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

Steven D Chambreau, David Townsend, Sridhar A Lahankar, Suk Kyoung Lee, Arthur G Suits

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12 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)C89-C93
Number of pages5
JournalPhysica Scripta
Issue number1
Publication statusPublished - Jan 2006


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