358 Nuclear Instruments and Methods in Physics Research B48 (1990) 358-360 North-Holland

LIMITS OF QUASI-RESONANCE FOR He-Pb

D.J. O’CONNOR and R. BEARDWOOD *

Department of Physics, University of Newcastle, Shortland, NS W 2308, Australia

The quasi-resonant charge exchange between He and Pb has been studied at small angles to establish the behaviour as the

distance of closest approach in the collision increases. The energies at which peak ion yields are observed have been found to vary

with scattering angle and when the distance of closest approach is greater than 0.884 A the ion yield from the quasi-resonant process

ceases.

1. Introduction

Since the first observation of the oscillatory depen- dence of ion yield on projectile velocity for a projectile and a solid [l] there have been numerous studies which have established the essential features of the quasi-reso- nant process [2-121, though most studies have been confined to a fixed scattering angle of 90 O. As the theoretical description of this interaction has been covered in detail by previous authors [2,3,11] it will not be repeated here. Early studies identified the combina- tions of projectile and target which yielded oscillatory ion yields [4] and further studies aimed at determining the role of the chemical and physical environment on the interaction [4-91. More recent studies have em- ployed higher energy resolution to identify the two contributions to the ion yield and the energy loss associ- ated with a reionisation event has been measured [13-211.

The only study of the angular dependence [11,12] of the oscillatory ion yield established theoretically with a crude model and experimentally that down to scattering angles of 40 o there was little departure from the be-

haviour observed at 90 O. Despite using the assumption that the energy difference between the potential energy curves was constant and the use of an unscreened Coulomb potential to describe the repulsive interaction between the projectile and the target, good agreement was found between the experimental observations and the model for the energies at which the peak ion yields would be observed. While no measurements were re- ported below scattering angles of 40” the model ex- trapolated into this region predicted that the peak yield energy would be seen at lower energies with decreasing scattering angle.

In an extension of that study, the quasi-resonant

* Current address: School of Physics, University of Sydney,

Sydney, Australia.

0168-583X/90/$03.50 0 Elsevier Science Publishers B.V.

(North-Holland)

process has been extended down to a scattering angle of 15” in an endeavour to verify the behaviour predicted and to ascertain the limits at which the oscillatory behaviour would be observed.

2. Experimental

The experimental system is a Leybold-Hereaus ion scattering system which is pumped by a combination of a turbomolecular pump and a titanium sublimation pump which achieve a base pressure of 5 X lo-” mb. The energy analyser is mounted on a goniometer which has two degrees of rotational freedom about the target surface. The projectile ions were produced by a 5 keV 3M ion gun (now manufactured by Kratos).

The target was a 99.999% Pb polycrystalline sample mechanically polished to a flat clean finish before intro- duction to the system. As the experiments involved small scattering angles, a further cleaning stage was undertaken with the sample being subjected to an in- tense Ar ion beam at an incidence angle of 5O to the surface to both clean and flatten it on an atomic dimen- sion. Analysis by ISS after this procedure yielded a surface free of adsorbates to the limit of detection (specifically less than 1% for carbon, sulphur and other common contaminants and less than 0.1% for oxygen). The sample was mounted on a goniometer which has three translational degrees of freedom and two rota- tional degrees of freedom.

Data acquisition was performed by a LeCroy 3500 computer interfaced by camac modules to the appara- tus. The data acquisition, control of the ion gun and manipulation of the target and energy analyser by stepper motors were all under computer control. The beam current was monitored with a beam current dig- itiser and stored as well to enable beam current normal- isation.

D.J. OYhmor, R. B~a~~~ / Limits of ~~-teson~nce~or He - Pb 359

3. Results and discussion

In a typical scan at 90° shown in fig. 1 the ion yield is seen to oscillate as a function of energy. If plotted as a function of velocity the period would be constant. While any point on the curve would be adequate to follow in this study, for convenience the energies of the peaks in the ion yield have been measured as a function of decreasing scattering angle. In this experiment only the peaks with energies less than 1 keV have been studied and these have been labelled as peaks 1 to 4 in fig. 1.

The incident energy at which the ion yieid peaks is plotted in fig. 2 where it is evident that the peak yield energy decreases with decreasing scattering angle in agreement with White et al. [11,12]. Below a critical

0 0.2 0.1 06 1.0 12 14 1.6 14 26

Fig. 1. The ion yield as a function of incident energy for He

scattered through 90° of Pb. The peaks followed in this study

are labelled 1 to 4.

i-Ill_r: 01 I I / i I i

0 IO 20 XJ 40 x3 60

SCATTERING ANGLE

Fig. 2. The peak yield energy plotted as a function of scattering angle for He off Pb. The yield vanishes for each peak at a

common distance of closest approach of approximately 0.884

A. The solid curve is the relationship between energy and angle

which corresponds to a 0.884 A distance of closest approach.

scattering angle associated with each peak the scattered ion yield drops to zero. This condition has been identi-

fied with a common distance of closest approach of 0.884 A, determined using the universal interatomic potential [22]. The curve of constant distance of closest approach, equal to 0.884 A, is included in fig. 2. This interatomic separation is well within the mixing dis- tance corresponding to the overlap of the two levels (He ground state and the 5d levels of Pb) involved in the * charge exchange (- 2.7 A). The loss of the constructive quantum mechanical interference at this distance of closest approach must therefore be related to the time spent in the phase difference region.

These experiments were performed on a polycrystal- line surface at low angles so it is not possible to estab- lish the role played by neighbouring atoms in this process. Further experiments on a single-crystal surface are under way to establish the importance of the ad- ditional perturbations introduced by weak collisions

with these neighhours.

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