CO internal excitation from the reaction : H+CO2→CO+OH

Time-resolved infrared diode laser absorption spectroscopy has been used to probe CO internal excitation following 193 nm photolysis of 300 K H2S/CO2 samples. Vibrations and rotations are colder than statistical, i.e., Eint(CO) is only ∼1500 cm−1 even though ∼10 000 cm−1 is available for product excitations, assuming modest collisional deactivation of the hot H atoms that undergo reaction. A [v=1]/[v=0] ratio of ∼0.4 is obtained and there is essentially no population at v≥2. Both the v=0 and 1 rotational distributions are cold, peaking at Jmax∼11 and 13, respectively. The vibrational distribution is nascent while the rotational distributions may be partially relaxed, but not enough to alter the main conclusions. Combined with earlier results for OH internal excitations and center-of-mass (CM) kinetic energies, we conclude that at high collision energies there is a propensity toward product CM kinetic energy. In this regime, the reaction cross section rises rapidly with energy and statistical unimolecular rate theory is not applicable, even with a HOCO° intermediate.