15min:
EVIDENCE FOR DISSOCIATION FROM THE S 0 GROUND STATE OF ACETALDEHYDE TO THE RADICAL PRODUCTS CH3 and HCO.

ALAN T. MACCARONE, B. R. HEAZLEWOOD, S. J. ROWLING, S. H. KABLE, School of Chemistry, University of Sydney, Sydney NSW 2006, Australia.

Recent experiments and theory have implicated a "roaming" mechanism as being important in the photodissociation of \chemCH3CHO into the molecular products \chemCH4 + \chemCO., 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 \chemHCO core, before intramolecularly abstracting the formyl \chemH 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, \chemHCO and \chemCH3 have never been observed from the ground state ( S 0) surface. In this seminar, we will summarize the evidence for roaming in \chemCH3CHO and then present new experimental evidence that \chemHCO and \chemCH3 are indeed formed on the ground state.

Pump/probe experiments were performed on acetaldehyde seeded in a supersonic expansion of helium. \chemHCO products were probed via laser-induced fluorescence (B leftarrowX) at a range of pump wavelengths (308 - 330 nm). When the pump energy was above the ( T 1) barrier for dissociation ( lambda sim320 nm), the \chemHCO product state distribution is characteristic of a reaction proceeding over a barrier. When the dissociation energy is lower than the triplet barrier, \chemHCO was still observed, which must then arise from reaction on the S 0 surface. In addition, the \chemHCO 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 \chemCH3 + \chemHCO channel from the \chemCH3CHO ground state supports the previous assignment of "\chemCH3 roaming" in \chemCH3CHO photodissociation to \chemCH4 + \chemCO.