THEORY OF LASER-INDUCED DESORPTION OF AMMONIA FROM CU(111) - STATE-RESOLVED DYNAMICS, ISOTOPE EFFECTS, AND SELECTIVE SURFACE PHOTOCHEMISTRY

A two-dimensional, two-state model is used to model the UV-laser-induc ed photodesorption dynamics of NH3 and ND3 from Cu(111) by solving the nuclear time-dependent Schrodinger equation. By projecting the asympt otic wave functions on the asymptotic (''umbrella'') eigenstates of NH 3/ND3, we find that the molecules leave the surface vibrationally hot, in agreement with experimental data. Within individual asymptotic tun neling doublets, however, the desorbates are clearly non-Boltzmann wit h molecules of ''gerade'' symmetry desorbing with increased probabilit y. Our study correlates this parity selection with details of the elec tronic ground state potential energy surface. An experimentally observ ed strong isotope effect in the desorption yields for the different is otopomers is traced back mainly to differences between the vibrational frequencies of the ''umbrella'' mode, in accord with earlier, classic al models. Additionally, small tunneling and moderate zero-point contr ibutions are observed. Finally, the possibility of bend and isotope se lective photochemistry at surfaces, based on a two-photon IR+UV strate gy is demonstrated. (C) 1995 American Institute of Physics.