UC Berkeley

The photodissociation and liquid interfacial scattering dynamics of neutral species are described. Photofragment translational spectroscopy is used to determine active product channels and their relative importance, as well as providing insight into the detailed dissociation mechanisms leading to each channel. Modifications to the photodissociation apparatus enable experiments on evaporation and scattering from the surface of volatile liquid jets. These new experiments are capable of probing the detailed reactive and non-reactive interaction dynamics between atoms and molecules at the liquid-vacuum interface. Reactive free radicals are important species in fields such as terrestrial and non-terrestrial atmospheric chemistry, circumstellar and interstellar astrochemistry, and in combustion. For their widespread importance to these fields through their impact on interfacial chemistry and photochemistry, neutral free radicals are featured prominently in the enclosed studies.

Fulvenallene (C$_7$H$_6$) and the fulvenallenyl radical (C$_7$H$_5$) are important intermediates in the decomposition mechanism of toluene, a model system for aromatic combustion. Fulvenallene photodissociation was studied at 248 nm and 193 nm, in each case leading exclusively to H-loss. The data support a mechanism involving internal conversion to the ground state and subsequent energy randomization, leading to statistical production of H + fulvenallenyl. The absence of a proposed \emph{c}-C$_5$H$_4$ + C$_2$H$_2$ channel indicated the necessary intersystem crossing to the triplet state must have a low transition probability. Fulvenallenyl radicals produced by fulvenallene dissociation absorbed a second photon and their photodissociation was thus determined. At both wavelengths, fulvenallenyl dissociation yielded \emph{n}/\emph{i}-C$_5$H$_3$ radical + acetylene (C$_2$H$_2$) and diacetylene (C$_4$H$_2$) + propargyl radical (C$_3$H$_3$). The translational energy distributions for these reactions were consistent with internal conversion of fulvenallenyl to its ground state, followed by statistical dissociation. The C$_4$H$_2$ + C$_3$H$_3$ channel accounted for 85 $\pm$ 10 \% and 80 $\pm$ 15 \% of fulvenallenyl dissociation at \mbox{248 nm} and 193 nm respectively. The branching between these two channels agrees well with statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations, supporting the proposed mechanism.

The methylsulfinyl radical (CH$_3$SO) is an important intermediate in the oxidation of biogenic sulfur-containing molecules in the terrestrial atmosphere, ultimately contributing to acid-rain formation. The ultraviolet photochemistry of methylsulfinyl was assessed at 248 nm, finding evidence for CH$_3$S + O, CH$_3$ + SO, and CH$_2$SO + H channels. The existence of the very high energy O-atom loss channel relies on a dissociation mechanism mediated by a repulsive excited state of methylsulfinyl leading to ground state products, and this channel accounted for 77 $\pm$ 8 \% of all methylsulfinaly photodissociation. The SO-loss channel had a bimodal translational energy distribution attributed to formation of SO in its X $^3\Sigma^-$ ground state and $a~^1\Delta$ excited state, along with ground state CH$_3$ radical in each case. The SO-loss channels accounted for 22 $\pm$ 8 \% of dissociation, approximately 12 \% from the ground state and 10 \% via the excited state channel. Finally, H-loss to form sulfine, CH$_2$SO, occurred on the ground state following internal conversion and energy randomization, and accounted for 1.5 $\pm$ 0.5 \% of dissociation. For the ground-state CH$_3$ + SO and CH$_2$SO + H channels, their branching ratio agrees well with RRKM calculations.

The cyclohexyl radical (\emph{c}-C$_6$H$_{11}$) is a cycloalkyl radical relevant in combustion and astrochemical environments. Cyclohexyl radicals were excited to their 3p$_z$ Rydberg state using 248 nm photons, leading to three channels: C$_6$H$_{10}$ + H, C$_5$H$_3$ + CH$_3$, and C$_4$H$_7$ + C$_2$H$_4$. The bimodal translational energy distribution for H-loss was attributed to `slow' statistical ground state and `fast' repulsive excited state channels for which cyclohexene is the major and exclusive C$_6$H$_{10}$ isomer respectively. The initially prepared 3p$_z$ state converts to the 3s state, which is separated from the ground state by a conical intersection that determines branching between impulsive H-loss and population of the ground state potential energy well, from which H-atom loss is a possible dissociation channel. The translational energy distributions for the C$_5$H$_3$ + CH$_3$ and C$_4$H$_7$ + C$_2$H$_4$ channels were both consistent with statistical ground state mechanisms initiating with ring-opening of ground state cyclohexyl to form 1-hexen-6-yl, which competes with direct H-loss. Experimentally determined branching ratios were (H-loss)$_{slow}$:(H-loss)$_{fast}$:(C$_5$H$_3$ + CH$_3$):(C$_4$H$_7$ + C$_2$H$_4$) = \mbox{1 : 0.7$^{+1.6}_{-0.4}$ : 0.015$^{+0.005}_{-0.005}$ : 0.86$^{+0.10}_{-0.07}$}. The slow:fast H-loss ratio of $\sim$0.7:1 is very similar to findings in other alkyl radicals, and the ground-state channels are formed in excellent agreement with RRKM calculations. The 1-buten-4-yl (C$_4$H$_7$) radical was formed with sufficient energy to dissociate further, proceeding exclusively to C$_4$H$_6$ + H within the experimental detection capabilities.

The dynamics of evaporation and scattering from aqueous and organic liquid surfaces has important implications for atmospheric chemistry occurring at surface coatings and aerosol interfaces. Modifications to the photodissociation apparatus enable the generation of free-flowing, flat liquid jets of water and hydrocarbons in vacuum. Measurements of the kinetic energy and angular distributions of evaporation of the neat liquid and dissolved species reveal dynamical signatures of the thermal desorption process governing evaporation. Fundamentally interesting in their own right, the manifestation of thermal desorption isolated in these experiments develops experience that can be applied during scattering experiments, where impulsive scattering and trapping-desorption are prototypical surface-scattering processes. Preliminary results on scattering from the liquid-vacuum interface show promise for this technique, and several future research directions are discussed.