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

The strong-field ionization in a number of light homonuclear diatomic molecules N2 ,O 2, and H2 irradiated by an intense laser field of low fundamental frequency Ip is considered theoretically and studied numeri- cally compared to their "companion" atoms, having nearly identical ionization potential Ip. The background applied strong-field approach is based on using the S-matrix formalism of conventional strong-field approxi- mation 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 super- position intramolecular interference of contributions from ionization amplitudes corresponding to photoelec- tron 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 in- ner 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 distribu- tions are fairly well consistent with available experimental data and, in particular, provide one with a trans- parent physical interpretation of the nature and origin of high suppression in ionization of the O2 molecule as compared to its companion Xe atom as well as no suppression in ionization of N2 molecules as compared to its companion Ar atom.