State-to-state rotational energy transfer in methane (13CD4) from infrared double-resonance experiments with a tunable diode laser

Abstract

An infrared double resonance laser spectroscopic technique is used to study state-resolved rotational and vibrational energy transfer in the isotopically substituted methane molecule,13CD4 . Molecules are prepared in a selected rovibrational state by CO2 laser pumping, with the quantum numbers v, J, and Cn completely specified. Measurements of both the total rate of depopulation by collisions, and the rates of transfer into specific final rovibrational states (v′, J′, Cn′ ) are carried out using time-resolved tunable diode laser absorption spectroscopy. The depopulation rates due to collisions between methane and the rare gases are on the order of the Lennard-Jones collision frequencies. Self-relaxation is slightly more efficient than the Lennard-Jones estimate. The rather small relaxation rates are characteristic of a short-range potential, or ‘‘strong-collision’’ regime, expected for a molecule without a dipole moment. The state-to-state energy transfer measurements reveal a dramatic selectivity of rotational energy transfer pathways with respect to the fine-structure rotational states Cn . Relaxation occurs through a surprisingly small subset of the energetically accessible pathways. Also suggested is a preference for population transfer to occur within the initial vibrational angular momentum sublevel of the ν4 (F2 ) vibrational state, which has three sublevels in consequence of Coriolis interaction. This preference can be formulated as a propensity for Δ(R−J)=0 transitions. We deduce that large changes of J(ΔJ∼5) can occur in single collisions between methane molecules, based on a simple kinetic model of the data. This is also characteristic of short-range collisions in which it is likely that no single multipolar interaction dominates. Collisional relaxation between the ν4 and ν2 bending vibrations proceeds more slowly than rotational relaxation, but as fast as transfer among the closely grouped stretching and bend-overtone levels, measured previously in CH4 . No evidence for rotationally specific V–V transfer is found. We discuss an exhaustive spectroscopic analysis of 13CD4 that provides unambiguous spectral assignments for use in detecting vibrationally excited molecules (v4 =1) in specific rotational states.