Internuclear-distance dependence of ionization of H2 in strong laser fields

The ionization yield of molecular hydrogen exposed to intense laser pulses is studied as a function of the internuclear distance R. The results obtained by means of the full solution of the three-dimensional time-dependent Schrodinger equation (TDSE) describing electronic motion are compared with those predicted by the quasi-static approximation (QSA). It is found that for laser pulses with a wavelength of 800 nm and peak intensities between 3.5 x 10(13) W cm(-2) and 1 x 10(14) W cm(-2) the QSA predicts an R dependence that is in qualitative agreement with the TDSE results, provided a vertical ionization potential is used in the QSA model. The quantitative agreement depends, however, strongly on the laser intensity. While the yields for 800 nm vary smoothly with R, this is not the case for 266 nm (and 3.5 x 10(13) W cm(-2)). As expected, for these parameters the ionization dynamics is better described as a multi-photon process; it is strongly influenced by channel closing and the appearance of resonantly enhanced multi-photon ionization.