Laser-induced deformation of triatomic molecules: Influence on tunnel ionization

Abstract

We consider a type of light-dressing effect in molecules which is due to laser-induced deformation of molecules. We derive general formulas which describe the change of the equilibrium bond lengths, bending angles, and vibration frequencies for a polyatomic molecule. Up to the second-order terms in the field strength, the normal coordinates of the molecule are not changed. Explicit expressions for the change of the geometric parameters and vibration frequencies are found for a nonlinear triatomic molecule of the ${A}_{2}B$ type. These results are applied to calculation of the tunnel-ionization rates for ${\mathrm{H}}_{2}\mathrm{O}$, ${\mathrm{H}}_{2}{\mathrm{O}}^{+}$, and ${\mathrm{SO}}_{2}$ molecules, both in ac and dc fields. The main influence of the laser-induced deformation on the tunnel ionization is due to the changes in Franck-Condon factors, which are determined by overlap of the nuclei wave function of the molecule and its residual ion. In a laser field with an intensity of $\ensuremath{\sim}{10}^{14}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$, the contribution of the laser-dressing effect to the ionization rate is within 5% for ${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{H}}_{2}{\mathrm{O}}^{+}$, while accounting for this effect changes the ionization rate of the ${\mathrm{SO}}_{2}$ molecule by up to 20 times depending on the molecule's orientation with respect to the electric-field vector. Such a large difference is due to the electronic structures of these molecules.