Analysis of laser-enhanced adsorption/desorption processes on semiconductor surfaces via electronic surface state excitation

Electronic surface states in semiconductors often lie between the valence and conduction bands and give rise to charge densities confined to the surface region. Laser radiation of frequency less than the energy gap can excite electrons from delocalized valence band states to these localized surface states leading to large changes in the charge distribution at the surface. Selective enhancement of adsorption/desorption processes involving ionic or polar adspecies can result from such a charge redistribution. Using a one dimensional model for silicon, the cross section for the laser induced electronic transition to surface states is show to be large. The interaction energy of an adspecies with the surface changes significantly with direct excitation of surface states in a semiconductor. For a one dimensional metal, however, direct transitions between bulk and surface states are not allowed, but phonon mediated transitions coupled with laser radiation lead to substantial charge transfer as for semiconductors.