Knowledge Bank

The technique of sub-Doppler optical-optical double resonance spectroscopy is used to probe one velocity component of rotationally relaxed . The process is $4\leftarrow 3\leftarrow 2\leftarrow 1:$ co-propagating 1 MHz bandwidth cw dye lasers excite the $4\leftarrow 3$ and $2\leftarrow 1$ transitions and $3\leftarrow 2$ rotational relaxation in the intermediate vibronic level is sampled. For out experiments on BaO (in a 1 torr $Ba + CO_{2} + $Ar flame) the dye lasers excite C-A (3,1) and A-X (1,0) bands. Velocity distributions along the propagation direction (z) of the $2\leftarrow 1$ laser are sampled in the optically pumped and collisionally relaxed levels as function of $J,\Delta J,vz$, and Ar pressure. No $\Delta J$ propensity rule is observed. The single collision rotation-ally inelastic cross-section is approximately 10{\AA}$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ and no $vz$ dependence has been, detected. When molecules with $vz=0$ are rotationally transferred the line width increases (but remains sub-Doppler), corresponding to either an increase in total velocity or a reorientation of transverse velocity. Linewidths increase monotonically with $\Delta $J. When $vz$ is selected such that $vz>>(vX,vy)$, rotationally relaxed levels appear with asymmetric lines skewed toward $vz=0$ in the $C\leftarrow A$ excitation spectrum.