Electron loss and capture to continuum from He and ... - Springer Link

Atoms,Molecules and Clusters

Z. Phys. D - Atoms, Moleculesand Clusters 9, 229-234 (1988)

fiJr Phy~k D

© Springer-Verlag1988

Electron loss and capture to continuum from He and Ne atoms bombarded with He +-Ions* O. Heil, J. Kemmler, K. Kroneberger, A. Kiiv6r**, Gy. Szab6 **, L. Guly~s**, R. DeSerio***, S. Lencinas, N. Keller, D. Hofmann, H. Rothard, D. Ber6nyi**, and K.O. Groeneveld Institut f/Jr Kernphysik der J.W. Goethe-Universitfit,D-6000 Frankfurt, Federal Republic of Germany Received 29 February 1988; final version2 May 1988 We have measured the cusp electron yield in coincidence with the transmitted charge state (He °, He ÷ and He ÷ ÷) when aHe÷ collides with He and Ne under single collision conditions. F o r the first time this enables the electron capture to the continuum (ECC) yield to be directly compared with that from electron loss to continuum (ELC). While the ECC contribution is smaller than that from ELC at high projectile velocities (Vp > 3 au) the data suggest that ECC will dominate below Vp=2.8 au. The relevance of the results to the projectile velocity dependence of existing capture theories is discussed. PACS: 34.50.Fa

Introduction

Experimental set-up

The study of electron loss to continuum (referred to as ELC) in energetic ion atom collisions has lead in recent years to distinct discrepancies between theoretical model calculations and experimental results at zero degree emission angle [1, 2]. In contradiction to a predicted, nearly independent projectile velocity dependence (Vp) of the loss cross section aL for He+-projectile ions incident on He, at low projectile velocities (Vp< 3 au) a tendency of an increasing measured loss cross section with decreasing Vp was observed. As pointed out by Lucas et al. [1], K6v6r et al. [-2] and Briggs et al. [3], such an increasing tendency might be explained by a contribution of electron capture to continuum (referred to as ECC) in the experimental results. Nevertheless the agreement between the experimental data and the theoretical calculations is excellent in the projectile velocity regime Vp> 3 au [4].

In order to decide clearly between the two different physical processes (ELC, ECC), an experiment was performed at the 2.5 MV Van de Graaff accelerator of the J.W. Goethe-University in F r a n k f u r t / M , to measure the electron energy distribution (Ie) of the cusp electrons in coincidence with projectiles of the emergent charge states (He °, He +, He + +). 3He-gas in the ion source was used instead of 4He to avoid possible separation problems of 4He + ÷ (which was needed for calibration purposes) from 2D+ or H~-. A sketch of the experimental set up is shown in Fig. 1. The magnetically analyzed ion beam aHe÷ enters the target region (T) through a set of apertures (A) and slits (S). Here, a dynamically controlled effusion gas target is crossed and the emerging projectiles pass through the electric charge analyzing field (P). Finally the charge states are registered by a position sensitive parallel plate avalanche detector (PPAD) [-5]. The complete charge state distribution is monitored by a copper anode consisting of 16 electrically isolated stripes (2 mm width). An incoming particle traverses the Mylar foil in front of the gas amplification region of the P P A D and a Townsend electron avalanche in the gas area is induced. The electron cloud accelerates towards the copper anode and delivers an electric pulse at one stripe corresponding to a certain position

* This worl~ has been funded by the German Federal Minister for Research and Technology(BMFT) under the contract number 060F173/2 Ti 476, DFG/Bonn, MTA/Budapestand NSF and DOE/ Washington ** Institute of Nuclear Research of the Hungarian Academy of Sciences(ATOMKI), Debrecen,Hungary *** University of Tennesseeand Oak Ridge National Laboratory, Oak Ridge, Tenn, USA

230 effusion gas target the pressure in the electron spectrometer chamber increased to 2 x 10 - 5 mbar. The data were collected in a listmode event procedure with a computer system. This allows us to monitor simultaneously the time spectrum, the charge state spectrum and the electron energy spectrum.

ESA - 13

1/tm,x a true coincidence start signal will have a certain probability to be stopped earlier by a random stop signal, for time delays (to) smaller than the time delay of the peak position. Since the detected rates ~/e and ~ra are very different (ie~ 10 -1 Hz, Na~ 1 MHz), it was necessary to gate the position output decoding modul of the PPAD with the start electron signal events. Thus, when a gate signal appears, it is likely to measure a sudden stop event, caused by the large detected particle rate. Accordingly, with (tm~x= 400 ns) only a weak dependence of the random background is observed, covered by a small contribution of efficiency loss for time delays (to) smaller than the time delay of the peak position. The large hatched area at t o ,,~0 (Fig. 2) is mainly caused by the position decoding modul itself. The full width half maximum of the time peak (AtFwnM= 15 ns) corresponds to the induced time shift from electrons measured with different energies (LV~-Vpl ~0.2 au). The time resolution of the system is mainly limited to A t = 7 ns by electronic circuits and the rise time of the channeltron involved in the electron spectrometer. A typical charge state spectrum is presented in Fig. 3. Here the coincident count rate (i.e. truly corre-

231 I

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~- 280 123

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200 300 TIME DELAY t 0 Ins] ,, Fig. 2. Time to amplitude spectrum of the 3He+ - >Me collision system. Marked area shows events of "lost"-coincidences, the time peak corresponds to coincident events (measured electron velocity interval IVy-Vpl< 0.2 au), the area below the dashed line represents random events (Ref. 8)

I

I 900

I

/ / / / / / / / / / / / / /

3He+llMeV)--m,.Ne

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sensitive avalanche detector (PPAD). The histogramm marks the count of the particular stripe

0.6

25 POSITION[mini

200

Fig. 4. 1/Ee corrected coincident cusp electron energy spectrum (Oe =0 °) for electron loss and capture to continuum. Non correlated events are subtracted. The solid and dashed lines guide the eye

2

15

. "° ~'3¢~o _

Ee [eV]

Fig. 3. Coincident charge state distribution obtained by the position

5

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35 ,-

l a t e d events) is p l o t t e d versus the p o s i t i o n d e c o d i n g of the P P A D . D u r i n g the m e a s u r e m e n t the c o n s t a n c e of the c o p p e r stripe efficiency (e) was c h e c k e d with a r a n d o m pulse g e n e r a t o r . S p e c t r a with v a r y i n g r a n d o m c o u n t rates were d i s c a r d e d in the d a t a r e d u c t i o n . By setting w i n d o w s in the list m o d e d a t a , c o i n c i d e n c e s b e t w e e n e l e c t r o n s a n d a c e r t a i n c h a r g e state (e.g. H e +, H e ++) were a n a l y z e d . T h e 1/Ee c o r r e c t e d e l e c t r o n s p e c t r a in energy (Ee) c o i n c i d e n t with H e + were att r i b u t e d to E C C events, w h e r e a s the e l e c t r o n s p e c t r a c o i n c i d e n t with H e + ÷ were a t t r i b u t e d to E L C events. Events c o i n c i d e n t with H e ° were neglected (Ycc< 1%) c o m p a r e d to events c o i n c i d e n t with H e +. A l s o events o r i g i n a t i n g f r o m the rest gas in the c h a m b e r c o u l d be neglected. Because of the low c o i n c i d e n t c o u n t rates (/'e < 1 0 - 1 Hz) the u n c o r r e l a t e d r a n d o m events

3He+~~He,Ne oHe INe

>_~ ~ + 0.6 0.2

o

2.6

3.0

3.4 Vp [au]

I

3.8 ,-

Fig. 5. 3He+-projectile velocity dependence of the normalized cap-

ture (Y~) and loss (YL) yield (middle part) and intensity ratio for He and Ne target systems (bottom). The insert (top) and solid line (middle part) refers to the applied theory (see text) h a d been carefully s u b t r a c t e d f r o m the m e a s u r e d elect r o n spectra. A w i n d o w c o n d i t i o n h a d been set in the b a c k g r o u n d to b o t h sides of the time p e a k . This a c c o u n t s for the w e a k time d e p e n d e n c e of the r a n d o m b a c k g r o u n d u n d e r l y i n g the time p e a k a n d the effi-

232

ciency loss contribution superimposed on the random background [8]. Figure 4 summarizes the analyzed events from the ECC and ELC electron energy spectra from He + (1 M e V ) - - > Ne collisions. The normalized intensity I~ is plotted versus the electron energy E~. A noticeable capture contribution (Y~=25 _+12%, Fig. 5 (lower part)) is even seen at this projectile energy, which shows the tendency of an asymmetric skewness towards smaller electron velocities (Ve< Vp) [2]. However, the drawn lines are just to guide the eye; - the limited statistics does not allow us any definite shape conclusion. The projectile velocity dependence Vp of the normalized capture (Y~)and loss (YL) yield (normalization at Y~(Vp = 2.8 au)= 1) for a constant electron velocity interval (]V~-Vpl