Angular and Energy Distributions of RH Atoms Desorbed in an Excited State from Ion-Bombarded Rh{100}
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ANGULAR AND ENERGY DISTRIBUTIONS OF RH ATOMS DESORBED IN AN EXCITED STATE FROM ION-BOMBARDED RH{ 1001 ROYA MABOUDIAN, M. EL-MAAZAWI, Z. POSTAWA, AND N. WINOGRAD Department of Chemistry, The Pennsylvania State University, University Park, PA 16802
ABSTRACT Multiphoton resonance ionization spectroscopy has been used to determine the polar-angle and the kinetic-energy distributions of rhodium atoms desorbed from ion-bombarded Rh{ 100) 4 surface in the fine-structure components of the a Fj (=9/2 and 7/2) ground-state multiplet. The peak in the energy distribution of the metastable level (4 F7/2 with excitation energy of 0.2 eV) is found to occur roughly at the same value as the ground-state ( 4 179/2) distribution but decays more gradually at higher energies. The measured spectra have been used to investigate the dependence of the excitation probability on the takeoff angle (0) as well as the emission velocity (v). It is shown that the excitation probability depends strongly on theseparameters, approaching an exponential dependence on l/[v cos(0)] at higher velocities (> 5x10 cm/sec). INTRODUCTION Energetic ion bombardment of solid surfaces gives rise to the ejection of target species. A fraction of the desorbed particles leaves the surface in an excited state [1] which decays to ground state or to metastable levels via optical radiation. To investigate the desorption mechanism of the excited-state atoms, the spatial distribution [2-4] and the Doppler-broadened line-shape profile [5,6] of the emitted light were originally measured and used to determine the energy distributions of the ejected excited particles. With these techniques, extremely high values of mean kinetic energies (1 to 2 orders of magnitude higher than that for the ground state) were obtained, hence placing serious constraints on any models attempting to explain their mechanism of excitation. Based on these measurements, several excitation models were proposed. Some of these include excitation in early stages of collision cascade via knock-on collisions with target atoms [6], existence of a radiationless deexcitation process near the surface [7,8], resonance neutralization of sputtered ions into excited neutrals in the vicinity of the surface [9] and excitation via inelastic energy transfer to a target atom in the final collision leading to desorption [10]. More recently, Doppler-shift laser induced fluorescence (DSLIF) spectroscopy has been used to measure the velocity distribution of atoms desorbed in various metastable states [11-16]. With this technique, the energy distributions of atoms ejected in sublevels of ground-state multiplet or in low-lying excited states (with excitation energies less than 1 eV) were found to be very similar to the ground-state distribution [11,14]. Metastable particles with excitation energies above 1 eV were also investigated and shown to have broader energy distributions than the ground state, with the most probable energies 3-4 times higher than the sputtered ground-state atoms [12,13]. Another experimental approach t
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