Photodissociation dynamics of methyl vinyl ketone oxide: A four-carbon unsaturated Criegee intermediate from isoprene ozonolysis

The electronic spectrum of methyl vinyl ketone oxide (MVK-oxide), a four-carbon Criegee intermediate derived from isoprene ozonolysis, is examined on its second pi* & LARR; pi transition, involving primarily the vinyl group, at UV wavelengths (lambda) below 300 nm. A broad and unstructured spectrum is obtained by a UV-induced ground state depletion method with photoionization detection on the parent mass (m/z 86). Electronic excitation of MVK-oxide results in dissociation to O (D-1) products that are characterized using velocity map imaging. Electronic excitation of MVK-oxide on the first pi* & LARR; pi transition associated primarily with the carbonyl oxide group at lambda > 300 nm results in a prompt dissociation and yields broad total kinetic energy release (TKER) and anisotropic angular distributions for the O (D-1) + methyl vinyl ketone products. By contrast, electronic excitation at lambda & LE; 300 nm results in bimodal TKER and angular distributions, indicating two distinct dissociation pathways to O (D-1) products. One pathway is analogous to that at lambda > 300 nm, while the second pathway results in very low TKER and isotropic angular distributions indicative of internal conversion to the ground electronic state and statistical unimolecular dissociation.