Energy dependence of the roaming atom pathway in formaldehyde decomposition.

Recently, a new mechanism of formaldehyde decomposition leading to molecular products CO and H(2) has been discovered, termed the "roaming atom" mechanism. Formaldehyde decomposition from the ground state via the roaming atom mechanism leads to rotationally cold CO and vibrationally hot H(2), whereas formaldehyde decomposition through the conventional molecular channel leads to rotationally hot CO and vibrationally cold H(2). This discovery has shown that it is possible to have multiple pathways for a reaction leading to the same products with dramatically different product state distributions. Detailed investigations of the dynamics of these two pathways have been reported recently. This paper focuses on an investigation of the energy dependence of the roaming atom mechanism up to 1500 cm(-1) above the threshold of the radical channel, H(2)CO-->H+HCO. The influence of excitation energy on the roaming atom and molecular elimination pathways is reported, and the branching fraction between the roaming atom channel and molecular channel is obtained using high-resolution dc slice imaging and photofragment excitation spectroscopy. From the branching fractions and the reaction rates of the radical channel, the overall competition between all three dissociation channels is estimated. These results are compared with recent quasiclassical trajectory calculations on a global H(2)CO potential energy surface.