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 CH radicals with single-photon VUV ionization show different dissociation dynamics in three distinct wavelength regions. At the longest photolysis wavelengths, λ > 318 nm, CH radicals are formed over tens of nanoseconds with a speed distribution that is consistent with statistical unimolecular dissociation on the S surface following internal conversion. In the range 292 nm ≤ λ ≤ 318 nm, dissociation occurs almost exclusively on the T surface following intersystem crossing and passage over a barrier, leading to the available energy being partitioned primarily into photofragment recoil. The CH speed distributions become bimodal at λ < 292 nm. In addition to the translationally fast T 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 CHCHO obtained probing CH 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 S, accessed via a conical intersection between the S and S surfaces at extended C-C distances. Time-resolved ion images of CH radicals measured using a picosecond laser operating at a photolysis wavelength of 266 nm show that product formation on T and Svia the conical intersection occurs with time constants of 240 ps and 560 ps, respectively.