INTERNAL-STATE DISTRIBUTION OF RECOMBINATIVE HYDROGEN DESORPTION FROM SI(100)

We have measured vibrational- and rotational-state distributions for H-2, D2, and HD thermally desorbed from the monohydride phase on Si(100) surfaces using resonance-enhanced multiphoton ionization detection. The nu = 1 to nu = 0 population ratio is roughly 20 times higher than that predicted by Boltzmann statistics at the surface temperature, T(s) almost-equal-to 780 K. In contrast, the average rotational energies of the desorbed molecules are significantly lower than kT(s), exhibit no isotopic dependence within experimental error, and are not significantly different in the nu = 0 and nu = 1 vibrational states. In the vibrational ground state, we find <E(rot)> = 345 +/- 83 K, 451 +/- 77 K, and 332 +/- 57 K for H-2, HD, and D2, respectively. The degree of vibrational excitation suggests that the H-H interatomic distance in the transition state is elongated compared with the gas-phase equilibrium bond distance. The low average rotational energy clearly rules out recombination from a highly asymmetric transition state or recombination from high-impact-parameter collisions. Our data may be interpreted as resulting from a preference for reactive trajectories that impart little angular momentum either through the effects of the corrugation of the potential-energy hypersurface or through the collision leading to the transition state, followed by prompt desorption of the newly formed molecular hydrogen from Si(100). We propose that pairing on Si dimers occurs prior to desorption; various models are discussed regarding the desorption mechanism subsequent to pairing.