Counting vibrational quanta with a diode laser probe: Bending and stretching excitation in CO2 caused by collisions with hot atoms from excimer laser photolysis

A high resolution (10−3 cm−1) cw diode laser probe technique has been developed and used to determine the number of CO2 vibrational quanta of each kind (ν3 antisymmetric stretch, ν2 bend, and ν1 symmetric stretch) produced as a result of collisions between translationally hot hydrogen atoms and CO2 molecules. The experimental method relies on probes of the CO2 vibrational transitions mnlp → mnl( p+1) all of which ‘‘ride’’ the large oscillator strength of the fundamental 0000 → 0001 antisymmetric stretching transition. Transitions with different values of m, n, l, and p are easily separated due to the narrow spectral characteristics of the diode laser and the small anharmonicities associated with different vibrational quantum numbers. The probability for excitation of a CO2 ν3 quantum by collisions with hot hydrogen atoms produced by 193 nm excimer laser photolysis of H2S is about 1% per gas kinetic collision. Bending (ν2) quanta are produced about 5–6 times more efficiently than (ν3) antisymmetric stretching quanta. A precise value for the excitation probability of symmetric stretching (ν1) quanta cannot be obtained due to rapid equilibration between ν1 and 2ν2, but the number of ν1 quanta is found to be roughly one third the number of ν3 quanta. The thermalization rate for cooling hot hydrogen atoms below threshold for excitation of a CO2 ν3 quantum corresponds to two H atom/H2S collisions or 16 H atom/CO2 collisions.