Comparative study of strong-field ionization in laser-irradiatedF2and other diatomic molecules: Density-functional-theory-based molecular strong-field approximation

Abstract

The puzzling phenomenon of no suppression observed in experiments on strong-field ionization of laser-irradiated diatomic ${\mathrm{F}}_{2}$ molecules (as compared to its atomic counterpart Ar of nearly equal ionization potential) is addressed within the velocity-gauge formulation of molecular strong-field approximation (SFA). The approach essentially exploits the density-functional-theory (DFT) method applied for numerical composition of initial (laser-free) molecular states using the modified van Leuwen--Baerends (LB-$\ensuremath{\alpha}$) intramolecular binding potential, which incorporates both the exchange and correlation local-spin-density approximation (LSDA) potentials and also allows for construction of initial (laser-free) wave function correctly reproducing molecular and/or atomic valence shells and respective binding energies. Unlike the respective results of earlier alternative strong-field considerations (all predicting a high suppression in ${\mathrm{F}}_{2}$ ionization), our DFT SFA based calculation results unambiguously demonstrate no suppression in strong-field ionization of ${\mathrm{F}}_{2}$ versus its atomic (Ar) and molecular $({\mathrm{N}}_{2})$ counterparts. Our presented results also suggest that the predominant contribution to ${\mathrm{F}}_{2}$ ionization will always be from the $1{\ensuremath{\pi}}_{g}$ highest occupied molecular orbital (HOMO, corresponding to the outermost valence shell) and allow for quite a transparent physical interpretation. Namely, the phenomenon of no suppression in ${\mathrm{F}}_{2}$ ionization is just explained by the closed-shell nature of its $1{\ensuremath{\pi}}_{g}$ HOMO (and thus by its substantially more enhanced and pronounced electron-correlated response to an incident laser field). Quantitatively, the latter becomes manifest through equally large contributions from the correlation and exchange parts of the intramolecular LSDA potential to ${\mathrm{F}}_{2}$ and ${\mathrm{N}}_{2}$ valence shells, in contrast to ${\mathrm{O}}_{2}$ valence shells, to which the exchange part of the LSDA potential proved to contribute well, predominantly resulting in a high suppression of ionization relative to the atomic counterpart Xe.