Channel-specific angular distributions of HCl and CH3 products from the reaction of atomic chlorine with stretch-excited methane

Abstract

A beam containing methane and molecular chlorine is expanded into a vacuum where the methane is excited with two quanta of C–H stretching (one quantum each in two of the four C–H bonds). The reaction is initiated by fast Cl atoms generated by photolysis of Cl2 at 355 nm, and the resulting CH3 and HCl products are detected in a state-specific manner using resonance-enhanced multiphoton ionization. Speed-dependent spatial anisotropies (βprod) of HCl and CH3 products allow identification of three major product channels. They are in order of importance: (a) HCl (v=0)+CH3 [ν1 (symmetric stretch) or ν3 (asymmetric stretch)=1]; (b) HCl (v=1)+CH3 [ν2(umbrella bend)=1)]; and (c) HCl (v=1)+CH3 (ν1=1). The CH3 (v=0) product cannot be detected, and the HCl (v=2) product is minor. Channels (a) and (c) proceed in a vibrationally adiabatic manner, whereas channel (b) appears to involve the nonadiabatic interaction involving the low frequency bending mode in methane that correlates to the bending mode in the methyl radical product. The angular distributions differ markedly for the three product channels. This behavior is explained by the propensity for reactive collisions involving H-atom transfer along the line of centers and the difference in the cones of acceptance. The rotational angular momentum vector of the HCl (v=1, J=1) product is aligned perpendicular to the line of centers, which is consistent with an impulsive energy release along the line of centers. Our results clearly demonstrate that the direct and local mode picture of the chemical reaction remains largely valid, which connects vibrational excitation to the scattering dynamics.