HCO (N,Ka,Kc,J) distributions from near-threshold photolysis of H2CO (J, Ka, Kc)

The dynamics of the reaction H2CO+hv(lambda approximate to 330 nm)-->H+HCO have been studied following excitation of formaldehyde into the (A) over tilde((1)A(2)) state, just above the dissociation threshold of the (X) over tilde((1)A(1)) state. Formaldehyde was excited via specific J, K-a, K-c rotational states and the ensuing rotational distribution of HCO measured by fully resolving N, K-a, K-c, and J = N+/-S of the fragment. When only the N and K-a quantum numbers of both formaldehyde and the formyl radical are considered, the distributions are generally modeled well by phase space theory (PST). Within approximate to 10 cm(-1) of the threshold, however, the PST predictions consistently exceed the experimental populations. This was accounted for by the inclusion of a centrifugal barrier in the PST model. The attractive part of the effective centrifugal potential was modeled by a dipole-induced dipole plus dispersion interaction. The barrier is weak and long range (>5 Angstrom). Resolution of K-c in the reaction, in both parent and product, gave large deviations from the PST model. The HCO population distributions separate according to whether K-c was the upper- or lower-energy state. Additionally, the upper/lower preference was sensitive to the choice of K, in the parent. Insufficient data are currently available to quantify this observation. The product state distribution was also found to be independent of the spin-rotation state of HCO. (C) 1997 American institute of Physics.