Complex-scaling generalized pseudospectral method for quasienergy resonance states in two-center systems: Application to the Floquet study of charge resonance enhanced multiphoton ionization of molecular ions in intense low-frequency laser fields

Abstract

We present a complex-scaling generalized pseudospectral method for accurate and efficient treatment of resonance states in two-center molecular systems, involving optimal nonuniform grid discretization of the Hamiltonian in prolate spheroidal coordinates. The procedure is applied to the first converged non-Hermitian Floquet study of multiphoton ionization of molecular ions in intense low-frequency (1064 nm) laser fields. We explore the underlying mechanism responsible for the ionization enhancement of ${\mathrm{H}}_{2}^{+}$ at some critical internuclear distances. Several features of the complex quasienergy states are observed. A detailed analysis of the nature and dynamical behavior of these quasienergy states reveals that the ionization enhancement is mainly due to the effect of charge-resonance-enhanced multiphoton resonances of the $1{\ensuremath{\sigma}}_{g}$ and $1{\ensuremath{\sigma}}_{u}$ states with excited electronic states at some particular internuclear distances. These ``critical'' distances depend on the details of molecular electronic structure and the laser frequency and intensity used in the study.