Competing pathways in the near-UV photochemistry of acetaldehyde

• Published in: Physical chemistry chemical physics : PCCP Vol. 19; no. 22; pp. 14276 - 14288
• Main Authors: Toulson, Benjamin W, Kapnas, Kara M, Fishman, Dmitry A, Murray, Craig
• Format: Journal Article
• Language: English
• Published: England 07.06.2017
• Subjects: Acetaldehyde; Intersections; Nanostructure; Photochemistry; Photolysis; Radicals; Wavelengths
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c7cp02573d

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 3 radicals with single-photon VUV ionization show different dissociation dynamics in three distinct wavelength regions. At the longest photolysis wavelengths, λ > 318 nm, CH 3 radicals are formed over tens of nanoseconds with a speed distribution that is consistent with statistical unimolecular dissociation on the S 0 surface following internal conversion. In the range 292 nm ≤ λ ≤ 318 nm, dissociation occurs almost exclusively on the T 1 surface following intersystem crossing and passage over a barrier, leading to the available energy being partitioned primarily into photofragment recoil. The CH 3 speed distributions become bimodal at λ < 292 nm. In addition to the translationally fast T 1 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 CH 3 CHO obtained probing CH 3 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 0 , accessed via a conical intersection between the S 1 and S 0 surfaces at extended C-C distances. Time-resolved ion images of CH 3 radicals measured using a picosecond laser operating at a photolysis wavelength of 266 nm show that product formation on T 1 and S 0 via the conical intersection occurs with time constants of 240 ps and 560 ps, respectively. Time-resolved ion imaging measurements have been performed to explore the photochemistry of acetaldehyde at photolysis wavelengths spanning the range 265-328 nm.