Photodissociation of CH3CHO at 248 nm: identification of the channels of roaming, triple fragmentation and the transition state.

Quasi-classical trajectory (QCT) calculations are performed on the molecular products CO + CH4via the tight transition state (TS) and global minimum configurations. With the aid of this theoretical evidence, we have re-examined the experimental results published previously to clarify the controversial issue of photodissociation dynamics of CH3CHO at 248 nm. For the CO (v = 0 and 1) bimodal rotational distributions obtained previously [K.-C. Hung, P.-Y. Tsai, H.-K. Li, and K.-C. Lin, J. Chem. Phys., 2014, 140, 064313], the low-rotational (J) component is re-assigned to the contribution of triple fragmentation (H + CO + CH3), whereas the high-J component is ascribed to the CH3-roaming pathway. The H-roaming pathway is not found in the calculations. Further, the QCT results have confirmed that the CO vibrational population especially at higher states and the low-energy component of CH4 vibrational bimodality obtained experimentally are mainly produced following the TS pathway, which has never been identified before. While taking into account both the theoretical and experimental results, the ratio of the molecular products (CO(v = 1) + CH4) obtained by the triple fragmentation/roaming/TS processes is evaluated to be 0.23 : 1 : 0.29.