Ultraviolet photodissociation dynamics of the cyclohexyl radical

Cycloalkanes are important components in conventional fuels and oil shale derived fuels and the combustion of cyclohexane fuels leads to the production of benzene, a pollutant precursor. One of the pathways from cyclohexane to benzene is through sequential hydrogen loss, including the cyclohexyl radical as an intermediate. The ultraviolet (UV) photodissociation dynamics of the cyclohexyl ($c$-C$_{\mathrm{6}}$H$_{\mathrm{11}})$ radical was studied for the first time using the high-$n$ Rydberg atom time-of-flight (HRTOF) technique in the range of 232-262 nm. The translational energy distributions of the H-atom loss product channel, $P(E_{\mathrm{T}})$'s, show a large translational energy release and a large fraction of average translational energy in the total excess energy, $\langle f_{\mathrm{T}}\rangle $, from 232-262 nm. The H-atom product angular distribution is anisotropic with a positive $\beta $ parameter. The most likely H-atom loss pathway is an axial H ejection from the $\beta $-carbon in cyclohexyl to form cyclohexene $+$ H, which along with the positive $\beta $ parameter, indicates that the transition dipole moment, $\mu $, is perpendicular to the ring. The $P(E_{\mathrm{T}})$ and anisotropy of the H-atom loss product channel are significantly larger than those expected for a statistical unimolecular dissociation of a hot radical, indicating a non-statistical dissociation mechanism. The dissociation mechanism is consistent with direct dissociation on a repulsive excited state surface or on the repulsive part of the ground state surface to produce cyclohexene $+$ H, possibly mediated by a conical intersection. Cyclohexyl is the largest radical so far showing a direct dissociation mechanism.