Knowledge Bank

Recent experiments and theory have implicated a "roaming" mechanism as being important in the photodissociation of $\backslash chemCH3CHO$ into the molecular products $\backslash chemCH4$ + $\backslash chemCO$.}, \textbf{103}, 16079 (2006).}$\mathrm{<mrow\; data-mjx-texclass="ORD">,</mrow>}$} (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 $\backslash chemHCO$ core, before intramolecularly abstracting the formyl $\backslash chemH$ 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, $\backslash chemHCO$ and $\backslash chemCH3$ have never been observed from the ground state ($S$$0$) surface. In this seminar, we will summarize the evidence for roaming in $\backslash chemCH3CHO$ and then present new experimental evidence that $\backslash chemHCO$ and $\backslash chemCH3$ are indeed formed on the ground state. Pump/probe experiments were performed on acetaldehyde seeded in a supersonic expansion of helium. $\backslash chemHCO$ products were probed via laser-induced fluorescence ($B~\leftarrow X~$) at a range of pump wavelengths (308 - 330 nm). When the pump energy was above the ($T$$1$) barrier for dissociation ($\lambda$ $\sim$320 nm),}, \textbf{220}, 175 (1997).} the $\backslash chemHCO$ product state distribution is characteristic of a reaction proceeding over a barrier. When the dissociation energy is lower than the triplet barrier, $\backslash chemHCO$ was still observed, which must then arise from reaction on the $S$$0$ surface. In addition, the $\backslash chemHCO$ 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 $\backslash chemCH3$ + $\backslash chemHCO$ channel from the $\backslash chemCH3CHO$ ground state supports the previous assignment of "$\backslash chemCH3$ roaming" in $\backslash chemCH3CHO$ photodissociation to $\backslash chemCH4$ + $\backslash chemCO$.