Rotationally resolved hot atom collisional excitation of CO2 0001 and 0002 stretching vibrations by time-resolved diode laser spectroscopy

A tunable infrared diode laser has been used to monitor the time-resolved absorption of rovibrationally excited CO₂ molecules produced in collisions with fast hydrogen atoms from the 193 nm excimer laser photolysis of H₂S. Nascent relative rotational population distributions were measured for the 00⁰1 and 00⁰2 vibrational states in CO₂ following collisional excitation. The nascent distributions for both of these states were found to be significantly displaced from the 300 K Boltzmann distribution with peaks at J=31 and J=34 for 00⁰1 and 00⁰2, respectively. An approximate diffusion treatment for the mass transport of fast moving hydrogen atoms out of the region of the diode laser probe beam was developed and used to obtain approximate absolute scattering cross sections. Sufficient energy is available in the H/CO₂ collision process to excite 00⁰1 rotational states with J greater than 140. The fall-off in excitation probability for J>31 is ascribed to the small impact parameter necessary to produce vibrationally excited CO₂ (00⁰1). This in turn limits the total angular momentum available in those collisions which actually produce 00⁰1. These results are compared with previous experiments measuring the total probability for collisional excitation of all ν₃ quanta as well as with various models for inelastic collisional scattering.