VUV spectroscopy of charge exchange processes in collisions of multiply charged ions

268

Section

Nuclear

VIII.

and Methods

in Physics

Research B23 (19X7) 26X-273 North-Holland. Amsterdam

Spectroscop_k

VUV SPECTROSCOPY OF CHARGE EXCHANGE OF MULTIPLY CHARGED IONS M. DRUETTA,

Instruments

S. MARTIN,

PROCESSES

IN COLLISIONS

and J. DESESQUELLES

Lhxm~tom de SpectromCtr~eIonique et MolPculuire (ti.A. CNRS 171) Unirwsitt4 Cluude Bernard, I,yv~ 1. Campus de la Doua, 69622. V~lleurbanne, Crdex. France

We present some of our results beam fractions for C4+ and N5+ deduced. Two-electron change has change and transfer ionisation) are The charge exchange excitation measurements on 0 III

concerning the VUV spectroscopic study of low energy charge exchange collisions. Metastable have been measured and absolute cross sections for corresponding emission lines have been been observed for N IV and Ne VII. where two processes (single capture with one core electron involved. method has also been applied to pure spectroscopic investigation of Kr VIII and to polarisation

1. Introduction

ple. Two-electron change collisions Finally our polarisation measurements

Photon spectroscopy is a well established technique for studying collision products, with sufficient resolution to allow the measurement of the selective population of n, I subshells. Actually fast beam spectroscopy dates back to the experiments of Wicn [l] (1919) on canal rays, the first technique for the production of fast ionic or atomic beams with particle velocities of the order of 107~10x cm s ‘. The canal rays experience enough collisions with the residual gas for excitation and emission of light. Early experiments included intensity decay, determination of Doppler effect and linear polarization measurements and its use for in beam Hanle effect. Then, for a long time, the interest for those experiments faded until the classical fast beam technique was rediscovered in the sixties giving rise to beam-gas [2] and bean-foil [3] spectroscopy. In beamfoil technique, highly charged ions are produced by collision of fast projectiles with a thin solid target, and time resolved spectroscopy is used for studies of structure, lifetime or polarisation. In beam gas tcchmque. atoms (or molecules), generally singly charged, go through a thin gas target yielding slow [4] and fast [5] excited particles. Spectroscopy is used mainly for studying the collision. In this paper we deal with beam gas UV spectroscopy using highly charged incident ions at keV energies. We shall begin with a presentation of our recent results on emission and excitation cross section determination using ground state and metastable fast particles, and of the technique we use for our experiment. Then we will show how the spectroscopic identifications are facilitated by the selectivity of the capture: the Krs+ + He collision spectrum will be taken as an exam0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

R.V.

are also studied. are mentioned.

2. Metastable beam fraction measurement cross sections for metastable states

and collision

In collision experiments it is important to know what fraction of the beam corresponds to metastable states if one wishes to carry out cross section measurements. An interesting example is the case of collisions between heliumlike ions with atoms or molecules. The multicharged ions have well known metastable states ls2s’S and ls2s’S. the lifetime of which may be calculated from the decay rate data given by Lin et al. [6] for 2 El and Ml transitions. The capture of an electron by these metastable states produced doubly excited (ls2snl) quartet or doublet states of the lithiumlike ion, which lie high above the ionisation potential of the ion formed [7.8]. Whilst the ls2snl doublets are readily autoionised, the quartets are metastable with respect to Coulomb autoionisation and transitions between these doubly excited quartet states may be observed by photon spectroscopy. The search for these lines is made easier by the fact that these transitions have been extensively studied by beam-foil spectroscopy. We present here our study of the C4’ + Hz and N 5 + H ,-He collision cases for which subshell one electron capture has been reported [9,10,11] and for which total cross sections have been calculated [12] and measured [13,14]. The experimental apparatus has been described in previous papers [15,16] and is schematized in fig. 1. The ion beams are produced by the Grenoble 10 GHz ECR source. magnetically analysed by two bending magnets,

E C R ion eeurce

chsmbsr

8

M.C.P.

detector

ki-

-

U.V. specttometer Fig. 1. Schematic

representation

of the experimental arrangc-

ment.

passed through a cell (A) at 5 X lo-’ mbar pressure and collected in a Faraday cup. Another collision cell (B) may be introduced between the two magnets. Light emitted as a result of the collisions in cell (A). is observed at 25” to the beam axis with a 3m grazing incidence (82”) spectrometer equipped with micro-channel plates. When we sent C4’ (40 kev) into H, (5 X 10-j mbar) in cell A. we observed (in addition to the transitions resulting from electron capture by the 1s’ ground states of C”+). a weak line at 28.4 nm which may he unambiguously identified as the ls2s2p 4P”-ls2s3d 4D transition between doubly excited states [17] (figure 2a). The very strong 1~~2s ‘S-ls23p “p” transition line is blended with the ls2s2p 4Po-ls2s3s 4S 31.5 nm line. With N5+ (50 keV) into H, or He (5 x lo-’ mbar) in cell A we observed the 19.3 nm ls2s2p4Po-ls2s3s4D and the 21.1 nm ls2s2p4P0 1~2~3s 4S transitions between doubly excited states of NV. The 21.1 nm line is blended with the very strong 20.9 nm ls22s ‘So-ls23p *P” transition line. The presence of these transitions between doubly excited states together with the ls2nl-ls2n’l’ transitions, is indicative of a metastable beam component in the C4+ and NSf ion beam. The lifetime of the C4+ metastable states as deduced from Lin et al. [6] are 3 ps (1~2~‘s) and 30 ms (IsZS~S). At our collision velocity (II- 9 X IO’ cm/s) the time of flight from the source to cell A is around 7 ~LSand only about l/10 of the initial ls2s’S C4+ beam reaches the collision cell. Then only the ls2s3S metastable compo-

nent need to be taken into account in the collision process. A similar conclusion may be drawn for N’+. In order to measure this metastable beam fraction we used the electron capture method [l&19] preparing the C4’/N 5+ ions by charge exchange from C5+/N6+ in the preparation cell B. Although the total C4’ or Nsc beam is greatly reduced, the metastable beam fraction is increased as shown in fig. 2. It may be seen that the relative intensity of the lines resulting from transitions between doubly excited states as compared to the ground state core lines is increased in the experiment which used the beam prepared by charge exchange. In this latter experiment, where the N5+ is prepared by charge exchange in the cell B, the ls2s’S state is prepared closer to the collision cell A, and around l/3 of the ions in this state will reach this cell. Therefore the possible contribution of this term may not be neglected if we assume a statistical excitation between singlet and triplet states in the primary collision of Nht which will give Ns+. However one electron capture from this singlet state gives only doubly excited doublet states which autoionise and are therefore not optically observable. As a consequence of this method we may identify the line at 37.7 nm (fig. 2) as a transition from a doubly excited quartet state. According to the energy level calculations of Dumont et al. 1171, it may correspond to the 37.48 nm 2s2p2 4P-ls2s3p 4P0 transition. This line has also been observed by beam-foil spectroscopy [20]. Assuming core conservation for electron capture into some excited states. we may write for the two experiments (direct C4+- N5+ beam from the source, referred to as 1, and C4+-N 51 beam prepared by charge exchange, referred to as 2): II = t18 -i- it,,,,

J,,/~z~ = fIg/i2,

i,=i _

1, ,,,/f2 ,,, =

lg + ilnr,

i I ,,,/i 2,,,)

where i, and i2 are total beam intensities, i,,, i,, the beam intensities of the ions in their ground state (Is’), fin,* f Z,,, the beam intensities of the ions in the metastable state (ls2s), I,,, I,, the intensities of a line with ground state core, I,,,,’ Izm the intensities of a line with metastable state core. Solving these equation gives the metastable beam fractions:

Taking into account that ratio, and that the beam the error on ill,, and iln, results we have obtained

we need only the line intensity intensity i, is higher than i,, may be estimated to 20%. The are shown in table 1. VIII. SPECTROSCOPY

270 Table 2 Emission cross sections + H: at 3.33 kev/amu

for C4+(1~2~3S)+HZ-C3t(ls2s314L) (error *50’%)

X(nm)

Identification

~,,(lOK’~cm~)

28.4 31.5 31.7

ls2s2p4P” ls2s3d4D ls2s2p4Po - ls2s3s4S ls2p’ 4P - ls2s3p4P”

7 15 6

Table 3 Emission cross sections N4 ’ (ls2sn14L) + He ‘/Hi X(nm)

15.1 15.5 19.3 21.1

for NS’ (1~2~‘s) + He/H2at 3.57 kev/amu (error i 40%)

Identification

ls2s2p4P”-ls2a4d4D l.\2s2p4P”-ls2s4s4S ls2s2p4P”-la2s3d4D la2h2p4Po-ls2s3s4S

n,,,,( 10

” cm’

He

HL

3 4.9

4 2 9.2 4

)

cross sections of transitions from 1~2~31 4L states both in C IV and NV have about the same values as for the corresponding transitions from the ls’31 ‘L states. This result implies that the excitation cross section from the metastable state is almost of the same order as from the ground state.

emission

31.5

20.4

3. Use of the highly ion&d troscopic source

A(4

Fig. 2. Part of the CJ ’ spectrum obtained during the C4’ -Hi collision in cell A (P = 5 x 10 ’ mbar - beam diameter X mm energy 3.33 kev/amu), (a) with C4’ extracted directly from the source (I - 2 CIA). (b) with C”’ obtained by charge exchange between C’ * and H, in cell R (I - 50 nA).

We may then conclude that the ECR source used only a few per cent of metastable states of C4 ’ and N”+, but we did not check the variation with the source parameters (HF power.. ). Thus, since we are able to measure the intensity of the ion beam in the metastable state, we may deduce the emission cross sections of the transitions between douby excited states (tables 2 and 3). The absolute calibration is based on the C“’ + H, emission cross sections measured by Dijkkamp et al. [21]. We may notice that the gives

Table 1 Mctastable

Produced Prepared

(ls2a3S)

beam fraction

by the ECR ~onrce by charge exchange

(in % of the total beam) c4+

NS’

5+2 30+6

x+3 25+5

ion-gas collision as a spec-

To characterize the highly charged ion-gas collision spectroscopic source, one can list specific features and compare them to those of some other light sources: (a) Numerous charge states of any element produced by the ECR source may be obtained by choice of the incident charge state and element from the ion source. However for the moment the variety of products is less extensive than in beam-foil excitation at big accelerators. (b) Partially selective excitation of n, I sub-levels gives cleaner spectra than most other techniques. (c) Low excited beam luminosity requires single photon counting. More precisely the use of microchannel plates is an important improvement enlarging the possibilities of the method. Compared to plasma and even beam-foil sources, this low luminosity is a handicap though the size of machines are not comparable. (d) The spectral resolution is Doppler limited depending on the beam velocity. For the same element in the same charge state, this Doppler limitation is less drastic than in beam-foil spectroscopy. However till now published spectra have medium spectral resolution, mainly limited by instruments. (e) The excitation mode can produce alignment of

excited levels giving rise to anisotropy and polarisation of light. Some properties of this spectroscopic source are

illustrated in our spectrum of 80 keV I@+ ions in helium. Intensity of the beam was 800 nA. Part of spectrum displayed in fig. 3, between 42 and 47 nm, shows that: - Lines are due to trantitions in e Kr VIII excited by charge exchange. The line at 446 A, from the spectrum of Kr VII is much weaker than in the corresponding beam-foil spectrum (fig. 3). No line from other charge state than VII and VIII has been observed. In particular the lines at 465 A from Kr VI apearing in beam foil spectroscopy is absent here. - This spectrum of Kr VIII (and Kr VII) has been registred in a short time using microchannel plates. For higher charge states experiments became more and more difficult due to lower beam intensity and worse signal/ background ratio. - Spectral resolution is about 1 to 1.5 A. This is comp~able to medium resolution beam-foil spectroscopy and far from the Doppler limitation,

4. Two electron

Xmmi

43

Fig 3. Upper: Part of the krypton spectrum obtained during the Kr’ ’ t He collision at 0.95 keV/amu in ceil A. P = 5 X lo-’ -- beam diameter 8 mm - i - 0,8 pA energy 0.95 keV/amu). Lower: Partial krypton spectrum obtained by beam-foil-spectroscopy at 2 MeV beam energy, from ref. [22]. (a) Kr VIII 4p2Po - 4d’D. (b) Kr VII 4s4p3Po - 4s4d3D. (c)

Kr VI 465.27 A 4p’P’ - 4d*D.

change

Two electron capture in low energy charge exchange collisions has been observed by energy gain spectroscopy of the ions and by electron spectroscopy. By VUV spectroscopy the only unambiguous determination of two electron capture deals with the Ns+ -t He collision with the observation of the very strong N IV 2s2 ‘S - 2s2p ‘P transition (111. Mostly the phcnomenum of two electron capture is masked by double collision processes and it is difficuit to separate the contribution of each process. Two electron change has been observed in the Be-like sequence as a result of the collision of N4+, F6+, Ne’+ with He or II,. In addition to the 1~~2s ni excited configurations obtained by one electron capture on the ground state 1~~2s. the so called displaced terms ls’2p nl have been observed by energy gain spectroscopy [23-261. By VUV spectroscopy we have observed transitions coming from these particular terms in the Ne7’ + H,collision [27]. We have obtained a similar result with N4’ + He where some Lines may be attributed to transitions from ls22p3 E ‘L levels. The origin of such a two electron change may either be capture into an nl level while the 2s electron is excited to the 2p state, or a transfer ionisation process: double capture into doubly excited states followed by autoionisation. The double coincidence measurements between Ne6+ (ls22p nl), He” and He+ in the Ne7” + He collision experiment 1281 has shown that both processes are involved in this excitation with two electron change. Multi electron change processes are now under extensive study by energy gain spectroscopy as well as by electron spectroscopy and VUV spectroscopy. VIII. SPECTROSCOPY

Table 4 Polarisation fraction P = (I - I, )/( I,, + I, ) of some 0 III lines in the OX+ +Hz + Ozii +Ht collision at 1.X7 keV/amu X(nm)

Identification

J--J’

P(S)

298.4 304.7 326.1 326.5 375.5 375.7 376.0 377.4

3s’P” - 3p’D 3a’P” - 3p?P 3p’D - 3d’F” _ 3s3Pa - 3$D

1-2 2-2 2-3 3 -. 4 1-2 0 -- 1 2-3 1-l

15 13 20 20 II

_ _

18 9

5. Polarisation measurement Some polarisation experiments have recently been reported with low energy multicharged ions in collision with gas [29-311. We have performed such measurements in O”+ + H, collision at 1.87 kev/amu in the 250-600 nm spectral range where convenient polarizers are available. The observation of light was made at 90” from the ion beam direction with a CzernyTurner spectrometer equipped with a photomultiplier. The results for the fractional linear polarisation

are shown in table 4. The correction to be applied to a direct determination of collision cross sections due to the anisotropy of emission is, in this case, sufficiently weak to be included in error bars for an isotropic assumption. However it is good to have in mind that the anisotropy of emission exists and in some cases has to be taken into account. Measurement of polarisation in the UV domain is more difficult but there are methods to realize it. This anisotropy could be used for experiments in the future.

Conclusion Based on recent results obtained in collisions of highly ionized atoms on He or Hz targets we have shown that the VUV spectroscopy is a high resolution technique allowing measurements of n, I subshell collision cross sections. Furthermore spectra obtained with high Z elements proved that this technique may provide a new spectroscopic source giving infornlation on energy levels in complex spectra.

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Molecular

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VIII. SPECTROSCOPY