Photodissociation dynamics of formaldehyde: H2 (v,J) vector correlations

Several bipolar moments of the H2 (v,J) correlated angular momentum and velocity distribution produced from the photodissociation of formaldehyde near the threshold for dissociation have been measured by analysis of Doppler-resolved LIF line shapes. It is determined that the fragment H2 〈v⋅J〉 correlation is not at the limit of v⊥J, but is closer to the limit of v⊥J than to v∥J. The rotation of the excited H2CO during the 10−7–10−8 s before dissociation does not completely wash out the lab-frame vector correlations. Anisotropy parameters as large as 0.85 and as small as −0.41 have been measured; these are outside the limits imposed by classical models of parent rotation. A quantum mechanical model for parent rotation is introduced that accounts for the large magnitude of the measured anisotropy parameters. Photolysis on the rR0(0) line of the 43 band produces fragments with β0 while photolysis on the same rotational transition of the 2141 band produces fragments with β0. It is not known if the different anisotropies are caused by differences in the parent transition dipole moment or by differences in the dissociation dynamics. The simple impulsive model that reproduces the fragment rotational distributions and product quantum-state correlations does not adequately describe the measured H2 (v,J) vector correlations.