Nascent rotational distribution for CO₂(00⁰0, J) states from collisions with excited HBr (X¹Σ⁺ v″ = 1, J″ = 12)

Rotationally state selective excitation of HBr (X¹Σ⁺ v″ =1, J″ = 12) was achieved by stimulated Raman pumping. The full state-resolved distribution of scattered CO₂ (00⁰0, J) molecules from collisions with excited HBr (E) was reported. The Coherent Anti-Stokes Raman Scattering (CARS) spectral technique was used to measure the density of HBr (1, 12), and the value is 0.54 × 10¹³ cm^-3. Under single-collision conditions, nascent rotational distribution for scattered CO₂ (00⁰0, J) was measured using high-resolution transient laser induced fluorescence (LIF) spectroscopy. The data yield the full state-resolved distribution of scattered CO₂ (00⁰0). The scattered CO₂ (00⁰0, J) molecules have a biexponential rotational distribution. Fitting the data with a two-component exponential model yields a low-energy distribution with T_a = 261 K and a high-energy distribution with T_b = 978 K. The cooler distribution accounts for 65% of the scattered population and results from elastic or weakly inelastic collisions that induce little rotational excitation in CO₂. The hotter distribution involves large changes in CO₂ rotational energy and accounts for 35% of collisions and results from strongly inelastic collisions that induce big rotational excitation in CO₂. The value of total rate constant for appearance of scattered CO₂ (00⁰0, J) is (1.3 ± 0.3) × 10^-10 cm³ molecule^-1 s^-1. We have obtained a collisional average rate of kdep = (2.9 ± 0.8) × 10^-10 cm³ molecule^-1 s^-1 by the population depletion measurements. The total appearance rate coefficient is smaller than the average depletion rate coefficient, but they remain the same in order of magnitude. For scattered CO₂ (00⁰0, J = 60-74) highly-rotational states, the average change of center of mass translation temperature and center of mass translation energy increase with the increase of J value. For CO₂ (00⁰0) lower-rotational states, the change of translation energy is difficult to determine. (authors)