Nuclear Instruments and Methods in Physics Research B27 (1987) 579-583 North-Holland, Amsterdam

579

ONE AND TWO ELECTRON TRANSFER AT LOW COLLISION ENERGY: SPECTROSCOPIC CASE STUDIES

S. BLIMAN, J.J. BONNET *, D. HITZ, T. LUDAC, M. DRUETTA * * and M. MAYO

DRF. G/Padsi & Agrippa, CENG 85X, 38041 Grenoble Cedex, France

A comparative study of single and double electron transfer is made considering ions with an initial charge of 8 + (08+, Ne+, Ar8+) colliding with He at energies of a few keV/amu. It is shown that even though single capture populates n = 4, the dynamics of the process is different. Furthermore double capture being possible, mostly via a two step process in one collision, it appears that stabilization changes significantly in these three cases from mostly autoionizing to mostly radiative.

1. Introduction

Among atomic collision processes, charge exchange between a multiply charged ion and an atom is prob- ably the one which is associated with the largest rates (u,,u). This makes it an important process in consider- ing ionization equilibria of hot plasmas both in the laboratory and in astrophysical situations [1,2]. Further- more, it is a state selective process, at least at collision energies less than 20 keV/amu; the probability is great that population sharing among substates of capture levels will not be statistical [3]. A consequence is that when different collision processes (ion excitation by electron collision and charge exchange and other recom- bination processes [2]) contribute in populating levels, line intensities will depart from normal.

Another important aspect of ion-atom collisions is that, with two-electron or quasi-two-electron targets, double electron transfer may play an important role: doubly excited states are created which stabilize either via autoionization [4] or radiation [5]. No simple pre- diction is possible because these doubly highly excited levels are not well known.

We consider now the charge transfer collisions in- volving He as a target and three different ion species with initial charge 8 + viz. OS+, Nes+ and Ar8+ at energies in the range 2-4 keV/amu. In such collisions one and two electron processes contributing to X and VUV emissions are (1) single electron capture (SEC)

A8++He+A7+(nZ)+He+

I A7+(n’Z’) + hv,

* Laboratoire de Physique des Collisions Atomiques, CNAM, 292 rue St. Martin, 75141 Paris Cedex 03, France.

* * Laboratoire de SpectromCtrie Ionique et Moltculaire, Uni- versitC Lyon I, 69622 Villeurbanne, France.

and (2) two electron processes, i.e., double capture followed by radiative decay (TDC),

A8’ + He + A6+( nl, n’l’) + He2’

L,A6+(nZ) +hv’,

and double capture followed by autoionization (ADC)

A8++He+A6+(nZ, n’Z’)+He2+

I A7+( n”Z”) + e-

LA7*(n”‘Z”‘) +hy”

Depending on the levels populated in the two electron processes, it should be noted that some of these might have branching ratios allowing decay along both chan- nels.

2. Experimental setup and procedure

An ion beam produced by the ECR ion source at the Agrippa facility is charge and mass analyzed to obtain a pure beam of well identified mass to charge ratio of a desired species. It is then passed into a differentially pumped collision chamber where the gas pressure is kept around 4-5 X lo-’ Torr. This allows the single collision condition to be met [6]. Two spectrometers look directly into the collision chamber: One covers the soft X-ray range, lo-110 A, the other, a grazing inci- dence spectrometer, covers the range 100-1000 A.

For each individual line, the emission cross section is measured:

u,,(nZ+ n’l’) = g S

i 1 ; NK(A) p,

where D is the observation solid angle; L is the length of beam viewed by spectrometer; I is the incident ion

0168-583X/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

580 S. Bliman et al. / 1 and 2 electron transfer at low collision energies

beam current; N is the target number density; K(X) is the spectrometer sensitivity factor for the observed wavelength; and S is the measured signal. P is a factor taking into account anisotropy and polarisation effects. Here after we take P = 1 [7].

In the exit channel of the simple capture collision for the considered ions, transition probabilities A,[,,rir are all tabulated [8,9]. Thus, utilizing a,,, under the as- sumption that T,[ (the lifetime of state (nl)) is short compared with the time of flight in front of the spec- trometer, the partial capture cross sections a,,,_,(&) are obtained:

-c u,,( ol’ -+ nz’)A,,~,,rr >

02-n AOl~,d,

where I’=I=l, 1-I and

For those lines originating from the capture level the first term in this expression is needed. The second term accounts for all cascades contributing to the population of the upper level of the transition (nl --f n’l’). Sum- ming all the partial cross sections gives the total capture cross section.

n I

Among the specific problems associated with this method, mention should be made of the absolute calibration of the spectrometer, the possibility of non- radiative processes contributing to level decay and, in the case of simple capture, the possibility that some transitions have an excess intensity due to an ADC end process. These points are discussed later on.

3. Single electron capture

The collision velocity is of order 0.3-0.4 a.u. for these ions. Spectra obtained in these three cases show that capture has gone to n = 4.

3.1. 08+ + He

The case of Opt is by far the most difficult to analyse, since the spectra in the X-ray range show three lines: 4p + Is, 3p + 1s and 2p --+ 1s. The line intensities as a function of sublevel populations are

I(4p + Is) = 0.840N(4p),

1(3p -+ Is) = 0.419N(4s) + 0.255N(4d) + 0.882N(3p)

and

1(2p + Is) = N(2p) + N(3s) + N(3d) + N(4f)

+0.581N(4s) + 0.745N(4d)

+O.O41N(4p).

Solving for populations requires four equations. In these conditions, further assumptions need to be intro- duced: Any population must be positive or zero [lo]; N(4p) is obtained directly from the line intensity. Nor- malized experimental values for intensities are: for 2p --) Is, I = 0.74; for 3p + Is, 1= 0.075, and for 4p + Is, I = 0.185.

By estimating the value for N(3p), then the values for N(4s), N(4d) and N = N(4f) + N(3d) + N(3s) + N(2p) are calculated within a variation range. The re- sults of this population estimate are in fig. 1, where theoretically calculated values are also shown [l] for N(4s), N(4p), N(4d). The theoretical points fall in the range estimated from the experimental data.

A discrepancy for N = N(4f) + N(3d) + N(3s) + N(2p) appears for which a tentative explanation is as follows: the excess intensity in 2p + 1s and as such in N is attributed to ADC. The ion 07+ being in that case left in the 2p state, capture to n = 3 being very small. From a VUV measurement, capture to n = 4 is con- firmed and total capture cross section is obtained: utotal = (1.2 + 0.4) X lo-” cm2. This is to be compared to the growth rate methods result which was: (1.8 f 0.4) X IO-l5 cm2. This last value is too large due to ADC [12].

N(41)v

Fig. 1. Relative populations of 07+ (n = 4) substates. & Molec- ular calculation corrected for ADC [15]. Vertical bar: experi- mental population range. N(3p) is assumed to be zero.

* Statistical distribution.

S. Bliman et al. / 1 and 2 electron transfer at low collision energies 581

3.2. Ne’ t f He

This collision has been analysed at 4 keV,/amu. Experimental data and theoretical calculation suggest a selective SEC into n = 4. Spectra in the X-ray range show the lines with normalized intensities as indicated in table 1. In this case the 4f -+ 3d transition at 292.4 A is in the VUV range. Considering the cascade scheme with line intensities the calculation gives N(4s), N(itp), N(4d) and N(4f) + N(3d). This is to be compared with the result of the VUV measurement: it gives 41 -+ 31’ transitions and 2p ---) 2s. The VUV and X-ray popu- lation measurements are shown in fig. 2 where the theoretical estimates are also shown [13]. All three agree within error bars. Fur~e~ore the total capture cross section is of order (1.65 f 0.5) X 10-‘15 cm’ from the VUV measurement and (1.1 f 0.4) x lo-l5 cm’ from the X-ray measurement, and it was measured previously by the growth method to be (1.9 f 0.4) X lo-l5 cm*. It should be noted that ADC contributes to a non-neghgi- ble increase in the apparent SEC cross section.

3.3. A@+ + He

The observation is performed in the VUV range. The capture is clearly seen to be selective on n = 4 spectra showing a limited number of Na-like transitions. Their wavelengths are above I\ = 87 A (ionisa~on limit of the normal Na-like spectrum). No cascades feed the n = 4 level [14].

The excitation cross sections are in this case given by the simple relation

CA 41-- 31’

a,, = %I (41+ 31’) i, ) 41- 31’

with

04, = %,(4S -+ 3P),

cQp = 1.83u,,(4p + 3s)

a, = LlOu,,(4d + 3p),

04, = %m (4f -+ 3d).

Table 1

X-ray lines with normalized intensities

Transition Intensity Wavelength [A]

3s + 2p 0.048 103.1 3p --) 2s 0.177 88.11 3d --j 2p 0.210 98.27 4s --) 2p 0.145 74.64 4p --) 2s 0.121 67.39 4d * 2p 0.169 73.55

N(41) r’

Fig. 2. Ne’+ (n = 4) substate populations in the collision Ne**

+ He -+ Ne7+ (n = 4) at 4 keV/amu. A Molecular calculation

1151. m Experimental estimate from X-ray measurement.

l Experimental estimate from VW measurement. * Statisti&

dist~butjon.

From line intensities, at u = 0_29u,, we deduce

z% = 0.12, @4

““FE032 . 2 04

2 = 0.41, fJ4

?fc! = 0.16, 04

where a, has been obtained also by the growth rate method and is (2.45 & 0.45) X lo-l5 cmz. These are to be compared with theoretical values [15]:

z??z zO.15; *4

O4P - = 0.345; 04

T = 0.316;

2 = 0.188 04

with u4 = 3 x 1O-15 cm’ in this case no ADC contribu- tion is expected.

4. Two electron capture

Considering potential curves for any one of these three systems, two electron transfer can take place

582 S. Bliman et al. / 1 and 2 electron transfer at low collision energies

either in a one step process,

A*‘+- He-+ A6”(nZ, n’i’) + He2+,

or in a two step process in a single collision,

As++He-,A7+(Ptl)+He’jA6’(P21, n’f’)+He’+.

This is depicted in fig. 4 where the crossing radius R, is the one associated with SEC, R, the one correspond- ing with the second step in the two electron transfer, and RI is the one describing the one step process.

With the use of different instrumental techniques, it has been shown that double capture occurs mostly via two successive one electron captures [16]. Furthermore the most populated levels are n = 3 and n’ = 3, 4. This is confirmed by energy gain translational spectroscopy of the projectile (through differential cross section) and Auger electron energy spectroscopy. Photon spec- troscopy reveals some of the differences in the behavior of the three systems in the present study.

In the exit channels the doubly excited states are quasi-loo% autoionizing for n = 3, n’ = 3, n = 3, n’ = 4 [17]. The analysis of the X-ray spectrum shows that the Lyman (Y transition of 0 li has an excess intensity. Thus,

06’(nl,n’l’) -+ 07+(n = 2) + e-

occurs. From energy considerations, stabil~~tion into Q7’(2p) is more likely than to O”(2s) [13]. This excess

bf41)

Gq,q-l I I /

0.5

OL 0 1 2 31

Fig. 3. Substate excitation cross sections for Ar *+ + He -+ Ar7’. (n = 4) at 2 keV/amu. A Molecular calculations results

[15]. l Experimental estimates from VUV measurement.

Rll RII *A R

Fig. 4. Schematic representation of potential energy curves for A*+ f He. (I)-(1’) Incident channel, (2)-(2’) Single electron

capture (SEC). (3)-(3’) Double electron capture.

wonld give for double capture a partial cross section of order 5 X lo-‘6 cm’ to compare with a SEC cross section of order IO-r5 cm’.

4.2, hre8 i- -I- He

In this case, the Auger electron spectroscopy shows evidence of partial double capture to n = 3, n’ > 4 [18]. These levels are mostly autoionizing [5]. There are, however, a small number of doubly excited levels n = 2, n’ > 3, 4 which decay radiatively. Some of these doubly excited levels with their branching ratios are given in ref. [19]. Since the density of doubly excited states at the crossing in the repulsive region is larger than in the first crossing, a small amount of population goes to radiative states.

In this case the signature is simple, since Ar6+(nl, ~1’1’) with n = 3, n’= 3 and n’ = 4 is a Mg I like ion for which the decay is fully radiative [19]. Identified in the spectra are for Ar VII: h = 585.9 A, 3s2 Is, f- 3s3p IPr; x = 501.7 A, 3s3p ‘PI +- 3s3d ID,; and h = 476.5 Ai, 3s3p 3P0 +- 3s3d 3D. Some other low intensity lines are left to identify.

5. Discussion

5.1. SEC

The cross sections which were measured for single capture and population sharing among n = 4 substates are consistent and in agreement with theoretical esti- mates [ 151. The behavior is significantly different among the three species studied here. In the case of O*+ i- He, population goes to n = 4 and is redistributed among the substates by the Stark effect. In the case of Ne8+ + He

and Ar8+ + He, there is a competition between the ion core and active electron interaction and the Stark inter- action when the two ions, A7’ and He ‘, separate. A small increase in the total capture cross section is also noted, as predicted, when Z increases.

5.2. Two electron processes

From experiment, there is evidence that double cap- ture is a two step process [16]. Since the most populated levels are essentially n = 3, R’ = 3 and n = 3, n’= 4, and, since for OS+ and Ne 8t these are not well known, a provisional conclusion might be: the density of states in the triplet configuration for 06+, Ne6+ and Ar6+ is larger than in the singlet one. Since the capture process populates from the next cont~uum, triplet states are more likely to be populated; this will give transitions of larger intensities when compared with ions collisionally excited by electrons.

6. Conclusion

The X-UV spectroscopic observation of charge ex- change collisions between ions of charge 8 -I- and a He target shows common gross features: the same SEC level (n = 4) and basically a two step process for double capture mostly to the same levels. The dynamical details of these processes are different: for a fully stripped projectile the basic process is the Stark interaction when the collision partners separate; with projectiles having core electrons, there is a competition between the active electron-core electron interaction and the Stark interac- tion when the partners are separating.

It should be underlined that some doubly excited lines in the Mg-like argon data are still unidentified and are under analysis by comparison with calculated transi- tions. Finally, it should be mentioned that from a spectroscopic point of view, with other ions of charge 8 -I- (Al*+, Ss+> SEC from He has shown capture to the same level, n = 4 spectra are now under analysis, since known data are scarce.

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