Electron excitation of Ar between 26 and 34 eV

153 Inrernnionaf founud of Afass Spcctrometry and Ion Ph_n-ks, 18 (197.5) L53-164 @ Ekvier Scientific Pubiishing Company. Amsterdam - Printed in The Netherlands

ELECTRON

(first

EXCITATION

OF Ar BETWEEN

26 AND

34 eV

received 27 January 1975; in re~iscd form 2 April 1975)

Using data with a high signal-t-noise ratio, clear perturbations are detected in the ionization efficiency curve of argon by electron impact between 26 and. 34 eV_ These

rest&s

make

it possible

to locate and interpret several 3s23p*nln’l’

and

related states.

Few experiments have been performed to detect energy IeveIs in neutral arzon in the 26-34 eV region_ Using scattering techniques a few excited Ievels of argon have been reported [I-4]_ Bergmark et al_ [S] identified single excitation levels of the form 3s3p6nf by the analysis of autoionization electron spectra.

BoIduc, Queminer

and Marmet

163 aIso report single excitation

levels in Ar.

Sanche and Schulz 171 in an electron transmission experiment report some structures in the same enera range. Madden, Ederer and Codling [8] observed and identified a large number of opticaIIyailowed (from ground state) configurations, Some levels are detected by ion or neutral atom impact [9-E]. More recently, Veilfette and Marchand [13] have observed and interpreted many structures in measurements of photon intensity emitted by argon excited by electron impact.

APP_4RXTUS AND

EXPERLUENT’

Monoener$etic

PROCEDURE

electrons from a 127O electrostatic

energ

selector

1141 are

accelerated and cross a molecular beam of argon at a right angle. The ions produced are mass-analyzed by a large quadrupole mass filter [IS], detected with an

electron multiplier f163 and individually counted with a multichannel analyzer operating in the multiscaling mode formin,0 the electroionitation curve- The apparatus and experimental procedures are the same as those used previously in the studyof inert sases He [17],Ar [6], Kr and Xc [IS] and in Ne [19] where they have been fully described_ It has been shown by Bolduc and Marmct [Xl] that autoionizing states may alter the ionization efficiency cucye accordin,0 to a Fano lineshape 1211for negativelv charsed atoms or to the inte_gal of such a lineshape PO] over the electron energy for neutral atoms- These perturbations give rise to structures of very small amplitude superimposed on the electroionization continuum_ However, in order for these structures to be e&ly detected, ZI very hish signal-to-noise ntio must be available and a particuiar technique called ‘straightenin,o through smoothings_* had to be used to extract the structures from the very large slope of the continuum_ This **straightening_~technique ws used for the first time by Bclduc, QukmEner and Marmet [6] and its general properties studied by Cxbonneau, BoIduc and Marmet 13’)]_ No attempt has been made to reduce the effects of the remaining statisticalnoise by direct smoothings of the straightened curve or by other means_ beeawe spurious Structures would then be the expected outcome_ The enew ulibration of the curves is the same as was used at a Iower cneey for Ar [6& The energy has been calibrated with respect to the maximum of the second derivative of the structure corresponding to the He ionization threshold to which the value of 2458 eV was attributed. Aitcrnate swaps of argon structures

and of the ionization thrahoid of argon were also used for comparison as described by Quimkner et al’_[17]_ Calibrations consistent within O-03 eVwere thus obtained_ These calibrations also :kgree closely with structures corresponding to spcctroscopic measurements made by Madden et al_ IS]_

Predictions from extrapohted

energies

In a previous paper, the interpretation of the doubly-excited configurations in neutml neon [19] led to the conciusion that the binding energies of the two outer excited electrons are almost independent of the configuration of the Is’2s’2p4 core_ It czm be seen in Table 1 that this obscxvation is also valid in argon- Let us examine TABLE

Arm ArZI = Refc XL

t

tt’D)_(‘P)I

1.74

l(.'SH'D)I

[(‘D~s(“Df)_(‘P.)Js(~P~)]

i-79

((‘S)~S(=S)-(~D)~S(~D;)J

2.39 2-31

155 the energy differences between the ‘grandparent-’ Ar III terms by removing the outer electrons_ Three terms exist, namely the K L 3~‘3p~(~P, ‘D and IS)_ From observed atomic spectra 1233 (see Table 1) the energy differences are [E[‘D)E(3P) = 1.74eV] and [,5(‘S)--E(‘D) = 2.39 eV], where E( ) means energy of the state having the configuration in parenthesis_ Let us next consider the “parent-’ Ar II terms obtained by adding a single 4s Three terms exist here also; the K L 3s’3pa4s(*P, ‘D and ‘S)_ We can see in Table I that the ener,oy differences are almost unchanged with the addition of- a 4s electron_These ener=v differences are also expected to remain approximately constant with the addition of a second 4s or of a 3p electron. This was the case in neon 1191 and led to a consistent interpretation of the observed energy Ievek So this is the basis for this interpretation of the doubly excited levels of argon. As in neon [19], where the binding energies of the two outer electrons in MgI gave an approximation of the coupling between doubly-excited electrons, let us evaiuate the binding energies in CaL Since the energy in ArII of the levels

Ar I

-fJa2S~ , 4P

CIPd

-

3d(tD)] 3d(%)]

3s23d t’og

3s23P413P,

-34_d

-32-G

33-32

-31-01 J -so_= “.

-33-m -32.9

-

1

x_6c

E -29-2 ... 29.0 -

28.8

C SW

I -

4pt3PJ-J

Ar I

29_9i

B -26Jf ..*

D --28-61

A ---26-R

6

E

is i’s)] a

B

Y

Fig- I_ Encr_gy Ievel diagram of 3s’3p%s 19 configurations of Ar_ (. _ _ otir data; - exfmpoiatcd energies)_ Letkn refer fo structures on our data and listed in Table X Three poaiblc limits of hri, one for each grandpaem. arc aligned with that of CaI at the 0 rV Line-_ m Ref_ X3_

3~‘3p*(~P) 4s(‘P)I3240 eV], 3s23p*(‘D)4s(‘D)[34_i9 eV], and 3s23p*(‘S)rls(‘S) [36SO eV] are known from Moore [23], it is possible by extrapolation to predict the energy of the doubly-excited configurations, 4s’, 4s4p, and 4s3d of Ari by subtracting from the corresponding limit 3s’3p’4s the binding energies of the ekctrons as evaluated in Cd_ This was done in Fig_ J_ The first column (2) indicates the coupiing between the two externa! electrons. The vertical distance gives the energy of each revel as ob%rved 1231in Car_ The over-all states with nzsp=ct to the limit (‘S)k(‘S) are found by a&in= these two excited ekctrons to the cores ghen in the upper part

of Fig. f . In column fi the energy with reSpect to the limit is indicated. The figure contains three other coIumns, one for tich 3~‘3p~(~& ‘D, ‘S) of ArI are atigned with Fig_ I, only the energies of the terms of the W and &Tare listed. It is done so because.

core of ArL Three possible limits the corresponding limit of CaI_ In spectroscopic series with the lowest as seen in previous data, the lowest

*‘n’* memthoris strongly dominant with respect to higher terms- For example all of the eight structures detected in the electroionization data of neon 1191 corresponding to doubly-excited conliguntions are first members of a spectroscopic series_The strong predlominance of the lowest “n” member is aJso seen in the electroionization efficiency curves of Nz [24], CO [ZS Jrand NO [26]_ TABLE2 ENERGY

SEPARATIOS

BEIU-EEX

DIFFEREST

J

COUPLING

OF THE (‘P)

E{K L Js=3p*[

J =

WJ-‘PJ- II wws(~Pr=P,-

,)I I~'~~~~~P~'~~l~'~,'P,-,~1

0.13 O-11 O-l?

tarp

(CV)

CORE

1)

J=

Iorf(eVl

0.06 O-06 0.10‘

It is -seeni;l Table 2 that the energy separations between the three J couplings of the K L 3st3p4(‘P) confi,outation (J = 2, 1 and 0) are approximately the same as between the tJuee J couplings (J = 3, Q and f) of the K L 3~‘3p”(~P)%(*P) configuration. It is then expected that the energy differences between the 3 couplings also remain approximately constant with the addition of a 4p electronnre 3s’3p”(‘P)[4s4p(‘P)o)13P con+guralicm Figure 2 shows our experimentai results for electron eneq$es between 2s and 33 eV_ Curve- CIand jl in Fig_ 2 are direct efficiency curves straightened respectively througJ~HOCUand 2.5smoothings in order to eliminate the slope and low frequency components [22]_

52

0.1

t

157

per charmet (in TOgI 5-4 5.8 5.6 -r--

Counts

CO

‘, _-> I -...8C

1

0

i

_--

--_I

!

1

-1 I’

0.2

1

W

-

-_

--‘-

;.-

- .

B

z.t 28

= 14

/

, 29

. 30

1

31

Energy

*

32

.

e

0.1

_J0

,

33

(eV1

Fig- 1_ Cur%-er and fl are the ionization efficiency curves straightened through 1500 and 25 smoothings respectively- The scale on top of the figure gives the number of ions per channel in the original data- The htcral scak may k used to obtain the relative amplitude of a straightened structure COthe continuum. Solid fines indicate the energy of neutral states- Dashed lines indicate the energy of resonant negative-ion states. Energy scale is 48 meV per channel.

Counts

per

(in IO’)

ChaMel

2.65 I

2.60 I

255 1

_ I -. _ -. .

a_ _ .

m 0

. .

*

.-

.

IO 0 t

,

28-2

I

,

2~5-6

29-O

,

29-4

Energy (eV) Fig. 3. Direct ionization efficiency CUIZZstraightened through 25 smoothings. same meaning as in Fig- ZLEnergy scaIe is 24 meV per channel_

Scales have the

158 Let us beg@ the interpretation of the spectra with structures in Fig. 3 which is a part of Fig_ 2 and gives a detaikd view of the first region of interest_ These structures are chosen to begin with, because of a highly reliable spectroscopic measurement made by Madden et al_ [S] in this energy range_ The energy position of this Jastnmsurcmcnt is given in Table 3 to facilitate comparison with the energy position of the structuxs reported here_ This iast table also contains the energy position aJong with rhe interpretation of the structures reported in Fig_ 2. From TABLE

3

LrsT OF OBSERWD 5LaTES

26_52*

pP)rs=(JP)

A

. ( JP&?s4pZ

!

B

C D

E F G N

I I

K

L LV N 0

1’PM~(WIL 3s3PQSP(‘P~ Ir’waP(JP,lL’ (‘D)k’I’D) (‘Pms4pvP)I ‘P2 t’P)[a4pl’PWf’, (3Pms4pc’P,lJr’~ (‘D)Wp’ (‘D)[&4plJPlJ 0’,3d4p J (‘DUd4s J (‘Sj%‘!‘S) ? 7 . 3

;‘SWsWWJW) a I

3 I D

c

ts

‘S-1S 25.61 -- 77 19 29-33 29-39

1 t

29-97

1 30-60 30-92

32-23

f

16-50

2S.05

76.00

2S.36 7s.67 -‘9 IO 29.12 29-32 29-5’8 29-99

25.35

30.69 1 I 31-05 31-3s 31-5’9 31-96 3234 3265 33.06

2S-64=, Td 27.9f zs-oda, ZSL 302-1

2S.O?* 2s. tc

29.225’ 29.30 29.36= 29-9’ 30.706 31.05

32-40

Ref- 13_ Not ail states

fromotherauthors are reportedReT.It Ref. I I. Set text.

ReC 7- See text for the discussion of this IewI_ Ref.. Z L R&T_3b ReT- 4I Ref- IJ Rd: IO- ihe authors dcstztibc the configuration of this level as (‘P)js(*PMp(‘P)k ReT- S- The rtuthors deseribc the configuration of this level as (‘P)js(‘P&lpI .-Ref. -- IO-_The authors&scribetk configuration of this level as (‘D)-Ss(‘bJ4p(‘P)6 Rel. S- Sa tat for the diet&on of this IemI_ a When the stratum is faint the e&mated error is I;usrr than &-O-O5eV due to sm;l&r S]NrJtio.* fbk =Iuc m&t be siiglhtly shifted upward if a (‘P)&%p Ievel is deteetabkc

r

159 Table 3 it is clear that structure F (see Fig_ 3) corresponds precisely io the vaiue reported by Madden et al. [S]. It also corresponds to agood approximation to the extrapolated energy from Cal (see Table 3) of the ekctronic configuration 3~‘3p~(~P)[4~4p(‘P)]_ This last structure is described by Madden et al. [S] as %‘Sp”(‘P)&(‘P&p_ FurthcrmorcinTaule 3 the energyseparation between structures E and F IS-O-12 eV_ In Fig_ 3 above structure F a faint structure might be seen atG at 29.32 eV_ Othernewdata 1131confirm the structure at G (Figs. 2 and 3). The s?.ruciurcsE, F and G, (Fig_ 3) show: (1) the same ‘J” spacing as predicted from theirparents and grandparents (seeTabIe2); (3) a regularlydecreasing amplitude with decreasing ‘J” value; (3) a striking agreement between the optically aliowed transition (J = 1) as reported by Madden et al_ ISJ with structure F (Fig_ 3 and Table 3). Consequently these three points show without doubt that the strnctures E, F, and G form a triplet with J = 2, I, 0 respectively, of the configuration 3~‘3p”(~P)[LIs4p(*P)]_ Among aII the possible final configurations of 3~‘3p”(~P)Lzs4p only two of them give a triplet: the 3D and 3P. Since the J observed are 2, 1 and 0 and from the agreement with the results of Madden and al_ [S] it is possible to write the compIete description of structure E, F and G as being 3~‘3p*(~P)[4~4p(‘P)]~P,, 3~‘3p~(~P)[4~4p(iP)]~P, and 3~‘3p~(~P)[4s4p(‘P)]‘PO respectively_ tD, IS) configurations

Tirt- 3~‘3p~Qr’(~P,

It is easy to realize that the lowest doubIy+xcited level in ArT is the 3~~3p’(‘P&‘(~P)_ It can be seen as structure A (Fig_ 4)_ Veiliette and Marchand

Counts per channel (in 10’ ) I_-0 20

L

1.15

l.20 I-

1 A

_--_

-_-.-

__

60 [

.-

. w

”

0

so

z

. .

.G

. _

20

. e

_ .

0 1, 26

-

.

_ -.

.

.

l

_

_-

w

-e _

. .

_ _

-

_ I 27 Energy

I 28

(eV1

T=i% d Direct ionization efficiency curve straightened through 50 smoothings- Scab same meaning as in Fig. 2. Energyscaleis 48 meV per channd.

have the

[13] also report a scrufzcureat 26_8OeV and &we the same interpretation_ Orher in J?g. 4 have been clearly interprctcd previously [6]_ From the analogy with other inert gases [19]. the energy positions of the 3~~3~~4s’ Jev&s in ArI arc expected to be sii*tJy above the extrapolated values obtained by comparison with CaL From column E in FIN_ I, as expected, the energy position of structure A in Fi=_ * 4 is slightty above the extrapolated energy from CaI_ A more precise evaJuation of the positions of these 4s’ levels is found by comparkon with the doubly-excited ns’ Ievelz of other inert _w_ This is done in Table4 by ckuiating the energy difference between the first doubIyex~t~Ievelofthc~_eaKs”(n-iI)s’(~zJ)p’(3P)m’(3P)“and thecorresponding limit of simple ionization u(n- I)+l)~~(~P)mas given by Moore [23]_ These vaiucs for Ne [J9], Ar, Kr P_], and Xc [ZS ] are tabulated in column j? of Table A In column y* the ionization potenti [37] of the alkaline-eanhs Mg, Ca, Sr, and J3ais given, from wvtich the energy position of the doubly-excited level of the corresponding rare gas can be extrapohted Column J is the ratio of there two coIumns_ The energy position of 26_S2 eV, (structure A in Fig_ 4). has been assumed for the moment to correspond to 3s’3pJ( 3P)4s’(3P) in order to caJcuJate the energy diffcrwce in Ar (column j? of Table 4)_ From the regularity in tie resulting vaJucs of cohunn 6 cf Table 4 it is easy to con&de that level A of Fis 4 corresponds to the 3~‘3p”(~P)&(~P) configuration_ TJ& is in disagreement with &rmHk et al_ [J2] who report this state at B-64 eV_ TJ& Jast value gives an tuteqxctcd ratio -JIB = J-63 instead of J-IO_ From the ekctrontmergy diagram of Gerber et aJ_ [I I] their reported structure- in the same energy range, can either be attributed to the 3s3p64p or to the 3sz3pi(sP)4s’ state_ suucfures

= Refm19 and 23_ b Rd_27and23_ =R&28and23_ aRcfL23_ = Ref.. 23 and present wxic, suucnt~

A.

161 It has been shown above that the addition of a 4s eiectron produced a negiigible change in the splitting between the three configurations of ArIII, the 3~‘3p*(~P, ‘D, and IS)- Since the energy position of the ~s’~P~(~P)~s~(~P) is known. it is possible to extrapolate from TabIe I the ener,o positions of 3s’3pJ(‘D)4s’(‘D) at 28.61 eV (26.82+1_79) and of 3s’3pz(‘S)4s2(‘S) at 30.92 eV (28-6 I -I-23 I )_ This is shown in column ‘J and S of Fig_ I _ Structure D (Fig_ 2 andTabIe 3) observed at 28.67 eV corresponds remarkably well with the extrapolated vaIue (28.61 eV) of the 3s23p4(‘D)4s2(‘D) Ievel. There is no other neutral state possible in the close neighbourhood of this energy. One might consider that structure D contains a contribution from the negative ion 3s23p’(3P&3dZ_ However since its neutrai parent is not detected, this hypothesis is indefensible. The 4s’(‘D) interpretation reported here is in disagreement with the interpretation of Cermak et al- 1121 who assign the cotiguraticn 3s’3pa(‘D)4s’ to their structure at 30.24 eV. A few configurations might be assigned to structure K (Fig- 2) located at 31-05 eV. From our extrapolated energies (Fig- I) the state 3s23p*(‘D)[4s4p(‘P)] is expected around 31-01 eV. This last state has actualiy been observed by Madden et al. [S ] with a different description as the 3~‘3p~(‘D)4s(‘D&Ip at 31.23 eV. The energy difference between the Ievel reported by Madden et al.-[S] and the structure K (Fig_ 2 and Table 3) is such that it must be concluded that they have a different origin_ Some configurations like 3s’3pJ(3P)3d’ are Iocated in that ener=y range but because a large change in the anguIar momentum of both electrons is invoIved to produce the transition, that conf&n-ation is unIikeIy to be responsible for structure K_ Finaily the state 3s’3pa(‘S)4s’( ‘S) IS - most IikeIyresponsible for structure K, in good a,mment with the extrapolated value (see Fig_ I)_ Tfe 3sz3p~~4s4p(‘P)]

configrrrafions and relared stares

By comparison with CaI in Fig_ I structure C at 25-36 eV is interpreted as Ss’Sp’( 3P)[4s4p( 3P)]. There is no other neutraf state possible in this energy range. It is not possibie to determine the final configuration in that case because a (‘P) core- coupled with two external electrons with a (‘P) configuration gives rise to a huge number of different IeveIswhen taking into account a11possible muItipIicities_ Ogurtsov et al. [IO] , giving the description of the electronic configuration as the 3s23p”(‘P)4s(‘P)?tp(‘P), report a IeveI at 2S.2 eV compatible with structure C (Figs. 2 and 3). Let us now examine structure B (Fig_ 2)_ Sanche et ai_ 173 in au electron transmission expriment observe a structure of fairIy strong intensity at 27-95 eV_ Because their measuring technique gives the derivative of the electron current it is necessary for comparison to take the derivative of the ebzctroionization spectra. A strong structure appears in that derivative at 27-95 eV as in the case of Sanche

162

et al_ [7]_ The strikingcoincidencebct\wxnthosestructurescoming from two independent sets of data proves that they are caused by the same phenomena_ From the assertion of Sanche et al. [7] that the electron tmnsmission technique is specialiy sensitiwz LO study negative-ion resonances it must bc l.3ncluded that structure B which correspondsto thciq isproduced by thenegative-ionstafe3s’3pa( ‘P)4s4p2_

Ho\~~~~rthepossibilityof ;Bcontributionfrom a neutralstate3~~3p~(~Pj[4~4p(~Pj] with a difkrent &xaf configuration than the one which gives rise LOstructureC must not be exduded- Neither must a slight contribution from the neutral state 3s3pb5p(‘Pj due to simpli excitationof the 3s cktron as observed by many authors [l-5] as a conscqucncc of the cxrremely important 3s3p64p(*Pj level reported in previous data [6]_ The smal1structureI at 29-99 eV in Fig 2 agrees quite we11with the extrapolated Ievci from CaT, the 3~‘3p~(‘D)[4s4p(~Pj] in Fis_ 1 at 29-97 cV_ No other low-lying series term is expcctcd in the ncishbourhood of that energy- The final conf@xtion, however, cannot be written for similar reasons to the case of the 3s’3pa(3Pj core_ Ogurtsov et al_ [IO] giving the description of the ekctronic confi,wtion as the 3~‘3p*(‘D)4s(‘D)4p(~Pj, report 3 Icvci at 29-9 CV compatible with our structtuc I_ In the case ofstruct~~~ I-I, there is an excellent a,geement between the results of Sanche et at_ [7] and the derivatiwz of fi=,_ = 2 in that enem nngc. The marrimum in the derivative in the ionization eficicncy curve is at 29-39 eV which is in excellent

acertmcnt\viththe structureobserved by SancheCLal_ 171at 2937 eV_ The strongest structureH can therefix-e be interpreted as a perturbation due to the negatk ion 3s’3p‘I(‘D)4s4pz_ Higher in energy at 3234 eV (0 in Fi-_ m 2). the third level of this group, the 3sz3p”(‘Sj[4s4p(‘p)1 level is detected. The excellent aagreementbetween its position and the extrapolated vake (Fig_ 1) gives much confidence in its interpretation. Otiter structures

Two confi,gnations might be responsible for structure J at 30-69 eV (Fig 2)_ Fmt of aii Madden et al [3] report a soup of close-packed narrow structures beginning at 30-70 cV which they interpret as alI 3s*3p*(3P)3d4p with different coupling_ Secondly because there is a fair agreement between the extrapolated vaIue (set Table 3) and the position of J the possibiiity of a contribution of a neu-

tr;ll state3s’3pt(‘D)cls3d cannot bc excluded_

IL is shown in this paper that a Iarge number of doubly excitedstates of argon perturb the ionizationefficiencycurve produced by ekctron impact-Each

163 of these perturbations

is much sharper at its threshoId

than at a higher energy,

so that the ionization

efficiency curve appears (when straightened) as a spectrum of those states. Although most of these states arise from transitions which are opticaiiy forbidden from the ground state they are allowed by e!ectron impact. From

the regularity

between the set of structures measured

other inert gases or with atoms having similar configurations, of several doubly

excited states of argoz

like 3s23p’4s2

and by anaIo,oy with the energy position

with configurations

3P,

‘D , ‘S can be determined_ Many other states which cannot be reached by photon impact are easily detected by this technique which appears to be very powerful in measuring highly excited states of atoms and moiecules.

XCKEO\VLEDGL\IEYXS

We want to acknowledge the constant co!Iaboration of Dr_ P_ Marchand and Mr P_ Veiilette and fruitful suggestions from Mr R_ Dutil leading to Table 4. This research has received support from the National

Research Council of Canada,

grant no. A-3169, the Minist&re de 1’Education of Quebec and from C.R.A.M. (Centre de Recherches sur ies Atsmes et Ies Mol&cules)_

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1039.

5 T- Bergmark et al-. Instituteof Ph_ysics. Uppsda University.Rept_ no. 589 (1969). See aIso

K.

Sicgbahn

et al_, ES.C.A_

AppCicd fo Free

.Wolecufcs,

North-Hotland.

Amsterdam,

p_ 149_ 6 E_ Bolduc.J_J_ QuCmtnerand P_ hiarmer. Can. J. Phyx-. 49 (1971) 309X

1969.

7 L_ Sache and G_ J_ Schulz, Phyx Ret-_ A_, 5 (1972) 1672S R_ P_ Madden, D_ L_ Ederer and K_ Codling, P&s_ Rec., 177 (1969) 136. 9 M_ E- Rudd, T_ Jorgensen and D. J_ Void. P&-s_ Rec. 151 (1966) 2% IO G- N_ Ogurwv. I- P- FIaks and S- V_ Avakyan. Z/Z_Eksp_ Theor_ Fiz_. 57 (1969) 27. [SUE_ P&x_ JETP. 30 (1970) 16]11 G_ Gerber. R_ Morgcnsrtm and A. Niehzms,J. Phrs B, 5 (1972) 1396. 12 V_ b-nxik. M_ Smulek, and J_ Sramek. J_ E;lctrron Specrrosc__. 2 (1973) I_ 13 P_ VciIIeltc r?nd P_ Marchand, IRK_J_ Xiass Specrrom_ Ion Ph_t-s_. IS (1975) 165. 14 P_ Marmct and L_ Kcrwin, Gun. J- P&s-s-. 35 (1960) 787. 15 P_ hbrmet. J_ Vuc_ Sci_ Tcchnof.. 8 (1971) 262. 16 P_ Marchand. C_ Paquet and P_ Marmet, P&x_ Rec., 180 (1969) 123.

17 J_J_ Qu&m&ner, C. hquet and P. hiarmet.Phw Rec. A. 4 (1971)-%99J. 18 P_ Mxmet. E_ Bolduc and J. J. QuemfZner. J. Chcnr. Phys.. 56 (1972) 3363. 19 E Bolduc, J_J_ QuCmener and P. Marmet.J_ Chem Phys__. 57 (1972) 1957. 20 E_ Bolduc and P_ Marmet. Can_ J. Ph_w_. 51 (1973) 2105_ 21 U_ F-0. P&x_ Rer_, I?$ (1961) 1866_ 22 R Carbomxau, E Bolduc and P_ Marmet. Curz-J. Phm. 51 (1973) 505 23 C_ E_ Moore, NarL Bttr_ Srand (US.) Circ- nc. 467 (1949).

164 Carbonnuts and P_ Marmet. Itrt- I- f2ftr.wSprcronr- ton Phpm, 10 (1972273) Carbonncau aad PmrbCwmct, Can- J_ Ph_m, 51 (1973) 22OT Carbonmxu and PmMarmet. C&z- I- Phr)J.. 52 (197?) ISSS_ 27 M. Vatin and P. Mamtct. to bc pubtiskd. 28 R_ Dutil and P- Manner, to k publiskd-

24 R 25 R 26 R

143_