dc.creator Maccarone, Alan T. en_US dc.creator Heazlewood, B. R. en_US dc.creator Rowling, S. J. en_US dc.creator Kable, S. H. en_US dc.date.accessioned 2008-07-15T13:38:38Z dc.date.available 2008-07-15T13:38:38Z dc.date.issued 2008 en_US dc.identifier 2008-FB-12 en_US dc.identifier.uri http://hdl.handle.net/1811/33363 dc.description P. L. Houston and S. H. Kable, PNASB. R. Heazlewood et al, submittedS.-H. Lee and I.-C. Chen, Chem. Phys. en_US dc.description Author Institution: School of Chemistry, University of Sydney, Sydney NSW 2006, Australia en_US dc.description.abstract Recent experiments and theory have implicated a "roaming" mechanism as being important in the photodissociation of $\chem{CH_3CHO}$ into the molecular products $\chem{CH_4}$ + $\chem{CO}$.}, \textbf{103}, 16079 (2006).}$^{,}$} (2006).} As much as 80\% of the flux for this chemical channel was attributed to roaming; the conventional transition state mechanism is a minor contribution. Quasi-classical trajectory calculations reveal that many of these roaming trajectories can be described as a methyl group roaming around the $\chem{HCO}$ core, before intramolecularly abstracting the formyl $\chem{H}$ atom. A crucial element to this mechanism is that the simple, barrierless, C-C bond cleavage to radical products must be open at the wavelengths used in previous experiments. While there is no doubt that the radical channel is open in an energetic sense, $\chem{HCO}$ and $\chem{CH_3}$ have never been observed from the ground state ($\textit{S}$${_0}) surface. In this seminar, we will summarize the evidence for roaming in \chem{CH_3CHO} and then present new experimental evidence that \chem{HCO} and \chem{CH_3} are indeed formed on the ground state. Pump/probe experiments were performed on acetaldehyde seeded in a supersonic expansion of helium. \chem{HCO} products were probed via laser-induced fluorescence (\tilde{B}\leftarrow\tilde{X}) at a range of pump wavelengths (308 - 330 nm). When the pump energy was above the (\textit{T}$${_1}$) barrier for dissociation ($\lambda$ $\sim$320 nm),}, \textbf{220}, 175 (1997).} the $\chem{HCO}$ product state distribution is characteristic of a reaction proceeding over a barrier. When the dissociation energy is lower than the triplet barrier, $\chem{HCO}$ was still observed, which must then arise from reaction on the $\textit{S}$${_0} surface. In addition, the \chem{HCO} internal energy distribution was different when dissociating above and below the triplet barrier, thereby confirming the presence of two different mechanistic pathways. The existence of the \chem{CH_3} + \chem{HCO} channel from the \chem{CH_3CHO} ground state supports the previous assignment of "\chem{CH_3} roaming" in \chem{CH_3CHO} photodissociation to \chem{CH_4} + \chem{CO}. en_US dc.language.iso English en_US dc.publisher Ohio State University en_US dc.title EVIDENCE FOR DISSOCIATION FROM THE \textit{S}$${_0}$ GROUND STATE OF ACETALDEHYDE TO THE RADICAL PRODUCTS CH$_3$ and HCO en_US dc.type Article en_US
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