Novel molecular elimination mechanism in formaldehyde photodissociation: the roaming H atom pathway

We present a state-correlated experimental investigation of formaldehyde (H2CO) dissociation to H-2 and CO following excitation to a series of vibrational bands in the first electronically excited state, S-1. The CO was detected by resonance-enhanced multiphoton ionization at various rotational states of CO (J = 5-45) and the CO velocity distributions were measured using state-resolved DC Slice Imaging. These high-resolution measurements reveal the internal state distribution of the correlated H-2 cofragments. The results show that the rotationally hot CO (J(CO) = 40) is produced in conjunction with vibrationally cold H2 fragments (nu = 0-3), consistent with dissociation through the celebrated skewed transition state. After excitation of formaldehyde at energies near and above the threshold for dissociation to radical products (H2CO -> H + HCO), a second molecular elimination channel appears which is characterized by rotationally cold CO (J similar to 5-15) correlated with highly vibrationally excited H-2 (nu = 5-7). These products are formed through a novel roaming H-atom mechanism that involves intramolecular H abstraction and avoids the region of the transition state to molecular elimination entirely. The current measurements give insight into the energy dependence of the branching of these different reaction mechanisms.