230 Nuclear Instruments and Methods in Physics Research B13 (1986) 230-234 North-Holland, Amsterdam STUDY OF SOLID-SURFACE-INDUCED MOLECULAR DISSOCIATION LEADING TO ATOMIC EXCITATIONS C.H. CHRISTENSEN, J.O. JENSEN, K. LEFMANN, C. S@LVSTEEN and E. VEJE zyxwvutsrqponmlkjihgfe Physics Laboratory II, H. C. Orsted Institute, Universitetsparken 5, DK-2100 Copenhagen 0, Denmark Solid targets of graphite or tantalum have been bombarded with H’, Hi, and Hi (accelerated to 30 keV/nucleon), N ‘, Ni (at 3.2 keV/nucleon), and He’, HeH’ (at 18 keV/nucleon) at an angle of incidence of 80”. Excitation of scattered projectiles has been studied with the use of optical spectrometry. The spectral line intensities per incoming nucleus depend on the nature of the incoming projectile, whether it is monoatomic or molecular. As an example, for hydrogen molecules on graphite, the Balmer line intensities per proton are approximately a factor of two lower than the corresponding signals obtained with incoming protons. For the heavier elements He and N, more complex ‘features are observed. The results are compared to similar studies of molecular effects in beam-foil interactions and discussed. 1. Introduction There is presently a great deal of interest in so-called molecular effects in beam-foil processes. By molecular effects, we understand here nonlinear variations in optical signal strengths when molecular projectiles are used instead of monoatomic ions at the same velocity. For projectiles containing only hydrogen, enhance- ments in level populations have been observed, the enhancement being either independent of the principal quantum number II of the excited level [l, 21, or a smoothly varying function of n [3]. However, when heavier elements are involved, irregular features have been reported [ 1,2,4,5], population enhancements as well as reductions have been observed. There exists as yet no model which rationalizes or explains the experi- mental findings satisfyingly well. For monoatomic projectiles, it is normally agreed, that the resultant excited-level population, as observed in beam-foil spectrometry, takes place when the pro- jectile leaves the back of the foil. Recent experiment [6] and theory [7] agree that the final level excitation results from transfer of electrons between the projectile and the valence band of the foil. The relative velocity between the atomic particles originally forming one and the same molecule will always be much smaller than the projectile velocity relative to the foil. Thus, for beam-foil processes with molecular projectiles, it is justified to regard the whole process as being composed of a fairly swift interaction between the particle cluster and the back of the foil, followed by a fairly slow dissociation taking place downstream from the foil [5]. However, in addition, there may well be interactions caused by convoy electrons or other kinds of secondary electrons [5], which are created in the bulk (admittedly close to the exit of the foil, but not at the back surface itself). Such interactions can well possess some molecular effect in themselves. Molecular effects have been observed in secondary electron emission yields [8] as well as for convoy electrons [9]. Summarizing, molecular beam- foil processes result from a multitude of processes which are initiated either in the bulk, at the foil exit, or downstream from the foil, so it need not be surprizing that there does not exist one simple model which rationalizes the experimental data. In an attempt to elucidate this complex situation, we have accelerated molecules as well as monoatomic ions, and studied the final atomic excitation which followed from ion beam surface interaction at grazing incidence, in an attempt to avoid bulk processes, so that only the fairly swift particle-surface interaction and the relative- ly slow particle-particle separation are present. 2. Experimental setup, data taking and treatment The experimental equipment has been described in detail previously [lo]. The projectiles hit a solid surface of either graphite or tantalum, at an angle of incidence of 80”, and a quantum efficiency calibrated optical monochromator with photomultiplier analyzed and de- tected photons emitted from scattered projectiles. The observation volume was a cylinder with radius 2 cm, as described in ref. [lo]. The residual gas pressure in the target chamber was below lo-’ Pa, making it possible to work with uncontaminated surfaces. The target surfaces were initially cleaned thoroughly with 80 keV Ar’ ions. The following projectiles were used. H’, Hl, and Hl accelerated to 30 keV per nucleon, He’ and HeH’ accelerated to 18 keV per nucleon, and N’ and N: 0168-583X/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)