UV Photodissociation Dynamics of the CH₃CHOO Criegee Intermediate: Action Spectroscopy and Velocity Map Imaging of O-Atom Products

UV excitation of jet-cooled CH₃CHOO on the B¹A′-X¹A′ transition results in dissociation to two spin-allowed product channels: CH₃CHO X¹A′ + O″¹D and CH₃CHO a³-Rfaut' + O″³P. The O″¹D and O″³P products are detected using 2 + 1 REMPI at 205 and 226 nm, respectively, for action spectroscopy and velocity map imaging studies. The O″¹D action spectrum closely follows the previously reported UV absorption spectrum for jet-cooled CH₃CHOO [ Beames et al. J. Chem. Phys. 2013, 138, 244307 ]. Velocity map images of the O″¹D products following excitation of CH₃CHOO at 305, 320, and 350 nm exhibit anisotropic angular distributions indicative of rapid (ps) dissociation, along with broad and unstructured total kinetic energy (TKER) distributions that reflect the internal energy distribution of the CH₃CHO X¹A′ coproducts. The O″³P action spectrum turns on near the peak of the UV absorption spectrum (ca. 324 nm) and extends to higher energy with steadily increasing O″³P yield. Excitation of CH₃CHOO at 305 nm, attributed to absorption of the more stable syn-conformer, also results in an anisotropic angular distribution of O″³P products arising from rapid (ps) dissociation, but a narrower TKER distribution since less energy is available to the CH₃CHO a³A' + O″³P products. The threshold for the higher energy CH₃CHO a³A' + O″³P product channel is determined to be ca. 88.4 kcal mol^-1 from the termination of the TKER distribution and the onset of the O″³P action spectrum. This threshold is combined with the singlet-triplet spacings of O atoms and acetaldehyde to establish the dissociation energy for syn-CH₃HOO X¹A′ to the lowest spin-allowed product channel, CH₃CHO X¹A′ + O″¹D, of 55.9 ± 0.4 kcal mol^-1. A harmonic normal-mode analysis is utilized to identify the vibrational modes of CH₃CHO likely to be excited upon dissociation into the two product channels.