MULTIDIMENSIONAL DYNAMICS IN THE ELECTRON-STIMULATED DESORPTION OF AMMONIA FROM PT(111)

We characterize the electron stimulated desorption of neutral ammonia (NH3 and ND3) from Pt(111) with vibrational and rotational quantum resolution by using (2+1) resonance enhanced multiphoton ionization detection. Two significant isotope effects appear: (1) the desorption yield of NH3 is three times that of ND3 and (2) NH3 desorbs with considerably more ''spinning'' rotational energy than does ND3. We find virtually identical translational energy distributions for each desorbate and roughly equal vibrational energy distributions. Vibrational excitation is found exclusively in the v(2) symmetric deformation or ''umbrella'' mode, independent of isotope. These effects cannot be explained by desorption induced by vibrational energy transfer. Instead, desorption is the result of excitation of a 3a(1) electron principally on the N atom, which causes the pyramidal NH3 adsorbate to rapidly invert. Ab initio calculations of two-dimensional potential energy surfaces (intramolecular bond angle and surface bond length) reveal that near-inverted molecules deexcite to a repulsive hard wall of the adsorbate-substrate interaction and desorb. Spinning excitation derives from the rotational barrier of the inverted molecule. Both isotope effects are direct consequences of desorption via inversion. in general, multidimensional dynamics must be considered in the study of stimulated surface processes. Our calculations also indicate that excited-state forces at equilibrium molecule-surface distances are an order of magnitude less than those derived from a currently accepted image-potential model.