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

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-Schrödinger 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 diagonalizing the full Floquet Hamiltonian. Explicit expressions are presented for the determination of intensitydependent total and partial rates and electron angular distributions. The theory is applied to a case study of multiphoton detachment of H for a range of laser frequencies ~corresponding to the absorption of a minimum of two photons! and laser intensities from 10 to 10 W/cm. It is found that a 16th-order perturbative Floquet procedure provides an excellent description of the two-photon-dominant detachment processes for laser intensity up to 2310 W/cm. The predicted electron angular distributions are in good agreement with recent experimental data.