Photodissociation dynamics of the CH2Cl radical: Ion imaging studies of the Cl+CH2 channel

The photodissociation of the chloromethyl radical, CH2Cl, to chlorine atom and methylene is examined following excitation at selected wavelengths in t he region 312-214 nm. CH2Cl is produced in a molecular beam by using pulsed pyrolysis. Cl and CH2 products are detected by laser ionization and their velocity and angular distributions are determined by using the ion imaging technique. The spectrum obtained by monitoring the Cl fragment yield as fun ction of photolysis wavelength shows that throughout this wavelength region Cl atoms are major products. With 312-247 nm photolysis, the angular distr ibutions are typical of a perpendicular transition (beta = -0.7) and the ma in products are CH2((X) over tilde B-3(1))+Cl(P-2(3/2)). The available ener gy is partitioned preferentially into the translational degrees of freedom. "Hot band" transitions are prominent in this region even in the molecular beam indicating that the geometries of the ground and excited states of CH2 Cl must be very different. With 240-214 nm photolysis, the angular distribu tions are typical of a parallel transition (beta similar to1.2), and the pr edominant products are Cl(P-2(3/2)) and Cl(P-2(1/2)), with CH2((a) over til de (1)A(1)) as the main cofragment. A large fraction of the available energ y is partitioned into internal energy of CH2((a) over tilde (1)A(1)). Compa rison with the ab initio calculations of Levchenko and Krylov presented in the accompanying paper enables the assignment of the perpendicular and para llel transitions predominantly to 1 (2)A(1)<--1 B-2(1) and 2 B-2(1)<--1 B-2 (1) excitations, respectively, and both upper states are probably repulsive in the C-Cl coordinate. The electronic states of the products obtained via these two transitions are in agreement with the predictions of a simple di abatic state correlation diagram based on the calculated vertical energies of the upper states. (C) 2001 American Institute of Physics.