We report a first principles study of the structure and the vibrational properties of the Si(100)-H(2 x 1) surface in an electric field. The calculated vibrational parameters are used to model the vibrational modes in the presence of the electric field corresponding to a realistic scanning tunneling microscopy tip-surface geometry. We find that local one-phonon excitations have short lifetimes (10 ps at room temperature) due to incoherent lateral diffusion, while diffusion of local multi-phonon excitations are suppressed due to anharmonic frequency shifts and have much longer lifetimes (10 ns at room temperature). We calculate the implications for current induced desorption of H using a recently developed first principles model of electron inelastic scattering. The calculations show that inelastic scattering events with energy transfer n (h) over bar omega, where n>1, play an important role in the desorption process. (C) 1999 Elsevier Science B.V. All rights reserved.