EVIDENCE FOR A 2ND-MOLECULAR CHANNEL IN THE FRAGMENTATION OF FORMALDEHYDE

This paper reports CO(v,J) distributions from photolysis on the 2(1)4(1), 4(5), 2(1)6(1), 2(1)4(3), and 2(3)4(1) bands of H2CO and the 2(2)4(3) band of D2CO. A significant fraction of the CO(v = 0) photofragment is found in low rotational states (J(CO) < 15) for excitation above the threshold of the H+HCO dissociation channel. Photolysis on the 2(1)4(1) band, which lies below this radical threshold, shows no measurable population in low-J(CO) states. The fraction of the total population in low-J(CO) states increases with increasing photolysis energy. In contrast, the CO(v = 1, J) distributions do not broaden significantly with increasing excitation energy. Similar results are found for D2CO. Two alternative models addressing the dynamics of this dissociation are proposed. First, anharmonic motion at the transition state may lead the molecule to dissociate from configurations with smaller impact parameters and thus produce a broadened rotational distribution. More likely, a second fragmentation path, related to the exit channel of the H+HCO --> H-2+CO abstraction reaction and accessed through the radical channel, may open.