Rotationally and translationally resolved hot atom collisional excitation of the CO2 Fermi mixed bend/stretch vibrational levels by time-dependent diode laser spectroscopy

High resolution IR diode laser absorption spectroscopy is employed to monitor the nascent rotational population distributions and transient linewidths in the Fermi mixed symmetric stretch/overtone bend vibrational mode of CO2 (1000/0200) following collisions with translationally hot hydrogen and deuterium atoms, produced from the 193 nm excimer laser photolysis of H2S or D2S. The nascent 1000/0200 rotational distribution produced by H* atom collisional excitation peaks at J∼26 and is well fit by a 747 K Boltzmann distribution. The transient linewidths are 1.5--3 times the ambient, room temperature CO2 Doppler linewidths, are∼0.002 cm-1 larger for D* atom collisions than H* atom collisions, and increase with increasing rotational quantum number. The experimentally determined relative cross sections for H* atom collisional excitation of CO2 vibrational states are as follows: 0001 antisymmetric stretch: 1000 Fermi mixed upper level: 0200 Fermi mixed lower level: 0220 bend∼1.0 : 0.6 : 0.6 : 0.4. The absolute cross section for inelastic collisional scattering of CO2 by H* atoms into 1000 J=38 is (1.4±0.8)x10-2 A2, and the total excitation cross section for the 1000 vibrational state is 0.37±0.21 A2. A statistical model and a simple quantized Landau--Teller model are unable to explain qualitatively the observed data; however, a breathing ellipsoid model, coupled with an IOS quantum scaling relation, reproduces the major features in the experimental data for both the 1000/0200 and 0001 states. The differences in the experimental data for distinct vibrational motions can be attributed to hot atoms sampling different regions of the potential surface