title: The roaming atom pathway in formaldehyde decomposition.
abstract: We present a detailed experimental and theoretical investigation of formaldehyde photodissociation to H(2) and CO following excitation to the 2(1)4(1) and 2(1)4(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) approximately 45) is produced in conjunction with vibrationally "cold" H(2) fragments (v=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(2)CO-->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(1)4(3) transition allow us to determine the relative contribution to molecular products from the roaming atom channel versus the conventional molecular channel.
authors.affiliation: Department of Chemistry, Wayne State University, Detroit, Michigan 48202, USA.
authors.forename: Sridhar A
authors.initials: SA
authors.lastname: Lahankar
authors.forename: Steven D
authors.initials: SD
authors.lastname: Chambreau
authors.forename: Dave
authors.initials: D
authors.lastname: Townsend
authors.forename: Frank
authors.initials: F
authors.lastname: Suits
authors.forename: John
authors.initials: J
authors.lastname: Farnum
authors.forename: Xiubin
authors.initials: X
authors.lastname: Zhang
authors.forename: Joel M
authors.initials: JM
authors.lastname: Bowman
authors.forename: Arthur G
authors.initials: AG
authors.lastname: Suits
mesh_terms: D002248:Carbon Monoxide / Q000737:chemistry; D002627:Chemistry, Physical / Q000379:methods*; D005557:Formaldehyde / Q000737:chemistry*; D006358:Hot Temperature; D006859:Hydrogen / Q000737:chemistry; D006860:Hydrogen Bonding; D008956:Models, Chemical; D008962:Models, Theoretical; D008968:Molecular Conformation; D013696:Temperature
chemical_list: D005557:Formaldehyde; D002248:Carbon Monoxide; D006859:Hydrogen