Zero-degree binary encounter electrons in fast collisions of highly charged F and O ions with H2 targets

NIOMI B

Nuclear Instruments and Methods in Physics Research B79 (1993) 11-14 North-Holland

Beam Interactions with Materials 8 Atoms

Zero-degree binary encounter electrons in fast collisions of highly charged F and 0 ions with H, targets * D.H. Lee ‘, T.J.M. Zouros ‘, J.M. Sanders, H. Hidmi and P. Richard J.R Macdonald Laboratory,. Department of Physics, Kansas State University, Manhattan, KS 66506, USA

Doubly differential cross sections (DDCS) for binary encounter electrons (BEe) produced by 0.5-2 MeV/u highly-charged F and 0 ions in collisions with H, gas targets have been studied at 0” with respect to the ion beam direction. The measured DDCS of the broad binary encounter peak was well described by a simple impulse approximation (IA) treatment for bare ions, and was demonstrated to provide in situ detection efficiency of the electron spectrometer. The projectile energy dependence of the BEe production for nonbare (clothed) projectiles is found to follow a scaled L4 prediction, in which a BEk enhancement is consistently exhibited for the collision energy range studied.

1. Introduction Extensive efforts have been made recently our understanding of the binary encounter

to extend electrons

(BEe) which are target electrons ionized by direct ion bombardment in fast ion-atom collisions [1,2]. These studies followed the earlier reports on 0” BEe measurements [3,4], in which the electrons produced by fast (l-2 MeV/u) projectile ions were detected at 0” with respect to the beam direction. The doubly differential cross sections (DDCS) of the produced BEe exhibited a broad distribution in the observed electron energy spectrum [S], reflecting the momentum distribution (Compton profile) of the target electrons. The binary encounter between a fast projectile and a “quasifree” target electron gave a stringent test [3] for an impulse approximation (IA), which has been successfully applied to electron-electron (e-e) interactions such as the well-known resonant transfer-excitation [6] (RTE), e-e excitation [7] (eeE), and e-e ionization [8,9] (eeI). The observations of the “enhancement” of 0” BEe production [4] and of the “oscillatory” d-electron production [lo] with partially stripped ion projectiles have attracted further theoretical and experimental investigations.

This work was supported by the Division of Chemical Sciences, Office of Basic Sciences, Office of Energy Research, U.S. Department of Energy. Present address: Brookhaven National Laboratory, Upton, NY 11973, USA. Present address: Physics Department, University of Crete and Institute of Electronic Structure & Laser, Heraklion 711 10, Crete, Greece. 0168-583X/93/$06.00

In this article we report on further evidence of the 0” BEe enhancement for a limited projectile chargestate and energy range. We then demonstrate, using the results of BEe-IA and BEe enhancement factor, a method of in situ efficiency normalization of the electron spectrometer used in both low-resolution (nonretarding) and high-resolution (retarding) modes. The normalization of the electron spectrometer is crucial for absolute cross section measurements.

2. Experiment The measurements were performed at the J.R. Macdonald Laboratory at Kansas State University using the 7 MV EN tandem Van de Graaff accelerator. The BEe data of OS+ and F6P7*9+ions were obtained simultaneously when a series of RTEA (RTE followed by Auger emission), eeE, and eeI studies were carried out by measuring high-resolution Auger electron spectra. A 0” tandem electron spectrometer [ll] has been used to take all the electron spectra. The BEe spectra were mostly taken in low energy resolution (AE/E = 2.8%), in which the electrons were detected without any energy retardation. In the high-resolution mode, both binary-encounter and Auger electrons were analyzed using a required energy retardation. Further experimental details were previously reported [ 11,3].

3. Results and discussion Fig. 1 shows a series of 0” electron spectra originating from the ionization of the Hz targets in collisions

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I. ATOMIC/MOLECULAR

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D.H. Lee et al. / Zero-degree binary encounter electrons

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28.5 t&V Fg+ + HZ

S-38 MeV Fg+ + Ii2

LABORATORY

ELECTRON

ENERGY (keV)

Fig. 1. Zero-degree electron spectra obtained for 0.5-2 MeV/u F 9f incident on H, target. Bi and Ci refer to the binary-encounter electron (BEe) and ECC cusp electron peak, respectively, at projectile velocity steps of l/4 MeV/u. Inset: DDCS of BEe measured (+I and calculated (solid cmve).

with 9.5 to, 38 MeV bare F9+ projectiles. For each projectile energy, both the cusp ECC (electron capture to continuum) and the broad BEe peaks, which are labeled by Ci and B,, respectively, are well separated from each other. As given in ref. [3], the DDCS of the broad BEe peak (see inset in fig. 1) has been successfully described by an IA calculation, in which the 0” BEe was treated as 180” elastic Rutherford scattering in the projectile frame including a convolution of the Compton profile. The IA calculation agrees well with all the measured BEe distributions, as indicated in the fig. 1 inset. Thus, the simple nature of the BEe scattering and its production has been utilized for the efficiency normalization of the electron spectrometer. Since the spectrometer efficiency is, in general, a function of the electron energy, the BEe production at various projectile energies is to be used. Furthermore, the IA calculation is expected to, in principle, best predict its DDCS values “around” the BEe peak. The spectrometer efficiency, thus, can be determined by normalizing the recorded DDCS in a relative scale to the IA-DDCS values at various BEe peak energies, whose positions can be determined by different projectile bombarding energies. Fig. 1 displays several BEe peaks which were used to find the spectrometer efficiency at the laboratory electron energy range of l-4.5 keV [3]. It is noted that the present BEe-IA normalization is good only for low 2 projectile ions (Z I 15) [l]. We note that the cusp ECC peaks show much steeper variation with projectile energy than the BEe

PROJECTILE ENERGY (M&I)

Fig. 2. Zero-degree BEe peak values measured (data points) and calculated by a scaled IA (solid lines) versus projectile energy in collision of 7-38 MeV F6,7P9+ions on H, targets. The scaling factor for the clothed ions shows BEe production enhancement.

peaks. This is understood if one considers that the projectile energy E, dependence of electron capture varies as m Ei4.’ [12]. This is compared to that of BEe production, which was found to be u Ep2.6 [3] in the present projectile velocity range. Figs. 2 and 3 display the measured DDCS at the BEe peak for the present projectile species and energies. Each DDCS peak value was extracted by fitting the measured DDCS values around the BEe peak with the IA calculation for the bare projectile ion. Further details were given in ref. [4]. The solid line in figs. 2

20

I

I

1

I

I

I

1

I 20

I 25

I 40

OS+

and H,

10 t

-2

5-

% 2 6

2-

y

l-

z ;

IAX1.3

0.5-

:: n 0.20.1 0.055

;

I a

I 10

I 15

50

PROJECTILE ENERGY (Me!/)

Fig. 3. Same

as fig. 2 but for 5-30 collisions.

MeV

D.H. Lee et al. / Zero-degree binary encounter electrons 6

1

1

1

20 MeV F8+ + Hz

13 I

,

I

13.5 MeVOQ+ + Hz 1800 1850 9!,.,,.““.“,,....“““~ 700 1750 LABORATORY

ELECTRON

1900 ENERGY

1950 (cV)

n(lllllll

PROJECTILE-FRAME

ELECTRON

ENERGY’(eV)

Fig. 4. Zero-degree BEe and Auger electron spectra obtained in high-resolution mode. Auger electron production cross sections for the various excitation and RTE states [6] can be obtained using the BEe-IA normalization (see text).

and 3 represents an IA calculation for the BEe peak, which was obtained by multiplying the bare-ion IA calculation by the scaling factor shown in the figures.

using the normalization procedure discussed above. The experimental fA can be determined when the data points are best fitted with the calculated DDCS (solid

This scaling factor gives an average enhancement factor, which is the increase of the BEe production for the

line), which is obtained by folding the theoretical DDCS formula [15] with the spectrometer response function. In summary, we have measured DDCS of the binary-encounter electrons (BEe) produced from H, targets in bombardment with various highly charged OS-2 MeV/u F and 0 ions at 0”. The BEe enhancement with partially stripped projectile ions is observed to be a constant for each charge state in this collision energy range. The in situ normalization of the detection efficiency of the spectrometer was demonstrated using the BEe whose production is well described by the impulse approximation. The measured enhancement factor can then be used in the normalization of the DDCS of high-resolution projectile Auger lines that appear around the BEe peak.

partially charged ions with respect to that for the bare projectile [4]. The B& enhancement is observed to be a constant for each charge state for the present ion projectiles and bombarding energy range. Furthermore, the scaled IA calculation predicts well the projectile energy dependence except for a few data points at higher projectile energies, where the enhancement starts to diminish 1131. Shown in 4g. 4a are typical raw spectra of projectile Auger and binary-encounter electrons (in the laboratory frame) obtained in a retarding, high-resolution mode of the electron spectrometer. The strong intensity of the binary-encounter electron background is observed at these collision energies. The spectrometer efficiency was observed to be further affected by focus-

ing and defocusing properties, which originated from the focusing lens between the two spectrometers and electron energy retardation for high-resolution, respectively [ll]. Therefore, the spectrometer efficiency function in this specific high-resolution mode is obtained by normalizing the measured BEe DDCS (see broken line in the figure) to the scaled IA calculation, whose scaling is done using the measured or calculated BEe enhancement factor. Auger electron production cross sections can, thus, be determined with this normalization procedure [14]. In fig. 4b is a precision measurement of the Auger branching rate r, of the F7+ (2p*)*D RTEA line produced in collisions of F*‘(ls) with H, target. The data points of the experimental DDCS were obtained

References [l] For example J.E. Miraglia and J. Macek, Phys. Rev. A43 (1991) 5919 and references therein. [2] C.P. Bhalla, R. Shingal and S. Grabbe, these Proceedings (12th Int. C&f. on the Application of Accelerators in Research and Industry, Denton, TX, 1992) Nucl. Instr. and Meth. B79 (1993) 170; S. Hagmann et al., presented at this Conference; J.L. Shinpaugh et al., presented at this Conference; J.W. Berryman et al., presented at this Conference; D.R. Schultz et al., presented at this Conference. M. Sataka, presented at this Conference. D. Schneider, presented at this Conference; M.P. Reaves et al., presented at this Conference; H.E. Wolf et al., these Proceedings, p. 64. [3] D.H. Lee, P. Richard, T.J.M. Zouros, J.M. Sanders, J.L. Shinpaugh and H. Hidmi, Phys. Rev. A41 (1990) 4816. I. ATOMIC/MOLECULAR

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[4] P. Richard, D.H. Lee, T.J.M. Zouros, J.M. Sanders and J.L. Shinpaugh, J. Phys. B23 (19901 L213. [5] N. Stolterfoht, D. Schneider, D. Burch, H. Wieman and J.S. Risley, Phys. Rev. L&t. 33 (1974) 59. [6] For recent review see T.J.M. Zouros, in: Recombination of Atomic Ions, eds. W. Graham and J. Tanis (Plenum, New York, 1992) p. 271; J.A. Tanis, Nucl. Instr. and Meth. B40/41 (1989170. [7] T.J.M. Zouros, D.H. Lee and P. Richard, Phys. Rev. Lett. 62 (1989) 2261. [8] H.-P. Hillskdtter, W.E. Meyerhof, E. Dillard and N. Guardala, Phys. Rev. Lett. 63 (1989) 1938. [9] D.H. Lee, T.J.M. Zouros, J.M. Sanders, P. Richard, J.M. Anthony, Y.D. Wang and J.H. McGuire, Phys. Rev. A46 (1992) 1374.

[lo] C. Kelbch, S. Hagmann, S. Kelbch, R. Mann, R.E. Olson, S. Schmidt and H. Schmidt-B&king, Phys. Lett. 139A (1989) 304. [ll] D.H. Lee, T.J.M. Zouros, J.M. Sanders, J.L. Shinpaugh, T.N. Tipping, S.L. Varghese, B.D. DePaola and P. Richard, Nucl. Instr. and Meth. B40/41 (1989) 1229. [12] AS. Schlachter, J.W. Steams, W.G. Graham, K.H. Berkner, R.V. Pyle and J.A. Tanis, Phys. Rev. A27 (19831 3372. [13] D.R. Schultz and R.E. Olson, J. of Phys. B24 (1991) 3409. [14] R. Parameswaran, C.P. Bhalla, B.P. Walch and B.D. DePaola, Phys. Rev. A43 (1991) 5929. [15] C.P. Bhalla, Phys. Rev. Lett. 64 (19901 1103.