The laser-induced desorption of ammonia from GaAs(100)-(4X6) and Cu(111) has been studied using (2+1)-photon resonance-enhanced multiphoton ionization as a state specific probe. A surprisingly marked isotope effect (NH3/ND3) in the photodesorption cross section of ammonia from GaAs(100) had indicated that internal degrees of freedom play a crucial role in the desorption dynamics. We find that NH3 desorbs with a mean translational energy, (Etrans), of 300 K and 600 K for GaAs(100) and Cu(111), respectively. The average extent of vibrational excitation is about 1000 K. The rotational temperatures are 520 and 170 K. These results are interpreted in terms of a direct mechanism for NH3 desorption in which a reaction path which initially follows the internal v2 coordinate and then bends towards desorption is dominating. A marked unequal population of the inversion symmetry doublet states is observed.
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