Stereodynamical Control of a Quantum Scattering Resonance in Cold Molecular Collisions

Cold collisions of light molecules are often dominated by a single partial wave resonance. For the rotational quenching of HD ($v=1$, $j=2$) by collisions with ground state para-${\mathrm{H}}_2$, the process is dominated by a single $L=2$ partial wave resonance centered around 0.1 K. Here, we show that this resonance can be switched on or off simply by appropriate alignment of the HD rotational angular momentum relative to the initial velocity vector, thereby enabling complete control of the collision outcome.

Figure 1

Integral cross section for the HD $(v=1,j=2)+{\mathrm{H}}_2$ ($v=0,j=0$) inelastic collisions as a function of the collision energy. Top panel: ICS for $\mathrm{\Delta }j=-1$ (solid red line) and $\mathrm{\Delta }j=-2$ (solid blue line). The contributions of the $L=2$ partial wave to the ICS are shown as dashed lines. Middle and bottom panels: Energy dependence of the $s_0^{\{2\}}$ integral alignment moment for $\mathrm{\Delta }j=-1$ (red) and $-2$ (blue). Middle panel shows the overall results and the $L=2$ contribution is shown in the bottom panel.

Figure 2

Integral cross section as a function of the collision energy for $\mathrm{\Delta }j=-1,-2$ for different preparations of the HD internuclear axis, $\beta =0°$ (red line), $\beta =90°$ (blue line), and the magic angle (olive line). The isotropic preparation (in the absence of external alignment) is shown in black. The inset shows the resonance region in a linear ordinate-axis scale.

Figure 3

Contour plots showing the collision energy dependence of the DCS for the $\mathrm{\Delta }j=-1$ transition with different preparation of the HD rotational angular momentum. The effect of the resonance is prominent for all preparations except for $\beta =0$, for which the resonance disappears. $\mathrm{Log}({\mathrm{E}}_{\mathrm{col}};\mathrm{K})$ refers to $\mathrm{Log}(E_{\mathrm{coll}}/E_{\mathrm{ref}})$ where $E_{\mathrm{ref}}=k_B$ 1 K. $S_0^{\{2\}}(\theta )$ for four representative energies are shown in the Supplemental Material [62].

Figure 4

Integral cross section as a function of the collision energy for $\mathrm{\Delta }j=-1$ along with the internuclear axis stereodynamical portraits. The reference frame is defined by the reactants approach ($\mathit{k}$) and the products recoil (${\mathit{k}}^{\prime }$) directions. The $z$ axis is parallel to $\mathit{k}$, the $x\text{−}z$ plane is the scattering plane, and the $y$ axis is parallel to $\mathit{k}\times {\mathit{k}}^{\prime }$.