Nonperturbative approach to photoemission by direct simulation of photocurrents

A procedure is presented for the ab initio calculation of the angle- and energy-resolved photocurrents emitted from an atom, molecule, cluster, or solid surface excited by a fs-laser pulse. The approach does not rely on perturbation theory. Instead, it is based on the direct simulation of the photoemission process in the time-and-space domain. Hence, though the focus of the present work is on single-photon photoemission from the Si(001) surface which is presented as a test case, we emphasize that the simulation inherently includes two- and multiphoton photoemission currents. The system is assumed to be initially in its electronic ground state. Its electronic structure is calculated within density functional theory using supercells and a slab geometry. The time evolution of the system is obtained by the integration of the time-dependent Kohn-Sham equations. In case of photoemission from solid surfaces discussed in this paper, the inelastic scattering of the photoelectrons is roughly accounted for by an absorptive gauge-invariant optical potential, which is acting on the excited-state admixtures of the time-dependent singe-particle wave functions only. Due to the omission of the inelastically scattered electrons from the calculated charge density, the effective potential cannot be updated any longer and has to be kept frozen during the simulation. Technically, the decoupling of the slabs is achieved by an absorbing potential in the vacuum region. The angle- and energy-resolved photoemission spectrum is obtained from the Fourier transform of the time-dependent single-particle wave functions. Photoemission spectra for the Si(001) surface are compared to experimental data from the literature.