Observation of Collisional De-Excitation Phenomena in Plutonium

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]