Analysis of two-photon photoemission from Si(001)

Abstract

We have applied our ab initio simulation approach for the photoemission process at solid surfaces to calculate two-photon photoemission spectra from the p(2 × 2)-reconstructed Si(001) surface. In this approach, the groundstate electronic structure of the surface is obtained within density functional theory. The subsequent timedependent simulation is carried through at frozen effective potential, while an optical potential is applied to account for inelastic scattering in the excited state. We have derived normal emission spectra for sand p-polarized light with photon energies in the rangehω = 3.85-4.75 eV. The dependence of the theoretical spectra on photon energy and polarization is analyzed and compared to experimental spectra from the literature. To unravel the role of the unoccupied states between Fermi energy and the vacuum level which are acting as intermediate states in the excitation process, we investigate the expression for the two-photon photocurrent from perturbation theory. The scattering states, which serve as the final states of photoemission, are obtained from a time-dependent simulation of a LEED-type experiment. The evaluation of the dipole matrix elements allows us to identify the relevant bulk band transitions and to address the influence of surface states.

Ultra-fast carriers relaxation in bulk silicon following photo-excitation with a short and polarized laser pulse

“…However ad-hoc parameters must be introduced to describe both the photo-excitation process and the specific material properties.First-principles simulations are commonly performed by using time-dependent Density-Functional Theory [13] or equilibrium Many-Body Perturbation Theory [14]. In the first case the coupling with the laser pulse is described but the dissipative processes are neglected [15,16] or described in an empirical way [17]. In the second case the laser pulse is replaced by some ad-hoc initial guess of the carriers distribution and, as a consequence, the scattering rates are derived from the equilibrium and static quasi-particle (QP) lifetimes [18].In this Letter we demonstrate that only by a careful and ab-initio description of both the photo-excitation process and the full time dependence of the nonequilibrium carrier scatterings it is possible to device a successful, parameter-free, accurate and predictive approach to the interpretation of real-time experiments.…”

“…First-principles simulations are commonly performed by using time-dependent Density-Functional Theory [13] or equilibrium Many-Body Perturbation Theory [14]. In the first case the coupling with the laser pulse is described but the dissipative processes are neglected [15,16] or described in an empirical way [17]. In the second case the laser pulse is replaced by some ad-hoc initial guess of the carriers distribution and, as a consequence, the scattering rates are derived from the equilibrium and static quasi-particle (QP) lifetimes [18].…”