Ionization and excitation of the excited hydrogen atom in strong circularly polarized laser fields

Abstract

In the recent work of Herath et al. [T. Herath, L. Yan, S. K. Lee, and W. Li, Phys. Rev. Lett. 109, 043004 (2012)] the first experimental observation of a dependence of strong-field ionization rate on the sign of the magnetic quantum number $m$ [of the initial bound state $(n,l,m)]$ was reported. The experiment with nearly circularly polarized light could not distinguish which sign of $m$ favors faster ionization. We perform ab initio calculations for the hydrogen atom initially in one of the four bound substates with the principal quantum number $n=2$, and irradiated by a short circularly polarized laser pulse of $800\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$. In the intensity range of ${10}^{12}\ensuremath{-}{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$ excited bound states play a very important role, but also up to some ${10}^{15}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$ they cannot be neglected in a full description of the laser-atom interaction. We explore the region that with increasing intensity switches from multiphoton to over-the-barrier ionization and we find, unlike in tunneling-type theories, that the ratio of ionization rates for electrons initially counter-rotating and corotating (with respect to the laser field) may be higher or lower than 1.