Ve sion:
Copy igh :
Righ s:
Righ s u l:
Please ci e he o iginal e sion:
CC BY 4.0
h ps://c ea i ecommons.o g/licenses/by/4.0/
Obse a ion o Collisional De-Exci a ion Phenomena in Plu onium
© 2022 by he au ho s. Licensee MDPI, Basel, Swi ze land.
Published e sion
Raggio, And ea; Pohjalainen, Ilkka; Moo e, Iain D.
Raggio, A., Pohjalainen, I., & Moo e, I. D. (2022). Obse a ion o Collisional De-Exci a ion
Phenomena in Plu onium. A oms, 10(2), A icle 40. h ps://doi.o g/10.3390/a oms10020040
2022


Ci a ion: Raggio, A.; Pohjalainen, I.;
Moo e, I.D. Obse a ion o
Collisional De-Exci a ion Phenomena
in Plu onium. A oms 2022,10, 40.
h ps://doi.o g/10.3390/
a oms10020040
Academic Edi o : Mus apha
Laa iaoui
Recei ed: 27 Ma ch 2022
Accep ed: 13 Ap il 2022
Published: 20 Ap il 2022
Publishe ’s No e: MDPI s ays neu al
wi h ega d o ju isdic ional claims in
published maps and ins i u ional a il-
ia ions.
Copy igh : © 2022 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
a oms
A icle
Obse a ion o Collisional De-Exci a ion Phenomena
in Plu onium
And ea Raggio * , Ilkka Pohjalainen and Iain D. Moo e
Accele a o Labo a o y, Depa men o Physics, Uni e si y o Jy äskylä, FI-40014 Jy äskylä, Finland;
[email p o ec ed] (I.P.); iain.d.moo [email p o ec ed] (I.D.M.)
*Co espondence: and [email p o ec ed]; Tel.: +358-04-57-833-2669
Abs ac :
A p og am o esea ch owa ds he high- esolu ion op ical spec oscopy o ac inide elemen s
o he s udy o undamen al nuclea s uc u e is cu en ly ongoing a he IGISOL acili y o he
Uni e si y o Jy äskylä. One aspec o his wo k is he de elopmen o a gas-cell-based ac inide
lase ion sou ce using ilamen -based dispense s o long-li ed ac inide iso opes. We ha e obse ed
p ominen phenomena in he esonan lase ioniza ion p ocess speci ic o he gaseous en i onmen o
he gas cell. The de elopmen and in es iga ion o a lase ioniza ion scheme o plu onium a oms is
epo ed, ocusing on he e ec s a ising om he collision-induced phenomena o plu onium a oms
in helium gas. The gas-cell en i onmen was obse ed o g ea ly educe he sensi i i y o an e icien
plu onium ioniza ion scheme de eloped in acuum. This indica es compe i ion be ween esonan
lase exci a ion and collisional de-exci a ion by he gas a oms, which is likely being enhanced by he
e y high a omic le el densi y wi hin ac inide elemen s.
Keywo ds: collisional de-exci a ion; ac inide elemen s; esonance lase ioniza ion; gas cell
1. In oduc ion
In adioac i e ion beam (RIB) acili ies based on he iso ope sepa a o on-line (ISOL)
echnique, lase esonance ioniza ion is a widely used me hod o p oducing isoba ically
pu e ion beams in combina ion wi h mass sepa a ion, hanks o i s high e iciency and
selec i i y. I s implemen a ion in acili ies ha use gas-cell-based echniques o ion he -
maliza ion and ex ac ion, such as he ion guide iso ope sepa a o on-line (IGISOL) [
1
]
echnique a he Uni e si y o Jy äskylä, he o me Leu en iso ope sepa a o on-line
(LISOL) in Lou ain-La-Neu e [
2
], o he KEK iso ope sepa a ion sys em a RIKEN [
3
], ha e
led o he de elopmen o he in-gas-cell lase esonance ioniza ion echnique. The combi-
na ion o his me hod wi h he as gas-cell ex ac ion ime and he chemical non-selec i i y
o he used noble gasses allows o he p oduc ion and s udy o e ac o y elemen s, which
a e no o iously challenging o he adi ional ISOL echnique. Lase esonance ioniza ion
is based on a mul i-s ep exci a ion scheme, u ilizing he unique a omic-le el inge p in o
he desi ed elemen . Resonance ioniza ion spec oscopy is a a ian o he lase esonance
ioniza ion echnique, whe eby he mass-sepa a ed ion yield is measu ed as a unc ion o
he wa eleng h o one o he lase s used in he exci a ion p ocess. By measu ing he iso ope
shi and spli ing (hype ine s uc u e) o he op ical esonance, a comp ehensi e p obe is
p o ided in o unde lying nuclea p ope ies, including nuclea spins, sizes and shapes [
4
].
When lase esonance exci a ion and ioniza ion is applied in a gas- illed en i onmen ,
a se ies o mechanisms s a o play a de imen al ole. Fo example, he spec al esolu ion
o a omic esonance in a gas cell su e s om p essu e b oadening and shi due o collisions
wi h he bu e gas a oms. Typical ope a ing p essu es o up o 500 mba limi he esolu ion
o a ew GHz, su icien o mask he hype ine s uc u e o ligh e elemen s. Ne e heless,
in-gas cell spec oscopy has he ad an age o being a e y sensi i e me hod and op ical
spec oscopy can be pe o med on iso opes p oduced wi h small c oss-sec ions. A ecen
highligh has been he success ul demons a ion o on-line, single-a om-a -a- ime, esonance
A oms 2022,10, 40. h ps://doi.o g/10.3390/a oms10020040 h ps://www.mdpi.com/jou nal/a oms
A oms 2022,10, 40 2 o 10
ioniza ion spec oscopy o nobelium in an a gon- illed gas cell [
5
]. A second, less-well-
documen ed e ec o he gas- illed en i onmen , also a ising om collisional in e ac ions,
is he po en ial educ ion in he lase ioniza ion e iciency h ough collisional de-exci a ion.
This e ec , which is o opical in e es and he ocus o his wo k, may be enhanced in
elemen s wi h high a omic-le el densi ies such as he lan hanides o ac inides.
Collision-Induced Popula ion T ans e
Collisions be ween a oms can cause changes in he elec on popula ion be ween a omic
le els. A well known example o his is he Bol zmann-dis ibu ed popula ion o low-lying
le els in a oms eme ging om he mal collisions wi h bu e gas a oms. Simila ly, collisions
wi h an exci ed a om can cause he exci a ion o be ans e ed o a di e en le el o
o he collision pa ne . In his wo k, we a e in e es ed in he collisional de-exci a ion
o he esonan ly exci ed s a es in plu onium and he compe i ion wi h lase -induced
esonan ansi ions. The dynamics o collisional de-exci a ion a e examined below o gi e
a quali a i e unde s anding o he collision p ocess in plu onium. Howe e , as he a omic
s uc u e o hea y elemen s can be exceedingly complica ed, he mechanism o collisional
de-exci a ion is in oduced he e wi h he help o simple sys ems.
T adi ionally, collisional phenomena ha e been s udied mos ly in alkali and alkaline-
ea h a oms and ions wi h noble gasses [
6
–
11
], since only ew loosely bound ou e elec ons
pa icipa e in he collision, esul ing in an easie heo e ical modeling. O he elemen s
s udied in his con ex a e epo ed, o example, in [
12
–
16
]. In gene al, i has been obse ed
ha he a e o popula ion ans e is highe o hea ie noble gasses.
A classic example and p ac ical applica ion o collisional exci a ion ans e is he
helium–neon (HeNe) lase sys em. The helium a oms a e exci ed om he g ound s a e
o highe -lying exci ed s a es by inelas ic collisions wi h ene ge ic elec ons, among hem
he i s wo me as able s a es 1
s
2
s3S1
and 1
s
2
s1S0
. Due o a o ui ous nea degene acy
be ween he helium me as able s a es and exci ed s a es in neon a oms, collisions esul in
an e icien and selec i e ans e o exci a ion ene gy om he helium o neon, as epo ed,
o example, in [
17
]. The subsequen decays o he exci ed s a es in neon a e esponsible o
he cha ac e is ic emission wa eleng hs o he lase sys em.
Ins ead o ans e ing he ene gy o he colliding sys em o he collision pa ne s,
a second scena io is o con e he exci a ion ene gy o he colliding a oms in o ansla-
ional ene gy. This can be unde s ood using he ba ium–a gon sys em as an example,
as ba ium has a a he simple elec onic s uc u e compa ed wi h hea ie elemen s [
18
–
21
].
The collisional de-exci a ion p ocess can be unde s ood by looking a he po en ial cu es
o he dia omic sys em, illus a ed in Figu e 1, o med by ba ium con igu ed in di e en
exci a ion s a es and a gon in he
1S0
g ound s a e. I such cu es ha e c ossing poin s
a gi en in e a omic dis ances, ma ked wi h a ci cle in Figu e 1, and i he symme y o
he c ossing s a es allows he o ma ion o an a oided c ossing, hen an inelas ic collision
can occu , ans e ing he popula ion o he lowe exci ed s a e and con e ing he ene gy
di e ence in o he ansla ional ene gy o he sys em. Fo his speci ic example, ba ium
a oms, exci ed o he 6
s
6
p1Po
1
s a e, o example, ia lase exci a ion, a e collisionally
de-exci ed o he 6
s
6
p3Po
2
le el h ough inelas ic collisions wi h a gon a oms. The c ossing
happens be ween he a ac i e
1Π1
po en ial cu e and he epulsi e
3Σ1
po en ial cu e,
he la e adiaba ically co ela ed wi h he 3Po
2ba ium le el.
The a e o collisional de-exci a ion can be compa able o he op ical de-exci a ion
ha occu s ia spon aneous emission. Fo example, he a e o collisional de-exci a ion
in ba ium o he 6
s
8
p1Po
1
le el by helium collisions a 833 K o he 6
s
7
d3Dj
mul i-
ple has been measu ed o be
∼
3
×
10
−9cm3/s
[
18
], while he a e o de-exci a ion
om he 3
s2
3
p4 1D2
s a e in a omic sulphu in a gon a 300 K has been measu ed o be
1.4
×
10
−11 cm3/s
[
22
]. Wi h a ypical gas cell p essu e (ei he helium o a gon) o 100 mba ,
hese eac ion a es esul in depopula ion a es o abou 3.3
×
10
91/s
and 4
×
10
71/s
o
ba ium and sulphu , espec i ely, compa able o a la ge Ai Eins ein coe icien .
A oms 2022,10, 40 3 o 10
Figu e 1.
Diaba ic po en ial cu es in he ba ium–a gon sys em. The le el c ossing indica ed by a
ci cle is esponsible o he popula ion ans e be ween he
1Po
1
and
3Po
2
le els o ba ium. Adap ed
wi h pe mission om [21]. 1992, Ame ican Physical Socie y.
In hea ie elemen s, namely he ac inide egion, a non-negligible collisional de-
exci a ion c oss sec ion is expec ed due o he high densi y o s a es compa ed wi h he
simple alkali o alkaline-like elemen s. Collisional phenomena ha e been obse ed in
ho ium [
23
] and exploi ed in lase spec oscopy measu emen s o singly ionized Th
+
ions [
24
,
25
]. The lase exci a ion o ho ium om he g ound s a e o a speci ic exci ed s a e
can be ine icien as spon aneous decays inc ease he popula ion o me as able (da k) s a es,
which a e no p obed by he lase ligh . The use o bu e gasses such as hyd ogen and
helium in adio equency aps helps o edis ibu e he popula ion o hese s a es o he
g ound s a e ia inelas ic quenching collisions, imp o ing he lase ioniza ion a e.
Collisional phenomena we e obse ed and also s udied in nobelium and i s chemical
homologue y e bium [
26
,
27
]. A wo-s ep esonance ioniza ion p ocess was used in a bu e
gas cell: he i s exci a ion s ep o esonan ly exci e nobelium om he
1S0
a omic g ound
s a e o he
1P1
le el, while he second s ep d o e a ansi ion o a Rydbe g s a e, which
was subsequen ly ionized ei he by esidual lase ligh , black-body adia ion o collisional
p ocesses. Impo an ly, analysis o he Rydbe g s a es e ealed con ibu ions om wo
in e media e s a es, he single
1P1
le el and a longe -li ed s a e lying
∼
300 cm
−1
below
he
1P1
le el. This lowe -lying s a e, assigned as he
3D3
le el, was popula ed ia a as
quenching om he 1P1le el due o collisions wi h bu e gas a oms.
He e, we epo e idence o such collision-induced phenomena o he case o plu-
onium ia lase esonance ioniza ion s udies. Al hough collisional e ec s o ac inide
elemen s wi h noble gasses ha e been epo ed, especially o low-lying exci ed s a es,
collisional de-exci a ion e ec s emain mos ly unknown in he exci ed s a es.
2. Expe imen al Me hod
This wo k was pe o med in he con ex o he de elopmen o a new gas cell o
he lase esonance ioniza ion o long-li ed ac inide iso opes ha can be p oduced in
su icien quan i ies a esea ch eac o s and anspo ed o acili ies elsewhe e [
28
]. In col-
labo a ion wi h he Nuclea Chemis y Depa men o he Uni e si y o Mainz, samples o
238−240,242
Pu and
244
Pu iso opes we e elec oly ically deposi ed on o a an alum subs a e
and deli e ed o Jy äskylä. The ilamen s, moun ed wi hin he gas cell illed wi h helium
a a p essu e o 80 mba , we e elec ically hea ed o a empe a u e be ween 1000 and 1200
◦
C. The e apo a ed plu onium a oms we e esonan ly ionized wi h lase ligh p o ided by
A oms 2022,10, 40 4 o 10
he FURIOS lase sys em [
29
], wi h b oadband i anium–sapphi e (Ti:sa) lase s ope a ing a
a epe i ion a e o 10 kHz in bo h he undamen al emission ange as well as equency
doubled. La e , a dedica ed g a ing-based Ti:Sapphi e lase was employed in collabo a ion
wi h Nagoya Uni e si y, ha ing he unique ea u e o in aca i y second ha monic gene a-
ion (SHG) [
30
]. This lase o e s a wide- ange scanning capabili y be ween 380 nm and
440 nm and hus is ideally sui ed o ioniza ion scheme de elopmen .
The di e en lase s we e spa ially o e lapped, anspo ed o he on -end o he
IGISOL a ge a ea and ocused slowly in o he gas cell h ough a qua z window. A maxi-
mum ou pu powe o
∼
1 W o undamen al in a ed (IR) ligh was a ailable a he en ance
o he gas cell. A e in a-ca i y equency doubling,
∼
660 mW om he s anda d b oad-
band esona o was a ailable, wi h
∼
105 mW om he g a ing-based lase . In ou s udies
o a wo-s ep blue–blue ioniza ion scheme, discussed below, bo h he JYFL Ti:sa lase and
he g a ing-based lase we e pumped by sepa a e Nd:YAG lase s o allow o a p ecise
iming con ol o he lase pulses o a ew ns.
The esonan pho o-ions we e e acua ed om he gas cell h ough an exi hole and
guided owa ds he high- acuum egion o he mass sepa a o using a adio equency
sex upole ion guide (SPIG) [
31
]. The ions we e accele a ed o a po en ial o 30 kV, mass
sepa a ed wi h a nominal mass esol ing powe o
M/∆M
= 500, and de ec ed in he ocal
plane o he sepa a o using a mul ichannel pla e (MCP) de ec o . A schema ic o e iew o
he expe imen al se up is shown in Figu e 2.
Figu e 2.
Schema ic igu e o he IGISOL beamline o ele ance o his wo k. The gas cell is loca ed
inside he on -end acuum chambe . Resonan lase ioniza ion o plu onium a oms occu s along
he gas cell axis; he ions a e guided h ough a sex upole ion guide and accele a ed owa ds he mass
sepa a o . The dipole magne is se o mass-sepa a e
244
Pu. An MCP de ec o in he sepa a o ocal
plane is used o measu e he ion coun ing a e. FC = Fa aday cup.
Ou ea lie wo k [
28
] ocused on he ob ained mass spec a, which p o ided use ul
insigh in o he gas-phase chemis y exhibi ed by plu onium. The esul ing mona omic
yields o iso opes we e su icien o high- esolu ion collinea lase spec oscopy [
32
],
wi h plu onium cu en ly he hea ies elemen s udied using his echnique o da e. Since
he publica ion o Re . [
28
], u he in es iga ions o elucida e a be e unde s anding o
he o iginal ioniza ion scheme ha e been made, using he same me hodology as discussed
in his sec ion. As he iso opic composi ion o he samples was no o pa icula in e es ,
he sepa a o was uned o he mos abundan iso ope,
244
Pu. The ollowing sec ion p esen s
he esul s o he ioniza ion scheme cha ac e iza ion.
3. Resul s and Discussion
Ini ially, a h ee-s ep ioniza ion scheme was es ed, using lase adia ion a wa e-
leng hs o 420.76, 847.26, and 750.24 nm, wi h he inal s ep esul ing in he popula ion o an
au o-ionizing (AI) s a e a 48,898 cm
−1
(Figu e 3). This scheme was o iginally de eloped by
Raede and collabo a o s o selec i ely ionize plu onium iso opes unde acuum o ace
analysis s udies o en i onmen al samples [
33
]. Su p isingly, in he gas-cell en i onmen ,
he wo IR s eps did no con ibu e o he ion coun a e. Ne e heless, a equency scan
o he i s blue s ep wa eleng h p esen ed a clea esonan signal and hus i was hy-
po hesized ha exci a ion and ioniza ion p oceeded ia a Rydbe g s a e loca ed a a ound

A oms 2022,10, 40 5 o 10
47,532 cm
−1
, wi h ioniza ion occu ing ia a omic collisions wi h He gas a oms. Only a
a subs an ially educed i s s ep lase powe was a small esponse o he ion coun a e
obse ed, wi h he addi ion o he wo IR s eps.
Figu e 3.
Resonance ioniza ion scheme ini ially used in his wo k, de eloped o ul a ace analysis o
en i onmen al samples [
33
]. The h ee s ep blue-IR-IR scheme d i es he elec ons o an au o-ionizing
s a e loca ed a 48,898 cm−1.
To explo e his hypo hesis, a second equency-doubled Ti:Sapphi e lase was imple-
men ed ins ead o he wo IR lase s. The wa eleng h o he second lase was scanned in he
icini y o he i s s ep 420.76 nm ansi ion, wi h a esonance obse ed a a wa eleng h o
422.53 nm which, in combina ion wi h he o iginal 420.77 nm, was ound o conside ably
enhance he ioniza ion a e. In e es ingly, bo h ansi ions we e ound o ionize plu onium
independen ly, albei wi h much educed coun a es. Al hough he e is no known le el ha
can be popula ed om he g ound s a e by he 422.53 nm lase , he low-lying i s exci ed
7F1
s a e a 2203.61 cm
−1
is expec ed o be he mally popula ed due o he empe a u e o
he ho ilamen (and he su ounding helium gas a oms). Exci a ion would hen p oceed
om his J = 1 le el o a known (J = 2) le el a 25,870.69 cm
−1
. Bo h 420.76 and 422.53
nm pho ons hen d i e he elec on ac oss he ioniza ion po en ial. The popula ion o he
he mally exci ed s a e explains he abili y o he wo lase s o independen ly ionize plu o-
nium, unde he assump ion ha he g ound-s a e ansi ion is connec ed o he ioniza ion
po en ial ia a high-lying Rydbe g s a e, as p e iously hypo hesized.
We no e ha he ioniza ion a e wi h bo h blue lase ansi ions was ound o be
∼
5 imes g ea e han he sum o he wo ion a es ob ained independen ly. This beha io
sugges s a connec ion be ween he wo ansi ions and we pos ula e a popula ion o he
low-lying s a e a 2203.61 cm
−1
om he
7Do
1
le el (Figu e 3) h ough collisionally induced
de-exci a ion. I his de-exci a ion p ocess is as compa ed wi h he o iginal IR ansi ions
o he AI-s a e li e ime, i could explain he negligible e ec o he wo IR s eps in he
o iginal scheme when applied wi hin he bu e gas en i onmen .
To u he s udy his beha io , a hi d expe imen was ca ied ou using he g a ing-
based Ti:Sapphi e lase om Nagoya Uni e si y, combined wi h a s anda d b oadband
equency-doubled Ti:Sapphi e. The la e was uned o he o iginal g ound-s a e ansi ion
a 420.77 nm, while he o me was used o pe o m a wide- ange wa eleng h scan a ound
he egion o he p e iously ound 422.53 nm ansi ion. The esul o his scan is shown
in Figu e 4, illus a ing he ion coun a e in he case o he p esence o absence o he
420.77 nm s ep. One can immedia ely see he e ec o in oducing he 420.77 nm lase
ansi ion, as he backg ound ion a e is conside ably enhanced, which we a ibu e o he
lase cons an ly ionizing plu onium independen om he g a ing-based lase , pos ula ed
o occu ia a po en ial Rydbe g le el, as no ed ea lie .
A oms 2022,10, 40 6 o 10
420.5 421.0 421.5 422.0 422.5 423.0
Wa eleng h [nm]
10 1
100
101
102
103
Coun a e [cps]
420.72
420.94
422.31
422.53
JYFL lase a 420.77 nm
JYFL lase o
Figu e 4.
Wa eleng h scan o plu onium ob ained om he g a ing-based Ti:Sapphi e wi h ( ed)
and wi hou (blue) he 420.77 nm lase ligh p oduced by he JYFL b oadband equency-doubled
Ti:Sapphi e lase . The e ical do ed lines indica e he peak maxima acco ding o he uppe spec um
o allow o compa isons wi h he lowe spec um.
In addi ion o he p e iously de ec ed ansi ion a 422.53 nm, bo h spec a indica e
he p esence o new esonances. The cen oid wa eleng hs we e compa ed wi h a ailable
a omic le el da a [
34
], wi h all possible elec ic dipole ansi ions close o he de ec ed
esonances conside ed. A summa y o he measu ed ansi ions is epo ed in Table 1along
wi h he li e a u e assignmen , he ini ial and inal ene gy o he s a es in ol ed, a omic
spins and s a e con igu a ions. In e es ingly, he new ansi ions all o igina e om he i s
ew low-lying s a es in plu onium. As he g a ing-based lase can exci e and ionize he
a oms wi hou he p esence o he 420.77 nm lase adia ion, hese low-lying le els mus
all be he mally popula ed due o he ilamen empe a u e. As an aside, we no e ha
he esolu ion o he esonances in he lowe spec um o Figu e 4is sligh ly wo se, likely
indica ing di e ences in he linewid hs o he wo lase s.
Table 1.
Lis o he wa eleng hs
λmeas
o he de ec ed peaks ob ained om he g a ing-based
Ti:Sapphi e scan as p esen ed in Figu e 4. Assignmen s om he li e a u e a e also gi en [
34
,
35
],
along wi h le el ene gies and con igu a ions. The epo ed wa eleng hs a e in acuum.
λmeas. (nm) λli . (nm) Ei(cm−1)Con igu a ion JiE (cm−1)Con igu a ion J
420.72 420.712 6313.866 5 66d7s2 7Ko
44 30,083.102 - 5
420.77 420.767 0 5 67s2 7F00 23,766.139 5 67s7p 7Do
11
420.94 420.942 2203.606 5 67s2 7F11 25,959.849 5 66d27s 1
422.32 422.306 4299.659 5 67s2 7F22 27,979.161 5 67s7p 2
422.53 422.528 2203.606 5 67s2 7F11 25,870.685 5 66d27s 2
422.539 6144.515 5 67s2 7F33 29,810.974 5 67s7p 3
The p esence o bo h lase s no only esul s in a highe backg ound coun a e, bu also
conside ably enhances he coun a es o he h ee mos in ense peaks. Simila o he hypo h-
esis made wi h ega d o he obse a ion o a conside ably highe ioniza ion a e seen in he
second expe imen (a po en ial enhancemen o he popula ion o he 2203.61 cm
−1
s a e
om he
7Do
1
le el), we sugges ha hese addi ional low-lying s a es a e also popula ed
ia collisional de-exci a ion om highe -lying s a es, ini ially accessed ia he g ound-
s a e ansi ion.
A oms 2022,10, 40 7 o 10
In addi ion o he ansi ions men ioned in Table 1, a ca e ul li e a u e sea ch e ealed
a candida e o exci a ion om he 5
6
7s
2 7
F
6
le el a 10,238.473 cm
−1
o he 5
6
7s7p J = 5
le el a 34,004.30 cm
−1
. This ene gy di e ence would esul in a esonance a a wa eleng h
o 420.772 nm, which lies only 8 GHz om he he o iginal 420.767 nm g ound-s a e
ansi ion. Due o he con olu ion o he equency-doubled lase linewid h (>5 GHz) wi h
he Dopple b oadening o a omic lines in he gas cell, we expec he a omic esonances o
ha e a b oadening o >8 GHz. In e es ingly, i he
7
F
6
le el is popula ed h ough collisional
de-exci a ions om he
7Do
1
le el, he non- esonan ioniza ion o plu onium can hen
p oceed om he 34,004.30 cm
−1
le el. This p o ides an al e na i e explana ion o he
ioniza ion wi h a solely 420.77 nm lase ligh ia an unknown Rydbe g le el. Due o he
spec oscopic linewid h o he a omic ansi ions in he gas cell, we ha e a s ong p e e ence
o his explana ion.
Combining all o he addi ional spec oscopic in o ma ion ga he ed wi h he g a ing-
based Ti:sapphi e lase , an ex ended ioniza ion scheme o plu onium is p esen ed in
Figu e 5.
Figu e 5.
Plu onium a omic-le el scheme de eloped wi h he g a ing-based Ti:Sapphi e expe imen .
Thick a ows co espond o de ec ed ansi ions, wi h he dashed a ows om he
7Do
1
s a e ep e-
sen ing he collisional de-exci a ion p ocess o he low-lying s a es. The hin solid a ows s a ing
om he a omic g ound s a e indica e he mal exci a ion due o he ho ilamen .
4. Ou look
This wo k has shown e idence o collision-induced phenomena in plu onium h ough
ilamen -based esonance ioniza ion s udies. A clea enhancemen in he ioniza ion a e o
mass-sepa a ed
244
Pu is obse ed when he g ound-s a e ansi ion o he exci ed
7Do
1
le el
is added o a lase esonance ioniza ion scheme ha elies on a single-colo pho on exci ing
(and subsequen ly ionizing) om he mally popula ed low-lying le els. The li e ime o
he
7Do
1
s a e in he gas-cell en i onmen appea s o be much sho e han he a e o he
pho oabso p ion o an IR second s ep o he o iginal h ee-s ep scheme ha was dominan
in a acuum en i onmen . The e is s ong e idence he e o e ha he
7Do
1
s a e is de-exci ed
by inelas ic collisions.
The exac na u e o his phenomena is di icul o ully cha ac e ize, in pa icula in
ac inide elemen s in which he elec onic le el densi y is high. Ve y simila e ec s o ha
discussed he e we e obse ed in ou wo k on ho ium, in which lase esonance ioniza ion
was pe o med in bo h acuum and in a helium bu e gas en i onmen [
23
]. Simila o
plu onium, in a epo ed wo k o he lase ioniza ion o ac inium, a educed in-gas-cell
A oms 2022,10, 40 8 o 10
lase ioniza ion e iciency was obse ed when compa ed wi h in-gas-je ioniza ion, which
could pa ly be a ibu ed o collisional quenching [
36
]. No expe imen al collisional c oss
sec ion da a o he quenching o exci ed le els o he ac inides ha e been epo ed, o ou
knowledge. Ne e heless, in compa ing he ioniza ion schemes p esen ed in his wo k, i is
p obable ha he a e o collisional de-exci a ion is compa able o o g ea e han op ical
abso p ion a es.
In he nea u u e, quan i a i e measu emen s will con inue hese in es iga ions in
neighbo ing u anium, bo h o be pe o med in acuum and in he gas cell. Impo an
in o ma ion ega ding he li e imes o he exci ed s a e in he di e en en i onmen s will
add o ou unde s anding o he collisional p ocesses. We no e ha despi e he ad an ages
o he gas-cell echnique compa ed wi h he adi ional ISOL me hod in e ms o chemical
non-selec i i y, he bu e gas en i onmen p esen s e ec s ha need o be unde s ood.
Lase esonance ion sou ces using ho ca i ies ha e epo ed high e iciencies o ioniza ion
schemes o a ious elemen s. In he adap a ion o hese schemes o in-gas-cell esonance
ioniza ion, conside a ion should be gi en o he po en ial o e y as collisional channels
ha may well be de imen al o he e iciency o he me hod.
Au ho Con ibu ions:
Fo mal analysis, I.P.; In es iga ion, I.P.; Supe ision, I.D.M.; W i ing—o iginal
d a , A.R.; W i ing— e iew & edi ing, A.R., I.P. and I.D.M. All au ho s ha e ead and ag eed o he
published e sion o he manusc ip .
Funding:
This p ojec has ecei ed unding om he Eu opean Union’s Ho izon 2020 esea ch and
inno a ion p og amme unde g an ag eemen no. 861198–LISA–H2020-MSCA-ITN-2019, as well as
om he Academy o Finland unde p ojec numbe 339245.
Ins i u ional Re iew Boa d S a emen : No applicable.
In o med Consen S a emen : No applicable.
Da a A ailabili y S a emen :
The da a p esen ed in his s udy a e a ailable upon eques om he
co esponding au ho .
Con lic s o In e es : The au ho s decla e no con lic o in e es .
Abb e ia ions
The ollowing abb e ia ions a e used in his manusc ip :
RIB Radioac i e Ion Beam
ISOL Iso ope Sepa a o On-Line
IGISOL Ion Guide Iso ope Sepa a o On-Line
LISOL Leu en Iso ope Sepa a o On-Line
SHG Second Ha monic Gene a ion
SPIG Sex upole Ion Guide
MCP Mul ichannel Pla e
FC Fa aday Cup
RIS Resonance Ioniza ion Scheme
Re e ences
1.
Moo e, I.; Dendoo en, P.; Ä je, J. The IGISOL echnique— h ee decades o de elopmen s. Hype ine In e ac .
2014
,223, 17–62.
[C ossRe ]
2.
Van Duppen, P.; B uyneel, B.; Huyse, M.; Kud ya se , Y.; Van Den Be gh, P.; Ve mee en, L. Beams o sho li ed nuclei by
selec i e lase ioniza ion in a gas cell. Hype ine In e ac . 2000,127, 401–408. [C ossRe ]
3.
Hi ayama, Y.; Wa anabe, Y.; Imai, N.; Ishiyama, H.; Jeong, S.; Jung, H.; Miya ake, H.; Oyaizu, M.; Kimu a, S.; Mukai, M.; e al.
On-line expe imen al esul s o an a gon gas cell-based lase ion sou ce (KEK Iso ope Sepa a ion Sys em). Nucl. Ins um. Me hods
Phys. Res. Sec . B Beam In e ac . Ma e . A oms 2016,376, 52–56. [C ossRe ]
4.
Campbell, P.; Moo e, I.; Pea son, M. Lase spec oscopy o nuclea s uc u e physics. P og. Pa . Nucl. Phys.
2016
,86, 127–180.
[C ossRe ]
5.
Laa iaoui, M.; Lau h, W.; Backe, H.; Block, M.; Acke mann, D.; Cheal, B.; Chhe i, P.; Düllmann, C.E.; Van Duppen, P.; E en, J.;
e al. A om-a -a- ime lase esonance ioniza ion spec oscopy o nobelium. Na u e 2016,538, 495–498. [C ossRe ] [PubMed]