High-order perturbation expansion of non-Hermitian Floquet theory for multiphoton and above-threshold ionization processes

Abstract

A high-order perturbation theory is presented for efficient and accurate computation of multiphoton and above-threshold ionization cross sections of atoms and molecules in weak to medium strength laser fields. The procedure is based on a Raleigh-Schrodinger perturbative expansion of the time-independent non-Hermitian Floquet Hamiltonian. The reduced Green function and generalized pseudospectral discretization techniques are extended to facilitate the calculation of complex quasienergy resonance states without the need of diagonaliz- ing the full Floquet Hamiltonian. Explicit expressions are presented for the determination of intensity- dependent total and partial rates and electron angular distributions. The theory is applied to a case study of multiphoton detachment of H 2 for a range of laser frequencies ~corresponding to the absorption of a minimum of two photons! and laser intensities from 10 7 to 10 12 W/cm 2 . It is found that a 16th-order perturbative Floquet procedure provides an excellent description of the two-photon-dominant detachment processes for laser inten- sity up to 2310 11 W/cm 2 . The predicted electron angular distributions are in good agreement with recent experimental data.