PHYSICAL REVIEW A VOLUME 41, NUMBER 5 1 MARCH 1990

Two-electron detachment and excitation processes in S collisions with atoms and molecnles

S. Boumsellek, Vu Ngoc Tuan, and V. A. EsaulovLaboratoire des Collisions Atomiques et MoJeculaires, BQ'timent 351,

UniUersite Paris —Sud, Orsay 91405, France(Received 21 March 1989)

Results of an electron-spectroscopy study of two-electron detachment (ionization) and excitationprocesses in S collisions with He, Ne, Ar, Kr, H2, Oz, and Nz are presented. The electron spectraindicate that singly excited autoionizing states of S ( D nl and 'P nl series) are populated in thesecollisions, and this should lead to substantial S+ production. Previously unknown autoionizingstates of the D( P)nl series are identified. In the case of the Oz target, charge exchange to thelowest 02 ( Hg ) shape resonance is observed.

I. INTRODUCTION

We present results of a study of excitation and two-electron detachment processes in collisions of S withatoms and molecules. In earlier studies we investigatedin some detail one-electron detachment in collisions of avariety of negative ions (see, e.g. , Esaulov ) with atomicand molecular targets at low collision energies where amolecular description is valid. The main motivation wasthe study of ionization processes for systems where thisprocess is accessible directly and not as a result of a priorinteraction with the Rydberg series of excited states. Thegeneral characteristics of this direct electron-detachmentprocess

now appear clear, though calculations for specific com-plex systems are still lacking.

Excitation processes in negative-ion collisions have notyet obtained a satisfactory description. There do not ex-ist any ab initio calculations. of known low-energy excita-tion cross sections. Attempts have been made to analyzequalitatively data on excitation within the usualmolecular-orbital diagram framework used in atom-atomcollisions; however, the applicability of such an approachis doubtful and approaches have been used in which theextra negative-ion electron and core collision are treatedseparately. At present no definitive description is inview. On the experimental side, early work had focusedmainly on the electron-spectroscopy study of autodetach-ing states of negative ions (see, e.g., the review of Ed-wards ). Until recently, the data on neutral excited-stateproduction in collisions of negative ions other than Hcame from energy-loss measurements' that gave gen-eral indications about excitation processes, but it is onlyin the past few years that optica1 and eleetron-spectroscopy studies have brought precise informationabout the excited states produced and some insight intotwo-electron detachment processes. Thus there are indi-cations 5 that ionization (two-electron loss) in low-energy collisions may be due to the population of singlyexcited autoionizing states. A large number of elements

possess such states. As an example one can considersome common elements with partially filled p shells(groups V, VI, and VII). For these, the groundconfiguration of the positive ion corresponds to severalstates, e.g., S', D', and P' for sulfur or P, 'D, and 'Sfor the halogen series. The upper members of the Ryd-berg series converging to the higher. ionization limits[( D)nl and P(nl) for sulfur] lie above the first (orsecond) ionization limits and are therefore autoionizing.In a negative-ion collision the detached electron energyspectrum will contain contributions from single- anddouble-electron loss. In the case of positive-ion produc-tion as a result of formation of autoionizing states, thespectrum will contain corresponding characteristic peaks.The detached-electron energy spectra (DEES) thus shedlight on excited-state formation and two-electron detach-ment. The study of these spectra as a function of col-lision energy can allow the estimate of the onset of two-electron detachment and its relative importance to one-electron detachment processes. These spectra are clearlyalso interesting from a purely spectroscopic point of viewsince they give information that is complementary to op-tical studies of autoionizing states, where dipole selectionrules impose a restriction on the possible excited states.

In this paper we present results of an electron-spectroscopy study of S collisions with inert gases andmolecules. Earlier studies of one- and two-electron lossin S collisions are limited to total cross-section measure-ments in the 12.S—240-keV energy range for the inertgases, H2, 02, and Nz targets. At a 12.5-keV collision en-

ergy the ratio of the 2:I electron detachment cross sec-tions were found to be of 22.7% (He), 8.5% (Ne), 26.5%(Ar), and 22.2% (Kr). Smaller values are obtained formolecular targets. At low collision energies there onlyexist total electron detachment cross-section measure-ments. ' For the H2 target there are also measurementsfor H production in reactive processes.

II. EXPERIMENTAL SETUP

The measurements were performed on a newly con-structed apparatus for gas-phase and surface studies. Aschematic diagram of the layout of the setup, consisting

2515 1990 The American Physical Society

2516 S. BOUMSEI.LEK, VU NGOC TUAN, AND V. A. ESAULOV

&6 BITDAC

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I

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FIG. 1. Schematic diagram of the experimental setup, show-ing the chamber configuration, ion source (IS), Wien filter (8"),quadrupole deflector (Q), various lenses {Ll—L3), deflects(D1—D5), Faraday cage (Fc), electrostatic analyzer (EA), andthe associated data-handling arrangement.

of three differentially pumped chambers, is shown in Fig.1.

A simple discharge ion source is used. For S ions ancarbon oxy-sulfide and argon (OCS)+Ar gas mixture was

used in the source. The source and the three cylindricalelement ion-extraction optics were readapted from anolder system. The extracted ions are mass analyzed by a10-cm-long, rare-earth —cobalt-permanent-magnet Wienfilter with a magnetic field of about 800 G within a 22-mm spacing. The Wien filter is enclosed in a cast ironbox to reduce magnetic fields along the beam line beyondit.

The ions are deflected through 90' by a quadrupoledeflector. This deflection prevents stray neutrals andphotons emitted by the ion source from reaching the col-lision region. Initially we made some attempts to use thequadrupole deflector without the shim plates present inthe original design. This resulted in a severe (50%) de-crease in transmitted beam intensity and attempts to re-focus the beam with an extra lens placed at the exit of thedeflector proved to be unsuccessful. It is interesting tonote that without the shim plates, the best performancewas obtained with strongly asymmetric voltages( V+ =2 V or vice versa). After inclusion of shim plates,essentially all of the beam could be collected through a5-mm-diam aperture placed at 20 cm beyond the quadru-pole and refocused by lens 1.2, using the normal sym-metric voltage mode.

The deflected ions enter the main chamber which isequipped with a secondary lens stack and a Menzinger-type' decelerator, which has allowed us to obtain beams

of energies down to 5 eV. The ions are monitored by aFaraday cage, which allows improved beam focusing, byusing a dual ring and plate system.

The main chamber contains a goniometer consisting oftwo coaxial rings. The inner ring supports the electronspectrometer, whereas the outer one will support an ionspectrometer. A continuous setting from + 135 to—135' with a 0.2 (inner ring) and 0.1' (outer ring) resolu-

tion may be obtained. A dual p metal magnetic shielding

is employed to reduce magnetic fields resulting in areduction of the vertical component to about 6 mG and

the horizontal component to 1 to 2 mG. A further reduc-tion in the vertical component of the magnetic field toabout 1 mG is achieved by using Helmholtz coils.

A 45' parallel plate electrostatic analyzer was em-

ployed in the present measurements and is described in

detail elsewhere. " The theoretical resolution for the sys-tem is approximately 30 meV for a 2.5-eV pass energy.In order to obtain low-energy electron spectra (below 2eV) the optics at the entrance of the analyzer was operat-ed in a zoom mode. The scanning was implemented us-

ing digital-to-analog converters. The data acquisitionand optics control were performed with an Intel 8086based computer, electronics, and program that are de-

scribed elsewhere. ' The voltages for the zoom optics forvarious electron energies were determined by manual op-timization and tabulated in the computer, which thensent out linearly iterated values of the voltages dependingupon the analyzed electron energy.

One of the main diSculties, which at present limitsthese measurements, consists of the determination of thetransmission function, i.e., the transmitted electron inten-

sity for a given electron energy. The use of zoom optics,which increases significantly the efficiency of data ac-quisition, also results in a strong sensitivity of the shapeof the acquired spectrum (for low electron energies) tothe size of the collision volume and to variations in theposition of the beam. In principle, the transmitted elec-tron intensity decreases with electron energy. However,small space charges can drastically modify the spectra forthe lowest transmitted electron energies. In earlier stud-ies attempts have been made' to determine the transmis-sion function by first measuring the electron spectrum inionizing electron-helium collisions, which in virtue ofWannier's threshold law, should be a constant. Thecorrection function obtained from these measurementswas then used to renormalize the electron spectra in ion-atom collisions. However, the results thus obtained arestill subject to a controversy and here we shall not dis-cuss at any length the shape of the spectra as a functionof electron energy, though in general very similar settingsof the optics were used and the transmission functionshould not be very di8'erent for the series of spectrapresented here, except at the lowest energies (E(200meV).

A last point to be noted is that for low electron ener-gies and forward observation angles (small angles be-tween the ion beam and analyzer) a substantial back-ground signal is observed. This is in part due to the in-cident ions hitting the analyzer. A fairly eScient meansof eliminating this is implemented by recording spectra

41 TWO-ELECTRON DETACHMENT AND EXCITATION PROCESSES. . . 2517

with the target gas passing through the main capillaryand through a secondary, similar, gas inlet placed farfrom the collision region, but which results in a similarbackground pressure. The two spectra are collected al-ternatively, sweep by sweep, and are then subtracted.Most of the results presented here were obtained in thismode.

The energy calibration of the analyzer was initiallyestablished by recording energy spectra of electrons emit-ted in 0 collisions with Ne. These were previously re-ported by Penent et al. , who gave the energies of peaksdue to autoionizing states with an accuracy of 20 meV.

III. RESULTS AND DISCUSSION

A. General characteristics of the electron spectra

occur, in particular, due to some quasimolecular decayduring the collision.

The position of the peaks in Figs. 2—5 is different fordifferent collision energies because of kinematics effects.The energies at which these peaks appear in the center-of-mass frame are given in Table I along with the pro-

l

(a)

The integral spectrum we measure contains contribu-tions from one- and two-electron detachment processes.Some of these, like the direct detachment process (1), leadto continuous electron energy distributions. Excitationof autoionizing states and autodetaching states on theother hand results in we11-defined peaks. The attributionof these to the projectile or target atom is based on theDoppler shift of the peaks due to the projectile as a func-tion of observations angle and (or) collision energy. Thespectra were therefore recorded for several angles and re-sults for a 25' observation angle are shown in Figs. 2-5for some of the studied energies.

As may be seen, the electron spectra consist of a con-tinuous distribution, on which is superimposed a series ofpeaks which we attribute to autoionizing peaks of sulfur.The width of the peaks in the spectrum is determined byseveral efFects: (i) the analyzer resolution; (ii) electronejection kinematics (5E&= 16 meV for a 0.63-eV electron,a 2-keV collision energy, a 8=25' observation angle, and18=5'); and (iii) "natural" broadening eIFects that may

I-CA

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Initial Final S+

TABLE I. Autoionizing states of S observed in S collisions(see Fig. 5 for peak labels). I

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A

BC

DEF

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0.2890.3350.493

0.6320.6980.782

0.952

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2.73

2.89

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4So

4So

/DO

S'

4So

4So

4S'

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r

(c)

I-CA

LUI-

I

1E (eV)

FIG. 2. Energy spectra of electrons ejected in S collisionswith (a) Ar and (b) and (c) Kr for the indicated collision ener-gies.

2518 S. BOUMSELLEK, VU NGOC TUAN, AND V. A. ESAULOV

(a)

1

2I

1E (eV)

0

posed assignments. The peaks at the lowest energies posesome problems. Here, the transmission function is gen-erally the largest and there also exists a large number ofstray scattered low-energy electrons. These effects resultin the appearance of an intense zero-energy peak. Thispeak is strongly reduced in the present spectra because ofthe subtraction procedure we employ. However, theidenti6cation of peaks in this lowest-energy region is ren-dered difficult. Nevertheless, in some cases, as in the 2-keV Ar spectrum, the large "noise" peak at zero energywas not observed (presumably because of a change in thetransmission function) and therefore the observed struc-ture (peak A ) was included in Table I and has a realisticassignment.

For some of the considered targets, the continuouselectron distribution, which is presumably mainly due toreaction (1), presents no significant features. However,for instance, for the Ne target the spectrum clearly ap-pears to be composed of different distributions. The ori-gin of this may be sought in the characteristics of the Splus inert-gas system, which is similar to the 0 inert-gasone discussed in detail by Esaulov et al." Here we re-call briefiy that there exist two entrance (or initial) molec-ular states in these collisions. These are the X(n. o ) and

l-CAKUJ

I

1E (eV)

FIG. 4. Energy spectra of electrons ejected in 2-keV col-lisions of S with (a) N& and (b) 02.

II(w o ) states that correlate to S ( P) plus inert gas('S) at infinity. Also there exist a number of exit chan-nels corresponding to S ( P, 'D, and 'S) plus inert gas atinfinity. These are (i) the II(m cr) and X(n o ) statesdissociating to S ( P) plus inert gas, (ii) the 'X+(m ),'II(n o ), and 'h(m o ) states dissociating to S ('D) plusinert gas, and (iii) the 'X+(n ) state dissociating to S ('S)plus inert gas.

Assuming" the detachment of an outer o electron wesee that scattering in the X(n. 0 ) state leads to the popu-lation of the 'X+(m ) state, whereas scattering in the2II( n.30 ) states will lead to the population of the3II(n o ) [and eventually the 'II(m o. )] states. Also quasi-molecular Auger detachment may occur from theII(m cr ) states leading to population of the 'X(m )

states. For the low collision velocities that we consider,one may expect that direct detachment (I) will lead to thepopulation of the lowest P and 'D states, as in the case of0 . We thus see that there exist several channels for de-tachment which can all lead to different electron energydistributions (as was indeed also noted' for 0 ). Thisthen may be the cause of the structured spectrum in theS —Ne case.

L K J I HG F EDC B

C/0

UJ

1E (.v)

I 8 l I

1E (eV)

FIG. 3. Energy spectra of electrons ejected in S collisionswith (a) He and {b) Ne.

FIG. 5. Energy spectrum of electrons ejected in a 2-keV col-lision of S with Ar. The peaks are labeled with reference toTable I.

41 TWO-ELECTRON DETACHMENT AND EXCITATION PROCESSES. . . 2519

Turning now to the question of peaks due to autoioniz-ing states we first note that these are found to increase inintensity with increasing collision energy and are moreintense for forward scattering angles. The latter featureis due to the following several effects: (i} electron ejectionkinematics, (ii) symmetry of the states involved, and (iii)additional anisotropic efFects such as the ones observedpreviously in H collisions, ' but for which there doesnot yet exist any explanation. A more detailed angulardependence study will have to await the knowledge of thetransmission function for different angles.

For the Ar and Kr targets these peaks appear at ener-gies below 500 eV (Fig. 2}. At this energy mainly thelowest states are observed. With increasing energyhigher-lying states are populated. Though we do notknow exactly the transmission function, an estimate ofthe relative magnitude of the processes responsible forthese peaks and for the broad electron distributions as afunction of collision energy may be made by integratingthe area under the peaks and the rest of the spectrum forwhich we assume a smooth, continuous form. This pro-cedure yields the following figures: 10%, 17%, 22%, and24% for 500 eV, 1000 eV, 2 keV, and 3 keV, respectively,for Ar; and 22% for collisions with Kr at 3 keV.

In the case of the He and Ne targets (Fig. 3} the onsetenergies for the autoionizing state production lie in the 1

keV laboratory collision energy range (this correspondsto 111 eV in the center-of-mass frame for S +He}. Thecorresponding cross section appears to grow very rapidlywith increasing collision energy, though in the Ne caseexcitation of these peaks remains small even at thehighest studied 3-keV energy. An interesting feature ofthe S +He spectrum is that it is not marked by any par-ticular selectivity in the excitation process. Indeed, peaksdue to quite high-lying states are present in the spectrum.This would suggest the opening up of a single commonexcitation channel. As for the Ar and Kr targets, we per-formed an integration of the areas under the peaks andthe broad distribution yielding a ratio of 22% for 3-keVcollisions with He. For Ne this ratio is less than 6% atthis energy.

In the case of the molecular 02 and N2 targets the elec-tron spectrum (Fig. 5) also contains a minor contributionfrom these autoionizing states. In the Hz case these werenot observed. Note that for Oz the spectrum alsodisplays peaks due to charge exchange to the Oz ( IIs)shape resonance. ' Decay of this resonance 02(v ~0)~Oz(v'~0) is at the origin of the sharp low-energy peaks with a spacing of 0.12 eV characteristic of02

It is interesting to compare the ratios of the known to-tal cross sections for one- and two-electron loss cited inthe Introduction with the ones obtained above. As maybe observed, the magnitude of the 3-keV data is con-sistent with the, somewhat higher energy, total cross-section data. Also, the fact that the two-electron totaldetachment cross section is smaller in the case of Ne andthe molecular targets is consistent with our finding of asmaller cross section of autoionizing-state production.This comparison should of course be treated with cautionsince the ratios may be affected by the transmission func-

tion, but it does indicate that excitation of autoionizingstates contributes significantly to ionization

B. The autoionizing states

The ground-state configuration of S+ corresponds tothe three S, D', and P states which are separated by1.84 eV ( S' D—) and 1.2 eV ( D' P—'). There thus ex-ist three families of singly excited states. On1y the 1owestD' (4s, 4p, and singlet 3d) and P' (4s) states lie below

the first S ionization limit. Other states lie in the con-tinuum region and some may autoionize. Autoionizingstates of sulfur have been studied experimentally' ' 'and theoretically' in several recent photoionization stud-ies. These authors report triplet states of the ns and ndseries. Singlet 3d' 'P', 3d' 'F, Ss' 'D', and 5s" 'P' stateshave been reported by Kaufmann. ' States of the D npand P np series were not known till now. A discussion ofvarious aspects of autoionizing states of S may be foundin these papers and will not be reproduced here. Itshould only be pointed out that in ease of sulfur (and oxy-gen} deviations from LS coupling are observed and one isled to consider less stringent rules, which follow from theinclusion of other interactions. Autoionizing transitionsmust obey angular momentum and parity conservation.Describing the continuum state in the same way as thediscrete state by specifying the spin and angular momen-turn quantum numbers for the state of the ion and freeelectron, the selection rules for LS coupling are KL=O,ES=O. Upon inclusion of effects of spin-orbit interactionone has' hL =0,+1 and AS=0,+I, while inclusion ofspin-spin interaction leads to hL =0,+1,+2 andAS=0,+1,+2. The inclusion of the effects of other in-teractions thus greatly increases the number of possibleautoionizing transitions. Note also that mixing with oth-er states may render some transition possible.

In order to assign the peaks in our spectra (Fig. 5) wefirst referred directly to earlier studies. This allowed theassignment of some of the peaks, though for the high-lying ones some ambiguities exist due to lack of resolu-tion. There also exists an ambiguity concerning the ass-ingment of peak C. According to the data of Gibsonet al. this could be the 5s' D' state whereas accordingto the data of Joshi et al. ' this could be the 3d' F' statewhich is forbidden to autoionize in the LS couplingscheme.

The peaks B, F-I, E, and M could not be identified inthis manner. Starting from the usual expression for theenergies (E) of states

R~ =Vion 2(n —5)

where V;,„ is the ionization potential, R the Rydbergconstant, and n the principal quantum number, we com-puted values of the effective quantum number n *=n —5,associated with these transitions. We assumed parentageto both the D nl and P nl series and a11owed for decayinto the S and D' continua. We also calculated n* forthe known' np' states which are found to be 2.33, 2.34,2.36, and 2.4 for the 4p' D, F, 'F, and P states, respec-tively. As reported in earlier studies' ' the quantumdefects for the ns' series are approximately 2.1. For the

S. BOUMSELLEK, VU NGOC TUAN, AND V. A. ESAULOV

nd' series these lie in the —0.46—0.3 range and are —0.46and —0.36 for the specific case of 3d' 'P' state and3d''F' states, respectively. ' A comparison of thesevalues suggests the assignments of the Sp', 6p', and 7p'states given in Table I. A possible assignment of peak Fcould be the previously unknown 3d' ' 6 states with aquantum defect of 0.17. Peak I could be due to the4d' 'P'. Alternatively, the peaks F and I could be due tothe P np series assuming that, as for oxygen, ' ' thequantum defect should be similar. At present peak 8remains unassigned.

IV. CONCLUDING REMARKS

The results here show that at low energies excitation ofautoionizing states is important in S collisions withatoms and molecules and that this channel contributessignificantly to S+ production. This result is in agree-ment with earlier measurements for 0 and halogenanion collisions. Thus it is clear that a significant part ofthe primary excitation mechanism is channeled into ion-ization. It would be most interesting to compare resultsfor S collisions with those for S collisions. This wouldbe helpful in particular in unveiling the eventual ex-istence of ionization channels, which result in a continu-ous electron energy distribution and which, in the case of

the negative ion, would be masked by the one-electrondetachment process. To achieve this a small modificationof the apparatus is now being performed to allow for pro-duction of a neutral S beam. An analysis of the excita-tion and ionization processes within a quasimolecularframework will be presented subsequently when the dataon the neutral system is available.

ACKNOWLKDGEMKNTS

This work was supported in part by the Direction desRecherches Etudes et Techniques, France, under Con-tract No. 84/154. We are greatly indebted to the Univer-sity of Kaiserslautern and Dr. F. Linder for invaluableassistance in the construction of the goniometer. Wewould also like to thank Dr. J. R. Peterson and D. H.Helm of the Stanford Research Institute for providingdata on the design of the quadrupole deflector. Specialthanks are due to A. Abadia, G. Le Bourhis, J. P. Guillo-tin, R. Paineau, D. Ragonnet, and P. Rudnyckyj, fortheir expert technical assistance in the construction andsetting up of the apparatus. The Laboratoire des Col-lisions Atomiques et Moleculaires is "Unite de RechercheAssocie au Centre Nationale de la Recherche ScientifiqueNo. 281."

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