| The results of an infrared-infrared pump-probe experiment investigating the rate of rotational self relaxation in carbon monoxide are presented. Room-temperature CO molecules in the ground electronic and vibrational state were excited into the J = 0, 7, and 14 rotational levels of v = 2 using a pulsed, narrow-band optical parametric oscillator. A cw lead-salt diode laser, tuned to resonant transitions between v = 2 and 3, probed subsequent population changes in the J = 0–18 rotational levels. The time dependent absorption linewidths of the probe transitions caused by the sub-Doppler OPO pump were also measured. The rotational populations were compared with theoretical predictions based on the statistical power gap, statistical power exponential gap, modified exponential gap, and energy corrected sudden rate constant models. The pump-probe data show that rotational energy transfer occurs faster and multi-quantum transitions occur more often than one would predict based on previously published CO-CO collision data. Homonuclear propensity is clearly evident in the pump-probe data but only for transitions with small energy gaps. The linewidth measurements indicate the energy gap between rotational levels rather than the difference in angular momentum controls rotational energy transfer in carbon monoxide. |
