Recombinative desorption of H2 on Si(100)-(2×1) and Si(111)-(7×7): Comparison of internal state distributions

Abstract

The dynamics of recombinative hydrogen desorption from the Si(100)-(2×1) and Si(111)-(7×7) surfaces have been compared using (2+1) resonance-enhanced multiphoton ionization to probe the desorbed H2. After dosing the surface with disilane (Si2H6), we performed temperature programmed desorption in a quantum-state-specific manner. The rovibrational-state distributions of H2 desorbed from both Si(100)-(2×1) and Si(111)-(7×7) are found to be the same within experimental accuracy. The rotational distribution is non-Boltzmann and has an average energy significantly lower than kTs, where Ts is the surface temperature. In contrast, superthermal energy is observed in the vibrational degree of freedom, and the v=1 to v=0 population ratio is approximately 20 times higher than that predicted by Boltzmann statistics. Our results imply that the details of the recombinative desorption process that affect the product state distribution are remarkably insensitive to the structural differences between the surfaces. We suggest that the transition-state geometry is similar on both surfaces and propose a model for hydrogen recombinative desorption localized at a single silicon atom.