Anomalous laser-induced ionization rates of molecules and rare-gas atoms

Abstract

Electron tunnel ionization is considered as the mechanism for producing free electrons in gases under laser radiation. The Keldysh result and the Ammosov-Delone-Krainov (ADK) formulation are amended by considering the excess forces due to the interaction of the electric field of the laser with the electron cloud in a simple mass-on-a-spring approximation. The result of this excess force is a kinetic energy that is directed along the polarization vector of the laser field and an induced potential energy that are proposed as a determining factor in electron tunnel ionization. Relative ionization rates for various pairs of gases are calculated and compared with reported figures. Comparisons were made between several combinations of ${\text{O}}_{2}$, Xe, Ar, ${\text{N}}_{2}$, ${\text{Cl}}_{2}$, ${\text{H}}_{2}$, CO, Kr, NO, ${\text{F}}_{2}$, and ${\text{D}}_{2}$. Predicted ratios of ionization rates between pairs of gases are compared to ADK predictions. Apparently anomalous ionization rates of ${\text{O}}_{2}$, ${\text{D}}_{2}$, and ${\text{H}}_{2}$ are explained. A simple expression is developed that reveals why the ionization rate of Xe is about an order of magnitude larger than that of ${\text{O}}_{2}$ even though their ionization potentials are nearly identical; why CO is only about half that of Kr even though their ionization potentials are nearly the same; why the ratio of ${\text{O}}_{2}$ to O is about ten times larger than predicted by ADK; and why the ratio of NO to Xe is about an order of magnitude less than predicted by ADK.