Journal of Chemical Physics

The roaming atom pathway in formaldehyde decomposition

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We present a detailed experimental and theoretical investigation of formaldehyde photodissociation to H 2 and CO following excitation to the 2 14 1 and 2 14 3 transitions in S 1. The CO velocity distributions were obtained using dc slice imaging of single CO rotational states (v=0, j co =5-45). These high-resolution measurements reveal the correlated internal state distribution in the H 2 cofragments. The results show that rotationally hot CO (j co - 45) is produced in conjunction with vibrationally "cold" H 2 fragments (u=0-5): these products are formed through the well-known skewed transition state and described in detail in the accompanying paper. After excitation of formaldehyde above the threshold for the radical channel (H 2CO→H+HCO) we also find formation of rotationally cold CO (j co=5-28) correlated to highly vibrationally excited H 2 (v = 6-8). These products are formed through a novel mechanism that involves near dissociation followed by intramolecular H abstraction [D. Townsend et al., Science 306, 1158 (2004)], and that avoids the region of the transition state entirely. The dynamics of this "roaming" mechanism are the focus of this paper. The correlations between the vibrational states of H 2 and rotational states of CO formed following excitation on the 2 14 3 transition allow us to determine the relative contribution to molecular products from the roaming atom channel versus the conventional molecular channel. © 2006 American Institute of Physics.