Collisional $X$- and $A$-State Kinetics of CN using Transient sub-Doppler Hole Burning

We examine the collisional kinetics of the CN radical using transient hole-burning and saturation recovery. Narrow velocity groups of individual hyperfine levels in CN are depleted ($X^{2}\Sigma ^{+})$ and excited ($A^{2}\Pi _{i})$ with a saturation laser, and probed by a counterpropagating, frequency modulated probe beam. Recovery of the unsaturated absorption is recorded following abrupt termination of an electro optically switched pulse of saturation light. Pressure-dependent recovery kinetics are measured for precursors, NCCN and CH$_{3}$COCN, and buffer gases, He, Ar and N$_{2}$. In the case of NCCN, similar recovery kinetics are observed for two-level saturation resonances, where the signal observed is a combination of $X$- and $A$-state kinetics, as well as for three-level crossover resonances, which can be chosen to probe selectively the hole-filling in the $X$ state or the decay of velocity-selected $A$ state radicals. However in the case of CH$_{3}$COCN, the $X$-state kinetics are faster than the $A$-state due to an efficient dipole-dipole rotational energy transfer mechanism. The observed recovery rates are 2-3 times faster than the estimated rotationally inelastic contribution and are a combination of inelastic and velocity-changing elastic collisions.