Velocity map imaging of O-atom products from UV photodissociation of the CH₂OO Criegee intermediate

UV excitation of jet-cooled CH₂OO X¹A′ to the excited B¹A′ electronic states results in dissociation to two spin-allowed product channels: H₂CO X¹A₁ + O ¹D and H₂CO a³A″ + O ³P. In this study, the higher energy H₂CO a³A″ + O ³P channel is characterized by velocity map imaging and UV action spectroscopy, in both cases utilizing 2 + 1 resonance enhanced multiphoton ionization detection of O ³P products, which complements a prior experimental study on the lower energy H₂CO X¹A₁ + O ¹D channel [Lehman et al., J. Chem. Phys. 139, 141103 (2013)]. Anisotropic angular distributions indicative of rapid dissociation are obtained at 330 and 350 nm, along with broad and unstructured total kinetic energy distributions that provide insight into the internal excitation of the H₂CO a³A″ co-fragment. A harmonic normal mode analysis points to significant vibrational excitation of the CH₂ wag and C–O stretch modes of the H₂CO a³A″ fragment upon dissociation. At each UV wavelength, the termination of the kinetic energy distribution reveals the energetic threshold for the H₂CO a³A″ + O ³P product channel of ca. 76 kcal mol⁻¹ (378 nm) and also establishes the dissociation energy from CH₂OO X¹A′ to H₂CO X¹A₁ + O¹D products of D₀ ≤ 49.0 ± 0.3 kcal mol⁻¹, which is in accord with prior theoretical studies. The threshold for the H₂CO a³A″ + O ³P channel is also evident as a more rapid falloff on the long wavelength side of the O ³P action spectrum as compared to the previously reported UV absorption spectrum for jet-cooled CH₂OO [Beames et al., J. Am. Chem. Soc. 134, 20045 (2012)]. Modeling suggests that the O ³P yield increases uniformly from 378 to 300 nm