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 ν₂ 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 3a1 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.