Temperature effects for vibrational relaxation of hydrogen adsorbed on Si(100): a stochastic multiconfigurational time-dependent Hartree (MCTDH) study

Using a previously developed system–bath model Hamiltonian for the vibrational relaxation of H atoms adsorbed on a Si(100) surface (Andrianov and Saalfrank in J Chem Phys 124:034710, 2006), temperature effects on the relaxation of excited adsorbate vibrations are studied non-perturbatively. In particular, the Si–Si–H bending mode, for which relaxation times in the order of picoseconds have been predicted theoretically, is considered. Also, the excitation by infrared laser pulses is explicitly modelled. To do so, a combined system–bath Schrödinger equation is considered, in which an anharmonic system describing the vibrating H atom is coupled non-linearly to a set of harmonic oscillators which represent the phonon bath of the solid. For the time evolution, the multiconfigurational time-dependent Hartree (MCTDH) method is used together with a stochastic approach to sample thermal initial conditions. In contrast to perturbative approaches (Golden Rule), it is found that the bending mode relaxes the slower, the hotter the surface is. Possible reasons for this “puzzle”, which also seems at variance with measurements for H:Si(100), are discussed.