Electron spectroscopy of doubly-excited helium-like ions

383

Nuclear Instruments and Methods in Physics Research B53 (1991) 383-386 North-Holland

Electron spectroscopy of doubly-excited helium-like ions S. Ohtani ‘v2,Y. Kanai, I. Yamada I, H.A. Sakaue 3, Y. Awaya, T. Kambara, T. Nabeshima 3, N. Nakamura 3, H. Suzuki 3, T. Takayanagi 3, K. Wakiya 3, A. Danjo 4 and M. Yoshino 5 The Institute of Physical and Chemical Research (RIKEN),

Wako, Saitama 351-01, Japan

Spectroscopic investigations of electrons coming from doubly-excited states of helium-like C and B ions have been made with zero-degree spectroscopies. These excited states are produced by two-electron capture collisions of bare ions with helium atoms. Electrons from the configurations of 2Inl’ are observed in a wide energy range. Total angular momentum distributions in the double-electron transfer collisions are briefly discussed.

2. Experiment

1. Introduction Recently,

multi-electron

highly-charged by

energy

ion-atom gain

spectroscopy Those charged

(1) ions,

multielectron ion-atom bare

as He and

to study

X-ray done

been

studied

ejected

electron

spectroscopy

[11,12].

for

the atomic

the

following

structure

in the highly-charged

In the double

projectiles H,,

only

states

electron

by collisions the singlet

an electron produced

of the

transfer

with targets

states

can be produced,

of the doubly the dou-

in such collisions are the

(21nl’) configuration, a possible final state after ejecting one electron should be the 1s state of H-like ions. Therefore, in these collision systems, the electron spectra should be simple enough to analyze easily. Here we report spectroscopic studies of ejected electrons from autoionising state of doubly-excited He-like ions [14].

’ Permanent address: National Institute for Fusion Science, Nagoya 464-01, Japan. ’ Permanent address: Institute for Laser Science, University of Electro-Communications, Chofu, Tokyo 182, Japan. 3 Permanent address: Department of Physics, Sophia University, Cbiyoda-ku, Tokyo 102, Japan. 4 Permanent address: Department of Physics, Niigata Uuiversity, Niigata 950-21, Japan. ’ Permanent address: Laboratory of Physics, Shibaura Institute of Techuology, Omiya, Saitama 330, Japan. 0168-583X/91/$03.50

+ C4+(21nl’)‘L + C’+(ls)

B’++ He(ls2)

+ He2+ + e-

+ B3+(21nl’)‘L + B4+(ls)

into such

unless the colli-

spin [13]. When

C6++ He(ls2)

of highly

transfer

processes

We have measured ejected electrons produced by the following double-electron transfer collisions at about 5 keV/amu;

two

the mechanism

configurations

excited

have

[l-3],

in slow

(2) to investigate

sion has changed bly

being

collisions.

nucleus

excited

and

are

processes

collisions

spectroscopy

[4-lo],

studies

purposes:

transfer

(1)

+ He2+ + e-.

(2)

The bare projectiles were produced by the ECRIS (electron cyclotron resonance ion source) which was wnstrutted as an ion source for the RIKEN AVF cyclotron [15]. Ejected electrons were measured by the technique of zero-degree electron spectroscopy. The experimental procedure will be published elsewhere [16].

3. Results 3.1. C6+ on He system Spectra observed for the collisions of 13C6+ with He are shown in fig. 1. The energy scale of the spectra was calibrated by using Auger peaks from the ls2s2p(‘P) and ls2s(3S)3d(2D) states of Li-like carbon ions which were produced by double collisions with target He atoms at high target pressure of 3 X lop3 Torr (fig. lb). The energy values of these Auger lines from Li-like carbon ions were determined by Mann [17]. The measured spectrum at a low gas pressure (3 x lop4 Torr), which

is considered

shown in fig. la.

0 1991 - Elsevier Science Publishers B.V. (North-Holland)

as a single

collision

condition,

is

tb)

250 Electron

300 Energy (eV)

324 328 328 330 Electron Energy (eV )

350

Fig. 1. Ejected electron spectra produced by the collision of C6+ with He atoms. (a) Spectra from C4’ (2M’) n L 3 configuration, obtained under the single collision condition. (b) Spectra obtained at the high target pressure, Auger peaks from the C’+(ls2lnl’) n 2 2 states are observed as well as the peaks from the C4+(21nl’).

There are two features of the spectrum shown in fig. la: (1) no peaks of the 2121’ confjguratio~s (around 270 eV) and (2) peaks of the doubly-excited states of 2t31’ configuration are strongly populated and peaks of 21nf’ (n 2 4) are observed for a wide range of n. These features can be explained in terms of a “reaction window”. From the estimation of the reaction window [N], the crossing radius R, to produce the 2121’ states in the double-electron transfer collisions is estimated to be out of -the reaction window for our experimental conditions. Hi& resolution spectra of ejected electrons from 213C’ states are shown in fig. 2. There are a few precise calculations [19-223 for the energy values of the 2131’ states. By comparing those calculations with observed spectra, we identify each peak as denoted by vertical lines in the figure, Experimental energy values of ejected electrons from C4+(2C31’) are listed in table 1 together with the theoretical ones. In table 1, the term represen-

Fig. 2. High resolution ejected electron spectrum from the 2E31 E configuration of C4+.

tations (K, T, and A) with the correlated classification scheme are shown as well as the single-particle notations. Theoretical values noted as Lipsky are extrapolated values from those of Lipsky et al. [IS] for higher Z ions by using a method described by Lin 1231. Energy values of the PC1 shift using the theoretical lifetime [24] are also listed in table 1. ~~~~rn~nt~ values of ejected electron energies from the 2631’ states (‘S, ‘P) are in good agreement with theoretical values of Ho [20] after the PCI correction. An accurate calculation of the ‘D and ‘F states is needed to compare with our results. 3.2. B5+ on He system Spectra observed for the collisions of the 11B5+ with He are shown in fig. 3. Similarly to those for C6* ion collisions, spectra measured at high- and low-pressure conditions of the target gas are shown in fig. 3a and 3b. As shown in fig_ 3b, which is thought of as a single collision condition, we observed an important shift of the populations towards lower members of the n-series in 21nJ’ configurations; a high probability of capture

Table 1 Energy of electron from the C4+ (2131’)

Observed values

PC1 shift

tev)

fe\3

Single particle notation

323.7 f 0.2 324.7 325.5 327.0 f 0.5 327.5 328.7 329.9 330.2 331.2

0.0 0.2

2~3s 2~3s

o*o

2p3p 2p3d

0.0 0.2 0.1 0.0 0.0 0.0

2~3~ 2s3p 2s3d 2p3d 2p3d

2s+1

IPO IS’ IF= ID” lDe lPO ‘De ‘P ‘P”

L”

(K, VA

(1, w fk (0, I)-(0%1Y

w+

(Lo)+ u411+

(O,lY (1,

w

(-Lojo

meQry Lipsky

Ho

323.91 325.36 325.71 327.56 328.17 329.41 330.34 330.71 331.70

323.88 324.96

328.90

331.36

HOI&en

Doyle

325.45

325.65 327.48

S. Ohtani et al. / Doubly-excited

( )I a

Double Collision

d’(2lnl’)

160

180

200

ELECTRON

220

240

(bli

260

280

He 50KeV

B&(2121’)

180

‘tv

185

ELECTRON Fig. 4. Ejected

electron

385

into 2121’ is observed. The shift of the populations in 21nl’ configurations can be explained in terms of the reaction window. The crossing radius for the production of the B3+(2121’) is estimated to enter a suitable region of the reaction window. Energy spectra of ejected electrons from 2121’ and 2131’ configurations are shown in figs. 4 and 5. Identification of each peak is made as shown by vertical lines in the figures, though statistics of the measured signals are somewhat poor. A more precise analysis for these spectra is in progress.

4. Discussion

ENERGY ( eV )

Fig. 3. Ejected electron spectra produced by the collision of B5+ ions with He atoms. (a) Spectra obtained at the high target pressure. Auger peaks from the B*+(ls2lnl’) n 2 2 states are observed as well as the peaks from the B3+ (2/n/‘) states. (b) Spectra from B3+(21nl’) n 2 2 configuration, obtained under the single collision condition.

Es-

helium-like ions

190

195

ENERGY

200

Total angular momentum L-distributions in the double electron transfer processes have been discussed for 06+(3131’), N5+(3131’) [8], C4+(3131’) [9], and 04+(ls22pnl) n = 6. 7 [25]. Those previous studies suggest that high-angular momentum states are dominantly populated by double-capture collisions of highly charged ions. In the present observations, most of the ejected electrons come from D and F states rather than S and P, as shown in figs. 2, 4 and 5. If we assume that all of the magnetic sublevels mL are equally populated, differential cross sections for electron emission in the forward direction will be proportional to the total emission cross sections for the respective L-states. Under this assumption, we could conclude that high L-states are dominantly populated in the present collision systems for C6++ He and B5++ He at about 5 keV/amu. At the present, however, since we have no accurate information on the mL population, systematic observation of spectral behaviors as a function of ejection angles is needed.

(eV)

spectrum from the 2121’ configuration of B’+.

Acknowledgement We acknowledge Dr. K. Hatanaka and Mr. H. Nonaka for managing the ECR ion source. This work was performed under the collaborative study program of National Institute for Fusion Science with RIKEN and Sophia University.

$+-

He 50keV B3’(2131’) mq I

References [I] S. Tsurubuchi,

222

224

ELECTRON Fig. 5. Ejected

electron

226

228 ENERGY

230

232

234

J

(eV)

spectrum from the 2131’ configuration of B3+.

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helium-like ions

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