CORRELATED STATE DISTRIBUTIONS FOR H$_2$ AND CO FROM FORMALDEHYDE PHOTODISSOCIATION USING DC SLICE IMAGING: THE "ROAMING H-ATOM MECHANISM"

Author Institution: Department of Chemistry, Wayne State University, Detroit, MI 48202We present a detailed experimental investigation of formaldehyde dissociation to H$_2$ and CO following excitation to a series of vibrational bands in S$_1$. The CO was detected by (2+1) REMPI 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 revealed the internal state distribution in the correlated H$_2$ cofragments. The results show that rotationally hot CO (J = 45) is produced in conjunction with vibrationally "cold" H$_2$ fragments ($\nu$ = 0-3): these products are formed through the celebrated skewed transition state. After excitation of formaldehyde above the threshold for the radical channel (H$_2$CO $\rightarrow$ H + HCO) we find formation of rotationally cold CO (J = 5-15) correlated to highly vibrationally excited H$_2$ ($\nu$ = 5-7). These products are formed through a novel roaming mechanism that involves intramolecular H-abstraction (D. Townsend et. al. Science \textbf{306}, 1158 (2004)), and avoids the region of the transition state entirely. The current measurements give us detailed insight into the energy dependence of the branching to these different reaction mechanisms