Competing pathways in the near-UV photochemistry of acetaldehyde

Time-resolved ion imaging measurements have been performed to explore the photochemistry of acetaldehyde at photolysis wavelengths spanning the range 265-328 nm. Ion images recorded probing CH₃ radicals with single-photon VUV ionization show different dissociation dynamics in three distinct wavelength regions. At the longest photolysis wavelengths, λ > 318 nm, CH₃ radicals are formed over tens of nanoseconds with a speed distribution that is consistent with statistical unimolecular dissociation on the S₀ surface following internal conversion. In the range 292 nm ≤ λ ≤ 318 nm, dissociation occurs almost exclusively on the T₁ surface following intersystem crossing and passage over a barrier, leading to the available energy being partitioned primarily into photofragment recoil. The CH₃ speed distributions become bimodal at λ < 292 nm. In addition to the translationally fast T₁ products, a new translationally slow, but non-statistical, component appears and grows in importance as the photolysis wavelength is decreased. Photofragment excitation (PHOFEX) spectra of CH₃CHO obtained probing CH₃ and HCO products are identical across the absorption band, indicating that three-body fragmentation is not responsible for the non-statistical slow component. Rather, translationally slow products are attributed to dissociation on S₀, accessed via a conical intersection between the S₁ and S₀ surfaces at extended C-C distances. Time-resolved ion images of CH₃ radicals measured using a picosecond laser operating at a photolysis wavelength of 266 nm show that product formation on T₁ and S₀via the conical intersection occurs with time constants of 240 ps and 560 ps, respectively.