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

Abstract

UV excitation of jet-cooled CH3CHOO on the B(1)A'-X(1)A' transition results in dissociation to two spin-allowed product channels: CH3CHO X(1)A' + O (1)D and CH3CHO a(3)A″ + O (3)P. The O (1)D and O (3)P products are detected using 2 + 1 REMPI at 205 and 226 nm, respectively, for action spectroscopy and velocity map imaging studies. The O (1)D 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 (1)D 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 X(1)A' coproducts. The O (3)P action spectrum turns on near the peak of the UV absorption spectrum (ca. 324 nm) and extends to higher energy with steadily increasing O (3)P 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 (3)P products arising from rapid (ps) dissociation, but a narrower TKER distribution since less energy is available to the CH3CHO a(3)A″ + O (3)P products. The threshold for the higher energy CH3CHO a(3)A″ + O (3)P 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 (3)P action spectrum. This threshold is combined with the singlet-triplet spacings of O atoms and acetaldehyde to establish the dissociation energy for syn-CH3HOO X(1)A' to the lowest spin-allowed product channel, CH3CHO X(1)A' + O (1)D, 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.