Angular distributions of NO in laser-induced desorption from Pt(111)

The angular distributions of NO photodesorbed from a Pt(111) surface by UV- laser irradiation at 193 nm are investigated as a function of velocity usin g a quantum state specific detection method, which combines resonance enhan ced multiphoton ionization (REMPI) and a two-dimensional-imaging technique. The angular distributions of NO in individual rovibrational states are obs erved to be strongly peaked in the surface normal direction and are consist ently reproduced by gaussian functions in the desorption angle theta. The a ngular width of these distributions is found to decrease substantially with the velocity of the desorbing molecules. The identical velocity dependence of the angular distributions is observed independent of the rotational qua ntum states of the photodesorbed NO molecules. This is indicative of a de-c oupling of the rotational motion of the molecular axis and the hindered par allel translation of the molecules in the adsorption ground state as well a s in the excited states probed by the desorption process. The implications of a simple impulsive model, which relates the amplitude of the frustrated translational motion to the velocity dependence of the observed angular dis tributions, are discussed. (C) 1999 Elsevier Science B.V. All rights reserv ed.