Exterior complex scaling method in time-dependent density-functional theory: Multiphoton ionization and high-order-harmonic generation of Ar atoms

Abstract

The exterior complex scaling (ECS) method is applied in the framework of time-dependent density-functional theory (TDDFT) to study multiphoton ionization (MPI) and high-order-harmonic generation (HHG) of multielectron atoms in intense laser fields. ECS allows one to impose correct (outgoing-wave) boundary conditions on the wave functions at large distances. In our implementation, ECS is combined with the time-dependent generalized pseudospectral method for accurate and efficient solution of the time-dependent Kohn-Sham equations. We make use of LB94 exchange-correlation potential which proved accurate in calculations of unperturbed electronic structure of Ar. Calculations of MPI and HHG are performed for the laser pulses with the wavelength of 800 nm and several peak intensities. The HHG spectrum exhibits an intensity-independent minimum corresponding to the photon energy of about 51 eV which is closely related to the Cooper minimum observed in the photoionization cross section of Ar. We found that MPI probabilities and HHG spectra calculated with the frozen-core potential (that is, not including dynamic response of the electron density) differ significantly from those obtained by TDDFT.