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

We present a state-correlated experimental investigation of formaldehyde (H₂CO) dissociation to H₂ and CO following excitation to a series of vibrational bands in the first electronically excited state, S₁. 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 H₂ cofragments. The results show that the rotationally hot CO (J_CO = 40) is produced in conjunction with vibrationally cold H₂ fragments (ν = 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 ̃ 5–15) correlated with highly vibrationally excited H₂ (ν = 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.