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

Li, Hongwei, Beames, Joseph M. and Lester, Marsha I. 2015. Velocity map imaging of O-atom products from UV photodissociation of the CH2OO Criegee intermediate. Journal of Chemical Physics 142 (21) , 214312. 10.1063/1.4921990

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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.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Publisher: American Institute of Physics
ISSN: 0021-9606
Date of Acceptance: 1 May 2015
Last Modified: 08 Sep 2020 15:30

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