Differential and integral cross sections for the rotationally inelastic scattering of methyl radicals with H₂ and D₂

Comparisons are presented of experimental and theoretical studies of the rotationally inelastic scattering of CD₃ radicals with H₂ and D₂ collision partners at respective collision energies of 680 ± 75 and 640 ± 60 cm^-1. Close-coupling quantum-mechanical calculations performed using a newly constructed ab initio potential energy surface (PES) provide initial-to-final CD₃ rotational level (n, k → n′, k′) integral and differential cross sections (ICSs and DCSs). The DCSs are compared with crossed molecular beam and velocity map imaging measurements of angular scattering distributions, which serve as a critical test of the accuracy of the new PES. In general, there is very good agreement between the experimental measurements and the calculations. The DCSs for CD₃ scattering from both H₂ and D₂ peak in the forward hemisphere for n′ = 2-4 and shift more to sideways and backward scattering for n′ = 5. For n′ = 6-8, the DCSs are dominated by backward scattering. DCSs for a particular CD₃ n → n′ transition have a similar angular dependence with either D₂ or H₂ as collision partner. Any differences between DCSs or ICSs can be attributed to mass effects because the PES is unchanged for CD₃-H₂ and CD₃-D₂ collisions. Further comparisons are drawn between the CD₃-D₂ scattering and results for CD₃-He presented in our recent paper [O. Tkáč, A. G. Sage, S. J. Greaves, A. J. Orr-Ewing, P. J. Dagdigian, Q. Ma, and M. H. Alexander, Chem. Sci. 4, 4199 (2013)]. These systems have the same reduced mass, but are governed by different PESs.