Velocity map imaging studies of the ultraviolet photodissociation of methyl chloride

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.