Ionic fragmentation of inner shell excited states of CO2 and N2O

Volume 66, number 1

IONIC

CIII:UICAL

FRAGMENTATION

OF INNER

PIIYSICS

SHELL

I October 1979

LETTI RS

EXCITED

STATES

OF CO-, AND

N,O

The rehtirc abundances oftragrnmt mm produced in the decay of the fC Is-‘, r* 1 mner _ shell exated stdtc of CO2 snd the (S Is-‘_ z*) states of S20 unolking promotion 01 &her the trrniind (1-l-J or czntr.d (XC) niiroprn 1s electrons, hare been imestigtcd b> an electron - wn coincidence techniqus.

1. Introduction The inner shell electron (C Is) and X10 and Brim

energy loss spectra of CO,

(N 1s) have been exunined

[I 1 using 23

keV electron

impsct

by W&t and by

using 1.5 kcV electron impact. The CO1 C Is spectrum is dominated by an intense peak at 290-7 eV arising from promotions of C Is electrons to the unoccupied T* orbital. In the N Is spectrum of NZO, t\\o sharp features of comparable intensity are observed at 401 _I and 404.7 eV, corresponding to promotions of the 20 [terminal N Is (KT)] and 3a [central N Is (NC)] electrons respectively to the unoccupied in* orbitn!. The present communication reports a study of the ionic decay of these (Is- t , TK*) states of CO1 And N,O using an electron-ion coincidence technique. A p.uticu1.u goal of this \iork wds to investigate the dependence of the tonic fragmentation on the location of the N Is hole in N,O. A comprehensive electronion coincidence study of the vatence-shell photoionisarion and ionic fragmentation of NT0 and CO, has been reported separately [4] _ Relatively few studies of the ionic fragmentation following molecular ionisation have been performed at soft X-ray ener$es- CarIson and Krause [5] have measured the photoionisation mass spectrum of CO, at 930 eV_ Van Brunt et al. [6] have studied the reiaTronc dt A

[2.3]

ti\e yields of thermal ions from photoionisation of CO, df energies of 0.23. I.25 and 1.5 keV. Hirsch et al. [7] h.~v,ereported similar data for N,O. None of these results study frsgment~tion folIo&g inner shell e_\citation to the n* levelsIn the clecrron-ion coincidence {(e, e + ion)] esperiment the continuousiy >sriable electron eners loss is analogous IO a tuneable Ii&r source. The ability of the electron-ion coincidence technique to “tune” the equivalent photon ener_gy has enabled examination of “photoionisation“ mass spectra at energies where promotions to specific inner shell excited states form the dominant signal. This has allowed a reasonably accurate determination of the characteristic ionic decay of these states- Prebious electron-ion coincidence measurements have studied the ionic fragmentation folIo\\ing inner shell excitation and ionisation of W, (N Is) [8,9]. CO (C 1s) [S,lO] and SF6 (S 2p) [I lf_

2_ Experimental The electron-ion coincidence apparatus and technique have previously been described in some detail [ 12,13]_ Modifications employed for the study of NT0 and CO, were discussed in the presentation of the valence-&e11 data for these molecules [4]_ Since 113

C‘lfl

\IIc‘.\L

WI\

SK

s

LI -l-r1 IiS

iu the present *.\urb.Since S LeV impxi energ .tnd scattering -shout /era degrees are used. the r&ti\c ion 1 irk niwsurtd I~\ the eIectron -1011 coincidence tccbmquc arc capxtcri ItI hc sinu1.u to tiwse wcasurcd in piiotoi~~nin~tiori 1ll~5s spxtruswpy ti~irlg pIlotor iillp.!ct_ It ~irould Iv iwted tli.!t dcvrations from tbc apprwAni.Aori of orlt~ electric &p& transitions in for\tard incI.tstic cfcctrctn stxttcring coufd bccinllc signifkmt at bi$irr ckxt.~tron rncrgics o\\ ing to tbc incrc;lsed II~~~II~I~~LIII~ tr.msfer_ 1Iowevcr. ;! prewous study of the X Is energ& Ioss spectrum of N1 11itli this apparatus [ ILtl is in ewxllcnt agreement wit11 Lktcr syncbrotron pbotoxbsorption results [ 15 I_ The sorrcctions for ion signals from b.iclgrour:d Jnd impurity X7 md 0, abicb were ;1ppIird to tbc ~de11cc-Shd &!tJ [aI_ codd !Wt be ev.dUJtCd for the inner shsit data due to tbr Iimitcd strttisticr rend to tbr Ixk of OS/O; ratios ;Lt tbcse energies_ Tbc ktlonrl presence of these impurities [3] impIies tI1.G the rclative yields of KT_ SC+ xxi 0’ from K\;,O and of CO” Xowever snd 0 + from CO, arc slightI>- overestinmtcd. thcsc systematic errors arc thought to be only of the order of the stAstica1 imprecision. judging from tbc fact tllat the impurity contributions to these ion sisWJh

Were

iin

kS

t!lJn

10%

St

tilt2 IligkSt

VC!kIKc-sile~1

pbotoionisarion energies studied [-I]_ In addition_ the inner shell data are espected to have sm.rller contributions from background gas than the vaIence-sheli data since the inner shell TOF spectra were recorded using somewhat higher sampie pressures-

t

I

1

405eV

! II ! NO’

N;

0’

It

-00

30

t 20

2l5 Ian

tune

I

10

of fhght

(US)

i‘~g. I_ Time of fli$t ixtss spectra XI rhr K Is-‘. r-1 . srrtie of co_ -

nnd the (N-’T . x’)

T *) .fnres of X20_ Thcmrrnsit~

adn

.st energies arc

corresponding

rehtire

and (Sz.

and dlffcr-

3_ Results and discussion

ent for each spectrum.

The time of flight (TOF) mass spectra recorded at 291 eV (C Is-t, a*) in CO-, and at 40_1eV (NTt. zz”) and 405 eV (NFL, a*) in N2 are presented in fig- I - The relatively large background of accidental coincidences in the CO2 (291 eV) and N,O N, (401 eV) TOF spectra arose from the overly large ion count rates employed in these runs. This is also the source of the rise in the background at 3 m time of flight in the N,O (NT) spectrum- The TOF spectra were recorded using the ion signal to start, and the delayed electron

sigrml to stop a time-to-amplitude converter and thus the 3 p TOFregion corresponds to short times between the ion and electron signals. Wren the accidental rJte is relatively high these short time events tend to inhibit the observation of longer time events (a dead time effect) causing non-Iinearity in the background of accidental coincidences- This does not affect the relative intcnsities of the true electron-ion coincidences. Additional TOF spectra were recorded at 200 eV in CO, and at 423 eV in N70_ The regions to lower

214

I979

case ntq be ;1s large as the S% yield measured in CO, at 930 eV [5]. The refariw peak areas mezlsurrd from the TOF spear& are fisted in tables 1 and 3 in comparison witii the soft X-r+ photoionisrttion mass spectroscopic data for these mofecuies [S-7]_ The relative abundances me,wred at :he energies of the (Is-‘. 7is) states C291. 101 .md 405 cV) are expected to be close to the rictu.d ion bmnching ratios for these states since the contribu-

time of flight containing sigxds from multiply charged atomic ions were not recorded in the present ~orh. Altfwug!x these ions are expected from the fragmcnration foilowing Anger decay of inner skIi ioniscd species fULc>l, they 9re not expected from v&nceshell ionisation or from the decay of (Is- t ~ P*) states.. Ttms the ncgkct of these species should affeut only tile N Is continwni data at 423 tV in X,0. The cuntribntion front multip:y charged atomic Lns in this

T3bk

1 October

CIII WC \I_ PII\ SICS t_I.Ti-CRS

v~*Iume 66. number 2

I

li&mw

abundances

and Linetx energies

-(eV) ~-_____XI0

rhis x
‘31 930 X0 1500

CO;

CO’

0’

0.36621 O.littt 0 05

O-1 IC2) 0.21(I) 0 09 003 0.01

0.3X?) O.-%l(i’) 0 52

0.18 0.01 ion

this mark trt

291

-0

930 ~___~_-__--_-___-_

in CO1 -_.__________

__- ______-__“-I__

ReIdtire ion .tbund.mces - _--_I-___ _.-- ---_---

Ref.

rnergy

&ssoclJtion for eon tkmation ----------- ---_

ot

--- --------.

hincric meqr

-

lLitr0 co+ic+, CO:+

C---.-.--.--w--L

0.75 0.95

O_lY(1) 027f I) 0.15

0.0X1) 0 0.01

-

0.01 0 01

I J

thermd

1.6 IJ 2.1

teV) 5 8

thhizrm41 OS thermst 15 --~~---

‘) Calson

wd l&me [5 f_ The rclaire ~bundxwes do not sum to umty because ofsn addaionalS6 >ieId of mL.ltipIv charged ~romtc iona (C’* Z_-l~,C3+0.3%. 0’+5%, 03+0.2’;). b) Van Brunt et A 161. Their therm.d abundances hxre been renormaliscd IO the rota1 number of ions viis their therm31 fractions. CO’.md

CO?

are assumed to hare essenthiig

cf See ref_ fl9jfor

Table r! Relative abundaxes Energy ieW

401 405 423 280 1500

details

of the deriration

thermal energies.

of the

awxge

ion kineticenergies

from the observed TOf

peak xxdrhs.

and binetic energies of dissociation for ion formation in NzO YP______ Ret-_

--~__---_--_ this work this work this work 3)

3)

Rehwe

Ratio

ion abundsnces --~

IYZO’

N;

0.05(l) 0.03(I) 0

02ot I ) 0.14(f) O.lO(3)

0.2 1 0.01

005 0

ion kinetic energy

~

0‘

x’

020(l) 0.18(l) 0.10(3)

0.17(l) 021(l) 027x3)

O-39(1) O-44(2)

2.3 2.1

-0.M

O-54(4) -

2.0

0.10

x0”

--__I-

099

-

(CV)

401

this work

thermal

0.7

0.8

405

this work

thermal

0.8

1.2

6 7

7 5

a) Hirsch et al. [7 I_ Their thermal ribundances have been renormalised to the total number of ions via their thermal fractions h’s and NO+ are assumed to have eaenti&y rhermal energies at 280 eV_ b, See ref. (19 1 for details of the derixxrion of the average ion kinetic energies from the observed TOF peak widths_

E~periment.ilI~ _ t!Le rcI.tti\c d-m~id~~~ccs of atomic ions \\a fourtd to he rtlmost ‘0% I.xger AL105 eV (XC- ) th.m at -IO f CV ( I\ r )_ There .Lrcrtlso su~~eslions of a slightI> greater yield of YO’ rtt -IO! e\’ 13 r) rtIthuug!L tile st.LtisticxI sigLLiikuLce of the dit~ierrnse is nix&d?hus the differences in the ionic fr3gLncnt~tion at Lhe Lwo energies are qualitative xi e_xIwekxI from core II& IocnIis3tiou_ The on!_\ other riotshI. differalec betaeeri the ionic fr2gnLcnt3tiorL of the tuo (h Is- i _ z*) st2tcs. aside from the sm;lIl increzsc in &herelrttiic ;rhundznce of atomic ions in the frrrgmentarion of (X‘;s’ _ h* )_ is the grcxttr peak width_ and thus Ixger average kineric energy, of SO+ from *he (SE*, 3if ) #state_ The origin of tILis ridditional xtidth is not understood aIdlough contributions from J double Auger process (XX:‘0 - NO* i- 0’ + ZC-) could possibty xcount for itMthough the diffcrenccs in the ionic fragmentation at the two energies are quaiitatively as expected, the rc!ative abundances for the (NT’, a’) and (Ne’ . c*)statcssccm rcmarkabIy similar- One possible intcrprctation of this obscrvrttion is that core Iocalisrttion is not compIcrc. This seems urdiktly since relatively strongcore hole localisation is thought to occur even in

N1 [ 161 where the uu and ug N Is levels are estimated to be separated by only 0-I eV [ 17]_ The 3.6 cV separation of the NT and i\ic levels in &O [I] suggests an even stronger cow hole Iocalisation in N,O that in N1_

‘Ihe abi!ity

to interpret

the N Is spectrum

of N-,0

the equivaknt core analogy also indieatcs strcng core hole localisrttionA second possible interpretation of the basic sirniIarity between the NT and NC relative ion abundances [ IJSJ

216

with

A third intcrpret~tion of the simLI~rir~ .)f the \ L .Lnd S, reIati\e ion .AxLnd.mccs is :h.Lt m.~ny difLScrent Lcmiz fr~r~mciit~ticrnchmnci~ coiltrlIllltt’ 50 Lh.11the bond hre.Ain~ arid chxze drsrrlhulicw m 111s:onLc ir.LgiiieiLt~tion of higlliy ewired X,0 .mtI C 0, WXLII~ in .m appro~lrllrtteIy r.mdom manner. Sc\erA otbcr lkiturcs of the mertsurcd rclati\c ~bundrtncea 5ub;c%f tflis ~iewpilitlt. OLle ch.irxteristic fe.ttLLre;Lt AI energies exul~ined is that tile S*iO* r.LtLoin X,0 .LLIJttw O’;C? r.Ltio in CO1 Are rcr> .zIow Lo 2 (see t.mI2s I md 2)_ TIus ~1s xIso obserred ill the x.dcrLcc-sIwII I&ttwonis;LtLt~L~ of SIO .nLd C 0, ;Lt .111 energies atxl~c -IO eV [i]_ In addition. at~tmxilmtcly equal reI.itnc xhund.LLtcesof A\; .md SO* are observed from Lhc ionicdec3J ofbo& (S Is-l _;i* )stAesxld in the S la eontiLluiLlll of %?O_ Al1 of thcsc fe.iLurcs polllt to J ‘-pseirdo-r~ndoLLL.*ionic fragmentation in X:,0 .uw CO,. Thus. the present resuIts suggest :hJt the ionic fr.kgmcntation pattern is not tremendously scnsiti\e to the initia1 excitation process \\ILenlarge excitation energies xc m~oIr_cd. Further studlcs of the chxwteristic fragmentation of inner shell exited st.ttes would be useful to test this tentative conclusion_ Although rile soft X-ray photoionkaion m.~ssspectra of CO1 and X20 btl\c bean imcstigatcd cxlier [S--7]. detailed comparisons are of limited value bec;lLLseof the different photon energies used. The data of Carlson and I-husc [5] for tile ion yields from 930 CV photoionisation of CO, is the mos: rcicvant to the present work since ions of all kinetic energies arc taken into account, as in the present apparatus_ However com-

parison is stilI limited by the fact that both carbon Is and oxygen Is ionisation occur at 930 eV. Even so, the relative abundances rrt 930 eV have some reseniblance to those for tbe (C Is-t,

zr*) stare in Lhat the 0+,X? ions (C’ + O+) is very iarge- The data of van Brunt et al_ [61 both at 2SO cV, beIow the inner shell threshold and at 1500 cV, in the carbon and osygcn inner she11continua, have been rcnormaIised using their estimates of the yields of thermal ions at these energies in order to provide a

ratio is two and the yieId of stotnic

tx ill Ul(‘.\L

PIIYsIcs

nwre \.Ld comparison to the present AILL The trends in the parent wn .md aromic fragment ion 1 iclds at 100. X0. 29 I and 1500 cV in CO1 (see i.tblc 1) seem c~u.dit.fri~cl~ consisrcnr since the rcnorni~iiscd 30 cV datJ of k3n kunt ct .11_[(,I shov. .I lov.cr ylcld of COG md a higha ywld of.uoniic ions than \\c dctcrminc 31 ‘00 cv. As ;I final comment wc now that the relative abundaxc of COT (36%) from rhc fC Is- 1 _x*) sfxc is consixtent with tile electron inip.wr*xLttcd carbon KLL Auger spectrum ofC0, [20] which shows d traisition at 172.6 CV arising from .mto~oniwtion Lo the 13swc of CO& at 1S.I cV binding energ)-. Since the B state does nlor fragmcnr fi]. rhis parricular zlutoionisrttion process results in CO; as obscrxcd in the cicctronion coincidence cxp&ment_ X second peah assigncd to tlutoionisdtion of the (C ls-I. z“) state is ohscrced in the KLL Xugcr spccrrum alt 268.5 cV [30]_ In rercportcd dipole (c , 32) mcrisurcmcnrs of CO, 1211 ;L pertk is obscrxcd af 23 CV in rhc binding cncrgy spectrum. This may be the find ion st.ttc for the second autoionising transition. The energy of this st.ife (22 cV) is such that fragmcntrition likeiy occurs in this pJrricuIa channc!. This is consisrenr with the i.irge yicki uf fmgmcnt ions from the decq of the (C Is-‘. -* II ) state. The Auger spectrum of NT0 in the region ofautoionisation of the (hy I. ;T*)and (KFI, x*) stam has not been reported to our knowledge. scntly

This work is pzrt of a coll;tborati\+ progrxn of the FOM Institute for Atomic scd ~!olecul~~ Physics. Amsterdam and thr Department of Chemistr_v, Universlt); of British Columbis, Vrtncouver, financially assisted by a NATO Scientific Exchange grmt. One of us (APH) acknowledges financial support from an NRC (Canada) postdoctoral fellowship. This work is part of the research program of the Stichting voor Fundamentcef Ondenoek der Uaterie (Foundation for Fundamgntal

1 October

LLI-rLRS

1979

Rcsc.lrch on Matter) and xv.15 made posslbk by &an&d support from the Ncdcrkndse Organiszltie voor Zuiwr-Nctcnschappclijk Onderzoeh (Netherlands Org.uus~tion for rhe Advmccment of Pure Rescrirch).

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331T_

[?I 1 C II Brian dnd K.H. Tan, Chcm. Phls. 3-l (1978)

111.

217