Predissociation of the Hydroxymethyl Radical in the 3pz Rydberg State: Formaldehyde + Hydrogen Atom Channel

Abstract

The photodissociation of the hydroxymethyl radical to hydrogen atom and formaldehyde was investigated following excitation to the origin band of the Rydberg 2A"(3pz) state of jet cooled CH2OD and CD2OH. D and H products were detected by 2-color laser ionization. The peaks in the photofragment yield spectrum of the D product from CH2OD correspond to peaks in the absorption spectrum, and this establishes hydroxymethyl as the source of the observed D atoms. D atoms appear as major products from CH2OD, but the H photofragment yield is not discernible above the background. On the other hand, in the dissociation of CD2OH, only H atoms are produced. It is concluded that isomerization to the methoxy radical is not important. Time-of-flight distributions of the D atom from CH2OD were obtained under core sampling conditions. The product translational energy distribution derived from the data is broad, indicating that the formaldehyde cofragment is produced in the ground electronic state but with an internal energy distribution that extends to the thermochemical limit. The recoil anisotropy parameter was estimated by comparing the intensity of the D signals obtained with parallel and perpendicular polarization of the photolysis laser. The observed anisotropic angular distribution suggests that the dissociation is fast, as also indicated by the line width. The results can be rationalized by a mechanism that involves nonadiabatic transitions from the initially excited Rydberg state to the ground state, with the final crossing occurring in a region of the potential energy surface that leads to direct O−D(H) fission without isomerization. Exit channel dynamics is probably responsible for the high vibrational excitation of the formaldehyde fragment.