Strong-field ionization of laser-irradiated light homonuclear diatomic molecules: A generalized strong-field approximation–linear combination of atomic orbitals model

Abstract

The strong-field ionization in a number of light homonuclear diatomic molecules (${\mathrm{N}}_{2}$, ${\mathrm{O}}_{2}$, and ${\mathrm{H}}_{2}$) irradiated by an intense laser field of low fundamental frequency $\ensuremath{\omega}⪡{I}_{p}$ is considered theoretically and studied numerically compared to their ``companion'' atoms, having nearly identical ionization potential ${I}_{p}$. The background applied strong-field approach is based on using the $S$-matrix formalism of conventional strong-field approximation supplemented by the standard linear combination of atomic orbitals and molecular orbitals method utilized for approximate analytical reproduction of the two-centered wave function of an initial molecular bound state. Accordingly, the ionization of a diatomic molecule is described as a quantum-mechanical superposition (intramolecular interference) of contributions from ionization amplitudes corresponding to photoelectron emission from two atomic centers separated by equilibrium internuclear distance. Besides the bonding (or antibonding) symmetry of the highest occupied molecular orbitals (HOMO) corresponding to the outermost molecular valence shell, its spatial configuration and predominant orientation with respect to the internuclear axis and polarization of incident laser field also proved to be of substantial importance and, thus, are taken into equally detailed consideration. Moreover, wherever appropriate, the comparable contributions from other (inner) molecular valence shells of a larger binding energy (closest to that of HOMO, but of different bonding symmetry and spatial configuration) are additionally taken into account. The related results for calculated differential and/or integral molecular ionization rates, molecular photoelectron spectra, and angular distributions are fairly well consistent with available experimental data and, in particular, provide one with a transparent physical interpretation of the nature and origin of high suppression in ionization of the ${\mathrm{O}}_{2}$ molecule (as compared to its companion Xe atom) as well as no suppression in ionization of ${\mathrm{N}}_{2}$ molecules (as compared to its companion Ar atom).