Ultraviolet photodissociation dynamics of 1-pentyl radical^†

The ultraviolet (UV) photodissociation of jet-cooled 1-pentyl radical is investigated in the wavelength region of 236−254 nm using the high-n Rydberg-atom time-of-flight (HRTOF) technique. The H-atom photofragment yield spectrum of the 1-pentyl radical shows a broad UV absorption feature peaking near 245 nm, similar to the 2p_z →3s absorption bands of ethyl and n-propyl. The center-of-mass translational energy distribution, P (E_T), of the H+C₅H₁₀ product channel is bimodal, with a slow peak at ~5 kcal/mol and a fast peak at ~50 kcal/mol. The fraction of the average translational energy release in the total available energy, ⟨f_T⟩, is 0.30, with those of the slow and fast components being 0.13 and 0.58, respectively. The slow component has an isotropic product angular distribution, while the fast component is anisotropic with an anisotropy parameter ~0.4. The bimodal translational energy and angular distributions of the H+C₅H₁₀ products indicate two H-atom elimination channels in the photodissociation of 1-pentyl: (i) a direct, prompt dissociation from the electronic excited state and/or the repulsive part of the ground electronic state potential energy surface; and (ii) a unimolecular dissociation of internally hot radical in the ground electronic state after internal conversion from the electronic excited state.