QUANTUM-MECHANICAL STUDIES OF PHOTODESORPTION OF AMMONIA FROM A METAL-SURFACE - ISOTOPE EFFECTS, FINAL-STATE DISTRIBUTIONS, AND DESORPTION MECHANISMS

The photodesorption dynamics of ammonia from a copper surface is studi ed quantum mechanically using empirical potential energy surfaces. The desorption is facilitated by substrate-mediated electronic excitation and subsequent de-excitation of the adsorbate, which are simulated in our model as Franck-Condon pump and dump between two electronic state s. The delayed de-excitation populates metastable predesorption states which lay above the desorption limit. The slow decay of these resonan ces via energy transfer from an internal mode to the desorption mode r esults in incomplete and rather slow desorption. The desorbed molecule s have significant vibrational excitation and their translational ener gy distributions are highly structured, due to the dominance of the pr edesorption mechanism. The desorption yield depends sensitively on the time delay between the excitation and de-excitation. Strong isotope e ffects are observed, consistent with experimental findings. The anomal ously large NH, yield relative to ND3 is attributed to its faster moti on along the inversion coordinate on the excited state. (C) 1995 Ameri can Institute of Physics.