Predissociation of the hydroxymethyl radical in the 3p(z) Rydberg state: Formaldehyde plus hydrogen atom channel

The photodissociation of the hydroxymethyl radical to hydrogen atom and for maldehyde was investigated following excitation to the origin band of the R ydberg (2)A " (3pz) state of jet cooled CH2OD and CD2OH. D and H products w ere detected by 2-color laser ionization; The peaks in the photofragment yi eld spectrum of the D product from CH2OD correspond to peaks in the absorpt ion spectrum, and this establishes hydroxymethyl as the source of the obser ved D atoms. D atoms appear as major products from CH2OD, but the H photofr agment 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 dist ributions of the D atom from CH2OD were obtained under core sampling condit ions. The product translational energy distribution derived from the data i s broad, indicating that the formaldehyde cofragment is produced in the gro und 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 perp endicular polarization of the photolysis laser. The observed anisotropic an gular distribution suggests that the dissociation is fast, as also indicate d by the line width. The results can be rationalized by a mechanism that in volves nonadiabatic transitions from the initially excited Rydberg state to the ground state, with the final crossing occurring in a region of the pot ential energy surface that leads to direct O-D(H) fission without isomeriza tion. Exit channel dynamics is probably responsible for the high vibrationa l excitation of the formaldehyde fragment.