Rotationally inelastic collisions of He and Ar with NaK: Theory and Experiment

Rotationally inelastic thermal collisions of NaK $A\,^1\Sigma^+$ molecules with He and Ar have been studied at Lehigh and Lyon. In both laboratories, a pump laser excites a particular ro-vibrational level $A\,^1\Sigma^+$($v, J$). Strong transitions from the pumped ($v, J$) level and weaker transitions from collisionally-populated levels ($v,J^{\prime} = J + \Delta J$) occur. Ratios of line intensities yield information about population and orientation transfer. At Lyon, we also identify $v$ changing collisions. A strong propensity for $\Delta J =$ even transitions is observed for He and Ar. Theoretical calculations are underway; we've calculated He-NaK and Ar-NaK potential surfaces using GAMESS and performed coupled channel scattering calculations for $JM\rightarrow J^{\prime}M^{\prime}$ transitions. Semiclassical formulas for the cross sections have been obtained and agree well with our quantum mechanical calculations. Using the vector model, where $J$ precesses with polar angle $\theta$ about the $z$-axis, we derived the distribution of final polar angles $\theta^{\prime}$ and final $M^{\prime}$ states. We identify a special case where the $\theta^{\prime}$ distribution is a Lorentzian centered at $\theta$.