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Rotationally inelastic scattering in CH4+He, Ne, and Ar: State-to-state cross sections via infrared laser absoption spectroscopy in crossed jets



W B. Chapman, A Schiffman


Integral and differential state-to-state cross sections are reported for rotationally inelastic scattering in crossed jets of CH4 with the rare gases He, Ne, and Ar, at center-of-mass collsion energies of 460+or-}90cm-1, 350+or-}70cm-1, and 300+or-}60cm-1, respectively. Ch4 seeded in Ar buffer gas is cooled in pulsed supersonic expansion into the three lowest rotational levels allowed by nuclear spin statistics corresponding to A (J=0), F(J=1),a nd E(J=2) symmetry. Rotational excitation occurs in single collisions with rare gas atoms form a second pulsed supersonic jet. The column-integrated densities of CH4 in both the initial and final scattering states are sunsequently probed in the jet intersection via direct absoption of light from a narrow bandwitch (0.0001cm-1), continuously turnable color center laser. Total inelactic cross sections for collisional loss out of the J=0, 1, and 2 methane states are determined in absolute units from the linear decrease of infrared absoption singals as a function of collider gas concentration, and demonstrate a clear sensitivity both to the initial quantum state and rare gas colliding species. Single-mode tuning of the IR laser source also permits probing of the collisonally excited rotational states with quantum state and velocity resolution. Column integrated scattering distributions are measured for all energetically accessible final states and used to infer absolute inelastic cross sections for state-to-state energy transfer. Though the observed trends are in good qualitative agreement with quantum state-resolved pressure-broadening studies of CH4, the dependence fo the rotationally inelastic cross sections on nuclear spin modification (i.e.,J) and rotational inelasticity (i.e.,Δ}J) is not well predicted by conventional angular momentum or energy gap models.1-5 To compare coworkers6-8 for each of the three CH4+ rare gas systems. Agreement for He+CH4 is remarkably quantiative, but significant discrepancies are noted for the heavier rare gases. Finally, the possible influence of sequential collsion channels on the measurement of the smallest observable cross section is investigated via a master equation analysis.
Journal of Chemical Physics


CH<sub>4</sub>, cross sections, direct absoption, high resolution, inelastic scattering, quantum state resolved, rotational energy transfer


Chapman, W. and Schiffman, A. (2008), Rotationally inelastic scattering in CH<sub>4</sub>+He, Ne, and Ar: State-to-state cross sections via infrared laser absoption spectroscopy in crossed jets, Journal of Chemical Physics (Accessed May 28, 2023)
Created October 16, 2008