The adsorption of D2 on Si(100) has been investigated by means of supersonic molecular beam techniques. We have succeeded in measuring the dependence of the molecular D2 sticking coefficient S on surface temperature Ts and nozzle temperature Tn. The sticking coefficient increases gradually in the range 300≤Tn≤1040 K. The influence of increased v=1 population has not been deconvoluted from the effects of translational energy alone. The dependence on Ts is more interesting. With an incident translational energy of 65 meV, S rises from a value insignificantly different from the background level to a maximum value of (1.5±0.1)×10−5 at Ts=630 K. The decrease in the effective sticking coefficient beyond this Ts is the result of desorption during the experiment. Having established that S increases with both increasing molecular energy and increasing sample temperature, we have demonstrated directly for the first time that the adsorption of molecular hydrogen on Si is activated and that lattice vibrational excitations play an important role in the adsorption process.

A marked quantum effect has been observed in the vibrational state distribution of photodesorbed ammonia. Namely, for quantum numbers larger than zero, symmetric and antisymmetric levels in the n 2 mode of the desorbed ammonia molecule are unequally populated. A strong propensity for symmetric levels is observed for NH 3 , whereas the reverse is found for ND 3 . Model calculations reproduce this effect. Moreover, it is found that the actual ratios probe the binding energy in the energetically less favorable inverted geometry with the H atoms pointing towards the surface. [S0031-9007(97)

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Abstract. Photodesorption of ammonia follows a curved reaction path. Electronic excitation of, or electron attachment to, the adsorbate molecule leads to a transition from the pyramidal ground state to an excited state with planar geometry. During the short residence time in the excited state, the hydrogen atoms are accelerated towards the surface, resulting in an inversion of the molecule and a strong repulsion from the surface which eventually leads to desorption. In this paper, we present evidence that in the inverted geometry the molecule feels the corrugation of the surface potential on rotation around its symmetry axis. This gives rise to a strong torque in cases where the 3-fold symmetry of the molecule is not matched by the lattice symmetry, resulting in energy transfer to the rotational degrees of freedom. Secondly, it is demonstrated that when both ammonia and oxygen are coadsorbed on a Ru surface, in which case the rotation of adsorbed NH, is frustrated, a large fraction of the zero point energy is released as rotational energy.