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

UV excitation of jet-cooled CH2OO X(1)A' to the excited B(1)A' electronic states results in dissociation to two spin-allowed product channels: H2CO X(1)A1 + O (1)D and H2CO a(3)A″ + O (3)P. In this study, the higher energy H2CO a(3)A″ + O (3)P channel is characterized by velocity map imaging and UV action spectroscopy, in both cases utilizing 2 + 1 resonance enhanced multiphoton ionization detection of O (3)P products, which complements a prior experimental study on the lower energy H2CO X(1)A1 + O (1)D 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 a(3)A″ co-fragment. A harmonic normal mode analysis points to significant vibrational excitation of the CH2 wag and C-O stretch modes of the H2CO a(3)A″ fragment upon dissociation. At each UV wavelength, the termination of the kinetic energy distribution reveals the energetic threshold for the H2CO a(3)A″ + O (3)P product channel of ca. 76 kcal mol(-1) (378 nm) and also establishes the dissociation energy from CH2OO X(1)A' to H2CO X(1)A1 + O(1)D products of D0 ≤ 49.0 ± 0.3 kcal mol(-1), which is in accord with prior theoretical studies. The threshold for the H2CO a(3)A″ + O (3)P channel is also evident as a more rapid falloff on the long wavelength side of the O (3)P 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 (3)P yield increases uniformly from 378 to 300 nm.