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 mn^lp → mn^l( 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.