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Abstract:
After UV‐laser‐induced desorption we observe bimodal velocity distributions independent of internal vibrational excitation [up to v=2 (4%)] applying resonance‐enhanced multiphoton ionization techniques. Both contributing desorption channels are of nonthermal origin. We introduce a model where the two desorption channels are correlated with the rupture of the molecule surface bond of the librating molecule either on the way toward or away from the surface. We have performed trajectory calculations to simulate the desorption processes. The calculated momentum distributions of the desorbing molecules show either one or two maxima, depending on lifetime, in agreement with experimental results. The vibrational distribution of the desorbing molecules can be reproduced by assuming transition into a state that is characterized by an altered N-O bond length as it is found, for example, in NO-. The model calculations both for velocity distributions and vibrational excitations result in similar lifetimes of the excited state, even though the translational and the vibrational degree of freedom of the desorbing molecules are decoupled.
