Strong Orientation Effects in Ionization of ${\mathrm{H}}_2^{+}$ by Short, Intense, High-Frequency Light Pulses

We present three-dimensional time-dependent calculations of ionization of arbitrarily spatially oriented ${\mathrm{H}}_2^{+}$ by attosecond, intense, high-frequency laser fields. The ionization probability shows a strong dependence on both the internuclear distance and the relative orientation between the laser field and the internuclear axis. The physical features are explained in terms of two-center interference effects.

Figure 1

Lower panel: Ionization probability in the parallel geometry ($\theta =0°$) as a function of the internuclear separation $R$ and the electric field strength $E_0$ with $\omega =2\mathrm{a}.\mathrm{u}.$ and $T=6\pi \mathrm{a}.\mathrm{u}.$. Upper panel: Convergence of the ionization probability versus $l_{\max }$ for $E_0=4\mathrm{a}.\mathrm{u}.$ and $R=3\mathrm{a}.\mathrm{u}.$ (solid curve) and for $E_0=8\mathrm{a}.\mathrm{u}.$ and $R=4\mathrm{a}.\mathrm{u}.$ (dashed curve).

Figure 2

Ionization probability as a function of the internuclear separation $R$ and of the angle $\theta$ between the polarization direction and the internuclear axis with $\omega =2\mathrm{a}.\mathrm{u}.$, $E_0=3\mathrm{a}.\mathrm{u}.$, and $T=6\pi \mathrm{a}.\mathrm{u}.$.

Figure 3

Upper panel: The electric field $E(t)$ of duration $T=6\pi \mathrm{a}.\mathrm{u}.$ (450 asec) and frequency $\omega =2\mathrm{a}.\mathrm{u}.$. The arrows indicate the instants of time at which the snapshots of the lower part of the figure are made. Snapshots of the wave function in the $xz$ plane at times corresponding to the beginning (top row), the middle (middle row), and the end (bottom row) of the pulse for parallel (left column) and perpendicular (right column) orientation. In all cases the internuclear separation is $R=3\mathrm{a}.\mathrm{u}.$. Both the polarization direction and the internuclear axis lie in the $xz$ plane.

Figure 4

Angular photoelectron spectra in the scattering plane for parallel (red lines) and perpendicular (blue lines) geometry as a function of the polar angle for a 6 cycle field with $E_0=3\mathrm{a}.\mathrm{u}.$ and $\omega =2\mathrm{a}.\mathrm{u}.$. The solid curve is obtained using Eq. (6) and the dashed one by using Eq. (5) with a “best fit” obtained for a wave number of $k=1.77\mathrm{a}.\mathrm{u}.$. The carrier envelope phase $\varphi$ is here zero. In all cases, the angle denotes the direction of the outgoing electron with respect to the internuclear axis.