Alignment-Dependent Strong Field Ionization of Molecules

We demonstrate a method to measure strong field laser ionization of aligned molecules. The method employs a macroscopic field-free dynamic alignment, which occurs during revivals of rotational wave packets produced by a femtosecond laser pulse. We investigate the dependence of strong field ionization of ${\mathrm{N}}_{\mathrm{2}}$ on molecular orientation. We determine that ${\mathrm{N}}_{\mathrm{2}}$ molecules are four times more likely to ionize when aligned parallel to the field than when aligned perpendicular to it.

Figure 1

Time dependence of the molecular alignment parameter $⟨{cos}^2\vartheta ⟩$ for ${\mathrm{N}}_{\mathrm{2}}$ following a 40 fs laser pulse with peak intensity $5\times {10}^{13}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. Fractional and full revivals are indicated on the figure. Within the first half revival, time delays corresponding to maximum alignment of molecules parallel and perpendicular to polarization of the aligning pulse are shown. $\vartheta$ is the angle between the projection of the molecular axis onto the circular polarization plane of the probe pulse and the polarization vector of the aligning pulse.

Figure 2

Experimental angular distributions $dN(\vartheta )/d\vartheta$ for maximum alignment along the field ($\tau =4.15\mathrm{p}\mathrm{s}$) (a) and perpendicular to the field ($\tau =4.33\mathrm{p}\mathrm{s}$) (b). The polarization direction is horizontal. Solid lines represent an analytical fit to the experimental distributions by the function $F(\theta )=({cos}^2\theta +{\epsilon }^2{sin}^2\theta {)}^{-1/2}+b$ with parameters $\epsilon =1.692$, $b=-0.415$ for (a) and $\epsilon =0.929$, $b=-0.916$ for (b).

Figure 3

Angular dependence of ionizaton probability. ${\Pi }_2(\theta )=(\epsilon {cos}^2\theta +1)/(\epsilon +1)$ with $\epsilon =3.5$ (solid line) and ${\Pi }_1(\theta )=({cos}^2\theta +{\epsilon }^2{sin}^2\theta {)}^{-1/2}$ with $\epsilon =4.5$ (long-dashed line) best reproduce the experimental ionization data. The ionizing pulse (40 fs, $2\times {10}^{14}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$) had the same polarization as the aligning pulse. Molecular ADK calculation for a 20 fs pulse with peak intensity of $1.6\times {10}^{14}$ (short-dashed line) and $3.2\times {10}^{14}$ (dash-dotted line) $\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ (from Tong and Lin [22]). All curves are normalized to unity at $\theta =0°$.