Velocity Dependence of Rotational Energy Transfer Rates inNa2-Xe

Using a technique based on the Doppler shift, we have measured the dependence of rate constants for ${\mathrm{Na}}_2$*-Xe rotation-changing collisions on the relative velocity. For ${\mathrm{Na}}_2$* in the $A{{}_{}{}^1{}_{}{}^{}\Sigma _u^{}{}_{}{}^{}}^{+}$ electronic state, we made measurements for four different values of the change in ${\mathrm{Na}}_2$* angular momentum, $\Delta =j_f-j_0$, for each of three initial $j_0$'s. Cross sections for $j_0=66\mathrm{}$ increase with velocity whereas those for $j_0=38\mathrm{} \mathrm{and} 16$ stay constant or decrease. A dramatic decrease is observed for $j_0=16\mathrm{}$ and large $\left|\Delta \right|$.