First-principles electron-ion dynamics of excited systems: H-terminated Si(111) surfaces

A first-principles molecular dynamics simulation is applied to an H-termina ted Si(111) surface subjected to Si-H sigma-->sigma* excitation which was m imicked by promoting the electron occupations. To take the finite Lifetime of the excited state into account, the time-dependent electronic Schrodinge r equation has been solved and coupled with the classical Newton equation o f ions. Density functional theory with use of the local density approximati on (LDA) and the generalized gradient approximation (GGA) was adopted to ex press the Hamiltonian. To study influence of localization of the excited st ate and height of the density of state (DOS) of the system on the simulated results, systematic calculations using clusters and slab models with sever al sizes were performed. Strong localization of the excited state in cluste r models caused stronger forces compared to those in slab models. Such over estimation of the localization indicates that cluster models have been foun d to be inappropriate for expressing electronic excited states on solid sur faces. On the other hand, the computed lifetime of the excited state was fo und to become shorter as the cluster became larger and as the slab became t hicker. This fact was attributed to higher DOS in extended systems that ope ned many decay paths. The obtained lifetime in slab models was on the order of (or less than) 10 fsec, which was too short to induce a direct H dissoc iation and consistent with the very low yield of the dissociated H atoms in recent experiments. The decay of the excited state was due to relaxation o f the Si-valence electrons to the SI-H sigma-hole state, but neither due to the direct recombination of sigma*-->sigma nor relaxation of the sigma* st ate into the Si conduction bands. Finally, detailed comparison between LDA and GGA results are also presented.