Abstract
We report a theoretical investigation of the relaxation of the umbrella vibrational mode (the ν2 mode) of the CH3 molecule in its ground X̃(2)A2 ('') electronic state in collisions with helium. We have calculated a four-dimensional potential energy surface (PES) for the interaction between CH3 with different umbrella displacements and a helium atom, using a restricted open-shell coupled-cluster method with inclusion of all single, double, and (perturbatively) triple excitations [RCCSD(T)]. With this PES we carried out full close-coupling scattering calculations including all CH3 umbrella-rotational levels with v2 ≤ 3. To our knowledge, this work represents the first fully quantum calculations of ro-vibrational relaxation of a polyatomic. In more detail, we investigate propensities in the calculated ro-vibrational cross sections and the dependence on initial rotational excitation, as well as determining thermal rate constants. Overall, ro-vibrational relaxation is nearly two orders of magnitude less efficient than pure-rotational relaxation, with a noticeable dependence on the initial rotational level. We predict the room temperature v2 = 1 vibrational relaxation rate constant to be 5.4 × 10(-12) cm(3) molecule(-1) s(-1), compared to the rate constants for pure-rotational relaxation of the lower rotational levels (~2.0 × 10(-10) cm(3) molecule(-1) s(-1)).
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