Velocity map imaging studies of the ultraviolet photodissociation of methyl chloride

Abstract

We report a high resolution velocity map imaging study of the ground state and spin–orbit excited Cl atoms and of vibrationally state selected CH3(v) fragments formed in the photodissociation of jet-cooled CH3Cl molecules at three wavelengths in the range 193.3 ≤ λ ≤ 212 nm (in its A band continuum) and when exciting various vibronically resolved absorption features in the ranges 146 ≤ λ ≤ 160 and 138 ≤ λ ≤ 140.6 nm, associated with the first two predissociated Rydberg states of this molecule. Excitation in all cases results in prompt C–Cl bond fission, on timescales shorter than the parent rotational period. Most of the excess energy is partitioned into product kinetic energy, ET, but the deduced Cl/Cl* branching ratios and favored CH3 product vibrational motions are excitation wavelength/excited state dependent. So, too, are the fragment recoil anisotropies which, even within one CH3(v) + Cl/Cl* product channel, are found to be sensitive functions of ET. The trends observed at longer λ reflect the wavelength dependent partial cross sections for excitation to the 3Q1, 3Q0, and 1Q1 components of the A band continuum, but full interpretation of the present data demands a much more detailed, quantum state resolved picture of the non-adiabatic population transfer probabilities between these dissociative parent states and, at higher excitation energies, the Jahn–Teller induced distortions within the photoexcited Rydberg states and their non-adiabatic couplings with the continuum states. Additional CH3+ and Cl+ signals evident in the images at lower ET are attributed to, respectively, two pump photon induced dissociative ionizations and one probe photon induced photodissociation of CH2Cl products formed via a competing primary C–H bond fission process.