Photodissociation dynamics of the reaction H2CO→H+HCO via the singlet (S0) and triplet (T1) surfaces

We have explored the photodissociation dynamics of the reaction H2CO+hν→H+HCO in the range of 810-2600 cm-1 above the reaction threshold. Supersonically cooled formaldehyde was excited into selected JKa,Kc rotational states of six vibrational levels (1141, 51, 2261, 2243, 2341, and 2441) in the A(tilde sign)(1A2) state. The laser induced fluorescence spectra of the nascent HCO fragment provided detailed product state distributions. When formaldehyde was excited into the low-lying levels 1141, 51, and 2261, at Eavail<1120 cm-1, the product state distribution can be modeled qualitatively by phase space theory. These dynamics are interpreted as arising from a reaction path on the barrierless S0 surface. When the initial states 2243 and 2341 were excited (Eavail=1120-1500 cm-1), a second type of product state distribution appeared. This second distribution peaked sharply at low N, Ka and was severely truncated in comparison with those obtained from the lower lying states. At the even higher energy of 2441 (Eavail≅2600 cm-1) the sharply peaked distribution appears to be dominant. We attribute this change in dynamics to the opening up of the triplet channel to produce HCO. The theoretical height of the barrier on the T1 surface lies between 1700 and 2100 cm-1 and so we consider the triplet reaction to proceed via tunneling at the intermediate energies and proceed over the barrier at the higher energies. Considerable population was observed in the excited (0,0,1) state for all initial H2CO states that lie above the appearance energy. Rotational populations in the (0,0,1) state dropped more rapidly with (N,Ka) than did the equivalent populations in (0,0,0). This indicates that, although individual rotational states are highly populated in (0,0,1), the total v3=1 population might not be so large. Specific population was also measured in the almost isoenergetic Kc and J states. No consistent population preference was found for either asymmetry or spin-rotation component