Alignment-dependent ionization of molecular hydrogen in intense laser fields

The alignment dependence of the ionization behavior of ${\mathrm{H}}_2$ exposed to intense ultrashort laser pulses is investigated on the basis of solutions of the full time-dependent Schrödinger equation within the fixed-nuclei and dipole approximation. The total ionization yields as well as the energy-resolved electron spectra have been calculated for a parallel and a perpendicular orientation of the molecular axis with respect to the polarization axis of linear polarized laser pulses. For most, but not all considered laser peak intensities, the parallel aligned molecules are easier to ionize. Furthermore, it is shown that the velocity formulation of the strong-field approximation predicts a simple interference pattern for the ratio of the energy-resolved electron spectra obtained for the two orientations, but this is not confirmed by the full ab initio results.

Figure 1

Ionization yields of ${\mathrm{H}}_2$ (internuclear separation $R=1.4a_0$) for a ten-cycle $\cos {}^2$-shaped 800-nm laser pulse and a parallel (blue circles) or perpendicular (red squares) alignment with respect to the laser polarization vector.

Figure 2

Ratio of parallel to perpendicular ionization yields of ${\mathrm{H}}_2$ ($R=1.4a_0$) as a function of laser peak intensity for different wavelengths and ten-cycle $\cos {}^2$-shaped laser pulses: without (left panel) and with (right panel) focal-volume averaging. For a better comparison to the experimental data from Ref. [12], the 800-nm result is also shown scaled down by a factor 1.18. Furthermore, the (simplified) MO-SFA-LG data in Ref. [12] are depicted.

Figure 3

Energy-resolved electron spectra for parallel (blue thick line) and perpendicular (red thin line) orientations of ${\mathrm{H}}_2$ ($R=3.0a_0$) for a ten-cycle $\cos {}^2$-shaped 800-nm laser pulse with a peak intensity of $2\times {10}^{13}\hspace{0.5em}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 4

Ratio of parallel to perpendicular energy-resolved electron spectra of ${\mathrm{H}}_2$ ($R=3.0a_0$) for a ten-cycle $\cos {}^2$-shaped 800-nm laser pulse and three different peak intensities. The ratios predicted by MO-SFA-VG in Eq. (5) are also shown using circles.