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.

Electronic spectroscopy of methyl vinyl ketone oxide: A four-carbon unsaturated Criegee intermediate from isoprene ozonolysis

“…The dissociation energy predicted for MVK-oxide to its lowest spinallowed asymptote is similar to that determined previously for CH 2 OO X 1 A to H 2 CO X 1 A 1 + O 1 D products. 40,41,61 Zero-point corrected dissociation energies for other MVKoxide conformers (Table S3) are evaluated by taking into account their relative ground state stabilities (Table S1) and the small energy splitting (84 cm −1 ; B2PLYP-D3/VTZ) predicted between the cis and trans conformers of the MVK product.…”

“…In addition, velocity-map images (VMI) of the O 1 D products are obtained following UV-vis excitation of MVKoxide, as described previously for other Criegee intermediates. 37,40,41,61 Again, UV-vis excitation is generated from the BBO-OPO source (∼2 mJ/pulse) and focused (70 cm focal length) in the interaction region. The O 1 D dissociation products are ionized via 2 + 1 resonance enhanced multiphoton ionization (REMPI) at 205.47 nm, 62 which is generated by frequency tripling of the output of a Nd:YAG (Continuum Powerlite 8010) pumped dye laser (Continuum, ND6000, Rh 610) and scanned over the O-atom Doppler profile (±0.4 cm −1 ).…”

“…The central region of the detector is protected using a beam block, which prevents ions with low translational energy (<150 cm −1 ) from striking the MCP plate. 40,41 The pancaked image is then analyzed using the pBASEX inversion method to extract the angular and velocity distributions of the O 1 D products. 63 An active background subtraction scheme is implemented by running the UV-vis OPO at 5 Hz, while the REMPI laser is operated at 10 Hz.…”

“…The UV spectra of simple Criegee intermediates have been recorded using a variety of techniques including direct absorption, 32,33,39 UV action spectroscopy with O-atom detection, 40,41 and UV-induced depletion of the associated VUV (vacuum ultraviolet) photoionization signal. [27][28][29] Here, we demonstrate 10.5 eV photoionization of MVK-oxide and detection on the parent m/z = 86 mass channel.…”

Communication: Thermal unimolecular decomposition of syn-CH3CHOO: A kinetic study

“…7 Because SCIs are readily consumed, they have very low steady-state concentrations, 8 and consequently their fate in the atmosphere is not well established. 1 To provide a relatively clean source of SCIs for spectroscopic, 9-21 dynamics, [22][23][24][25][26] and kinetics studies, [27][28][29][30][31][32][33][34][35][36] SCIs are almost exclusively made from photo-oxidation reactions of (R 1 R 2 )CI 2 in the presence of O 2 (where R 1 /R 2 represent hydrogen atoms or alkyl radicals To provide accurate rate constants and to aid experimental measurements, unimolecular decomposition rate constants of syn-CH 3 CHOO under atmospheric conditions are computed here from first principles using Miller's semiclassical transition state theory (SCTST) [41][42][43][44][45] in combination with a two-dimensional master equation (2DME) approach. 7,46 This chemical kinetics analysis is performed using a high accuracy potential energy surface that is constructed with a modification of the HEAT protocol.…”