Influence of the initial angular distribution on strong-field molecular dissociation

We study few-cycle, strong-field dissociation of aligned ${\mathrm{H}}_2^{+}$ by solving the time-dependent Schrödinger equation including rotation. We examine the dependence of the final angular distribution, the kinetic energy release spectrum, and the total dissociation yield on the initial nuclear angular distribution. In particular, we look at the dependence on the relative angle ${\theta }_0$ between the laser polarization and the symmetry axis of a well-aligned initial distribution, as well as the dependence on the delay between the “pump” pulse that prepares the alignment and the few-cycle probe pulse. Surprisingly, we find the dissociation probability for ${\theta }_0={90}^{\circ }$ can be appreciable even though the transitions involved are purely parallel. We therefore address the limits of the commonly held “ball-and-stick” picture for molecules in intense fields as well as the validity of the axial recoil approximation.

Figure 1

Schematic representation of the pump-probe scheme considered. The pump pulse creates a distribution at $t_i$ aligned along ${\widehat{\mathcal{E}}}_{\mathrm{pump}}$. After a delay $\mathrm{\Delta }t$, a probe pulse polarized at the angle ${\theta }_0$ to the pump pulse dissociates the molecule.

Figure 2

The pump-probe delay dependence of: (a) $\langle {cos}^2{\theta }_K\rangle$ (solid red line), the dissociation yield (dashed green line), and the field-free $\langle {cos}^2\theta \rangle$ (dot-dashed blue line); (b) the angular distribution $dP/d{\theta }_{K}sin{\theta }_K$; and (c) the kinetic energy release spectrum.

Figure 3

The alignment angle $\left({\theta }_0\right)$ dependence of: (a) the total dissociation probability, and (b) the angular distribution of the dissociation fragments $dP/d{\theta }_{K}sin{\theta }_K$ for $1.5\times {10}^{13}$ W/${\mathrm{cm}}^2$.

Figure 4

Kinetic energy release spectrum of the dissociation fragments in terms of the alignment angle ${\theta }_0$ for (a) $1.5\times {10}^{13}$ W/${\mathrm{cm}}^2$ and (b) $2\times {10}^{13}$ W/${\mathrm{cm}}^2$.

Figure 5

Dissociation probability as a function of the alignment angle ${\theta }_0$ for initial alignment widths 0.89 (dashed green line) and 0.81 (solid red line).

Figure 6

Dissociation probability (dashed blue and solid green lines) and field-free $\langle {cos}^2\theta \rangle$ (dashed red and dot-dashed black lines) in terms of the time delay $\mathrm{\Delta }t$ for $\langle {cos}^2\theta \rangle =0.89$ and $\langle {cos}^2\theta \rangle =0.81$, both with ${\theta }_0=0$.