Rotationally inelastic scattering of CD3 and CH3 with He: comparison of velocity map-imaging data with quantum scattering calculations

Abstract

Rotationally inelastic scattering of methyl radicals (CD 3 and CH 3) in collisions with helium is examined by a combination of velocity map imaging experiments and quantum scattering calculations. In the experiments a beam of methyl radicals seeded in Ar intersects a beam of He atoms at 90 at a collision energy of 440 AE 35 cm À1 (CD 3 + He) or 425 AE 35 cm À1 (CH 3 + He). The methyl radicals are prepared photolytically in a gas expansion that cools them to 15 K, giving a distribution over a small number of initial (low) rotational angular momentum states. By resonance-enhanced multi-photon ionization detection, we obtain velocity map images which are specific to a single rotational angular momentum quantum number n 0 of the methyl radicals, but averaged over a small subset of the projection quantum number k 0. We extract resolved angular scattering distributions for n 0 ¼ 2-9 (for CD 3). We compare these to predictions of scattering calculations performed based on a recent potential energy surface [P. J. Dagdigian and M. H. Alexander, J. Chem. Phys. 2011, 135, 064306] in which the methyl radical was fixed at its equilibrium geometry. The fully (n, k) / (n 0 , k 0) resolved differential cross sections obtained from the calculations, when combined in weighted sums over initial (n, k) levels corresponding to the 15 K experimental radical temperature, and final k 0 levels that are not resolved in the spectroscopic detection scheme, show excellent agreement with the experimental measurements for all final states probed. This agreement gives confidence in the calculated dependence of the scattering on changes in both the n and k quantum numbers.

Rotationally inelastic scattering of methyl radicals with Ar and N2

“…This collision energy corresponds to experimental conditions for our earlier study of CD 3 -He scattering. 1 The computed CD 3 -Ar DCSs at a collision energy of 440 cm −1 are not much changed from those obtained at 330 cm −1 , which is reasonable considering the steep repulsive wall of the CD 3 -Ar PES. The spacings of the diffraction oscillations evident at small scattering angles are smaller for CD 3 -Ar than for CD 3 -He because of the larger mass of the collider Ar, 65 as well as a larger impact parameter (b).…”

“…Changes in the scattering dynamics leading to population of different k ′ sub-levels for a given n ′ level can be explored by examination of the state-to-state DCSs obtained from the QM scattering calculations. We reported this type of analysis previously for CD 3 -He scattering, 1 and Fig. 9 compares the computed DCSs for CD 3 -Ar for scattering from the initial state n k = 1 1 into the n ′ k ′ = 4 1 , 4 2 , and 4 4 final levels.…”

“…1,5 A brief description is provided here. We used the HIBRIDON suite of programs 62 to carry out time-independent quantum close-coupling spin-free calculations of state-resolved integral and differential cross sections.…”

“…47,48 Determination of rotational level populations in the incident radical beam required comparison of experimental REMPI spectra with spectra simulated using the PGOPHER program. 49 We previously showed that the distribution of methyl radicals in the beam corresponds to a rotational temperature of ∼15 K, 1 and relative populations of the rotational levels were presented in Table 1 3 . Therefore, the work presented here concentrates on the inelastic scattering of CD 3 ; measured images and corresponding DCSs for the CH 3 -Ar system can be found in the supplementary material (Figs.…”

“…1 In brief, supersonic expansion of gas mixtures through a pair of pulsed valves (General Valve Series 9) and collimation by skimmers produced the two molecular beams. Methyl radicals were generated in the primary beam by 266-nm photolysis of CH 3 I or CD 3 I entrained as a 3% mixture in Ar at a stagnation pressure of 4 bar.…”