Quantum-state-resolved investigation of the UV photodesorption of NH sub 3

The photodesorption dynamics of ammonia from Cu(111) and GaAs(100)-(4x6) has been studied using (2+1) resonance-enhanced multiphoton ionization (REMPI) as a quantum-state-specific probe. Hyperthermal excitation of the v sub 2 mode is observed for both substrates. Photodesorption from Cu is characterized by a marked underpopulation of antisymmetric v sub 2 levels, whereas for NH sub 3 /GaAs the population of the v sub 2 mode follows Boltzmann statistics. The two systems also differ in the amount of rotational excitation and translational energy. The experimental results are discussed referring to a model which was developed recently to account for the large isotope effect in ammonia photodesorption. The model describes correctly the different quantum state distributions within the v sub 2 manifold for the two systems.