Photodissociation Dynamics of the Cyclohexyl Radical from the 3p Rydberg State at 248 nm

The photodissociation of jet-cooled cyclohexyl was studied by exciting the radicals to their 3p Rydberg state using 248 nm laser light and detecting photoproducts by photofragment translational spectroscopy. Both H-atom loss and dissociation to heavy fragment pairs are observed. The H-atom loss channel exhibits a two-component translational energy distribution. The fast photoproduct component is attributed to impulsive cleavage directly from an excited state, likely the Rydberg 3s state, forming cyclohexene. The slow component is due to statistical decomposition of hot cyclohexyl radicals that internally convert to the ground electronic state prior to H-atom loss. The fast and slow components are present in a ~0.7:1 ratio, similar to findings in other alkyl radicals. Internal conversion to the ground state also leads to ring-opening followed by dissociation to 1-buten-4-yl + ethene in comparable yield to H-loss, with the C₄H₇ fragment containing enough internal energy to dissociate further to butadiene via H-atom loss. A very minor ground-state C₅H₈ + CH₃ channel is observed, attributed predominantly to 1,3-pentadiene formation. The ground-state branching ratios agree well with RRKM calculations, which also predict C₄H₆ + C₂H₅ and C₃H₆ + C₃H₅ channels with similar yield to C₅H₈ + CH₃. If these channels were active it was at levels too low to be observed.