Photochemistry in the Far Ultraviolet

Ausloos, P.; Lias, Sharon G.

doi:10.1146/annurev.pc.22.100171.000505

Cited by 27 publications

(3 citation statements)

References 97 publications

(125 reference statements)

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“…Earlier investigations on VUV photolysis of methane reported Φ H ≥ 1.0 for λ near 121.6 nm. 14,38,39 More recent studies under collision-free conditions resulted in much lower yields. By referencing to another molecule with known quantum yield, the reported results are 0.47 ± 0.11, 16 0.45 ± 0.10, 19 and 0.31 ± 0.05, 17 respectively.…”

Section: Channel Branching Andmentioning

confidence: 99%

Unraveling the Photodissociation Branching and Pathways of Methane at 118 nm by Imaging the CH₃, CH₂, and CH Fragments

Wang,

Lin,

Xiao

et al. 2024

J. Phys. Chem. A

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Under irradiation of a vacuum ultraviolet (VUV) photon, methane dissociates and yields multiple fragments. This photochemical behavior is not only of fundamental importance but also with wideranging implications in several branches of science. Despite that and numerous previous investigations, the product channel branching is still under debate, and the underlying dissociation mechanisms remain elusive. In this study, the photofragment imaging technique was exploited for the first time to map out the momentum and anisotropy parameter distributions of the CH 3 , CH 2 , and CH fragments at the 118 nm photolysis wavelength (10.48 eV photon energy). In conjunction with previously reported results of the H atom fragment at 121.6 nm (10.2 eV), a complete set of product channel branching in both two-body and three-body fragmentations is accurately determined. We concluded that extensive nonadiabatic transitions partake in the processes with two-body fragmentations accounting for more than 90% of overall photodissociation, for which the channel branching values for CH 2 + H 2 and CH 3 + H are about 0.66 and 0.25, respectively. Careful kinematic analysis enables us to untangle the intertwined triple fragmentations into the CH 2 (X ̃3B 1 and ã1A 1 ) + H + H and CH(X 2 Π) + H + H 2 channels and to evidence their underlying sequential (or stepwise) mechanisms. With the aid of electronic correlation and prior theoretical calculations of the potential energy surfaces, the most probable or dominant dissociation pathways are elucidated. Comparisons with fragmentary reports in the literature on various photochemical aspects are also documented, and discrepancies are clarified. This comprehensive study benchmarks the VUV photochemistry of methane and advances our understanding of this important process.

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Section: Channel Branching Andmentioning

confidence: 99%

Unraveling the Photodissociation Branching and Pathways of Methane at 118 nm by Imaging the CH₃, CH₂, and CH Fragments

Wang,

Lin,

Xiao

et al. 2024

J. Phys. Chem. A

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“…The core isoprene scheme in GECKO-A is adopted from the Master Chemical Mechanism (MCM) v3.3.1 (Jenkin et al, 2015), while the meta-xylene oxidation mechanism follows MCM v3.2 (Jenkin et al, 2003, Bloss et al, 2005, typically until ring-breaking occurs, whereupon the GECKO-A mechanism generator implements the standard SAR protocols as described by Aumont et al (2005), Camredon et al (2007, and . Under the zero-NO conditions employed in this study, we find that, in two of the four m-xylene reaction channels (xylenol, 17 %; and MXYLO2, 4 %), some product species persist anomalously owing to a lack of alternative reaction pathways in the MCM.…”

Section: The Gecko-a Modelmentioning

confidence: 99%

Evolution of OH reactivity in NO-free volatile organic compound photooxidation investigated by the fully explicit GECKO-A model

et al. 2021

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Abstract. OH reactivity (OHR) is an important control on the oxidative capacity in the atmosphere but remains poorly constrained in many environments, such as remote, rural, and urban atmospheres, as well as laboratory experiment setups under low-NO conditions. For an improved understanding of OHR, its evolution during oxidation of volatile organic compounds (VOCs) is a major aspect requiring better quantification. We use the fully explicit Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) model to study the OHR evolution in the NO-free photooxidation of several VOCs, including decane (an alkane), m-xylene (an aromatic), and isoprene (an alkene). Oxidation progressively produces more saturated and functionalized species. Total organic OHR (including precursor and products, OHRVOC) first increases for decane (as functionalization increases OH rate coefficients) and m-xylene (as much more reactive oxygenated alkenes are formed). For isoprene, C=C bond consumption leads to a rapid drop in OHRVOC before significant production of the first main saturated multifunctional product, i.e., isoprene epoxydiol. The saturated multifunctional species in the oxidation of different precursors have similar average OHRVOC per C atom. The latter oxidation follows a similar course for different precursors, involving fragmentation of multifunctional species to eventual oxidation of C1 and C2 fragments to CO2, leading to a similar evolution of OHRVOC per C atom. An upper limit of the total OH consumption during complete oxidation to CO2 is roughly three per C atom. We also explore the trends in radical recycling ratios. We show that differences in the evolution of OHRVOC between the atmosphere and an environmental chamber, and between the atmosphere and an oxidation flow reactor (OFR), can be substantial, with the former being even larger, but these differences are often smaller than between precursors. The Teflon wall losses of oxygenated VOCs in chambers result in large deviations of OHRVOC from atmospheric conditions, especially for the oxidation of larger precursors, where multifunctional species may suffer substantial wall losses, resulting in significant underestimation of OHRVOC. For OFR, the deviations of OHRVOC evolution from the atmospheric case are mainly due to significant OHR contribution from RO2 and lack of efficient organic photolysis. The former can be avoided by lowering the UV lamp setting in OFR, while the latter is shown to be very difficult to avoid. However, the former may significantly offset the slowdown in fragmentation of multifunctional species due to lack of efficient organic photolysis.

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“…Taking the activation energy for reaction g to be zero and Sex,* Sex,*, simultaneous solution of eq 6 and 11 gives Sd* = 42 kcal mol"1 and Sex* = 9 kcal mol"1.…”

Section: D(clmentioning

confidence: 99%

Exchange reaction of methane-d4 with hydrogen chloride

Kern¹,

Nika²

1972

J. Phys. Chem.

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A shock tube coupled to a time-of-flight mass spectrometer was used to study the exchange reaction, CD4 + HC1 CDsH + DC1, over a density range 1.7-4.1 X 10"6 mol cm"3 in the temperature region 1600-2500°K. In the lower temperature range (1600-1900°K) DC1 exhibits a quadratic time-dependent growth rate while at higher temperatures (2200-2500°K) increase of DC1 is a linear function of time. The only chlorine-containing product is deuterium chloride. Rate constants extracted from the quadratic and linear regions yielded activation energies of 51 and 30 kcal mol"1, respectively. An atomic mechanism for the exchange predicts both time dependencies and the transition between them. Pyrolysis of methane occurred along with the exchange but the growth rate of the products ethane, acetylene, and diacetylene was shown not to be catalyzed by the presence of HC1. Acetylene was the major product of the pyrolysis. Exchange was observed in both major and. minor products. The ratio CHACEE was less than one when pyrolysis products were not detected and greater than one when they were present. This finding suggests the presence of methyl radical in the reaction mixture. The results are discussed in terms of an atomic mechanism.Professor Bauer and his coworkers studied the CH4 + D2 reaction in the temperature range 1440-1755°K (SPST) under conditions of negligible 02 impurities.6 Samples extracted from the reflected zone were analyzed with a mass spectrometer for CH4, D2, CHgD, and HD. Evidence for methane pyrolysis was not found. The reaction is reported to proceed through a four-center complex in accordance with the vibrational excitation mechanism proposed by Bauer.3a Burcat and Lifshitz (BL)6 studied the exchange system CH4 + CD4 in the temperature region 1340-1745°K (SPST). They report a zero-order dependence with respect to inert gas although Bauer found an order of 0.6 for the inert diluent. BL interpreted their results in terms of a methyl radical chain mechanism resulting from the pyrolytic initiation step CH4 CHg + H, followed by the exchange reaction CHa+ CD4-> CH3D + CD3.Several questions are apparent from a comparison of these two methane exchange reactions. Both studies are in the same temperature range, yet Bauer argues against the importance of pyrolysis in the exchange (1) (a) Support of this work by the National Science Foundation under Grant No. GP-23137 and also funds for equipment from NSF Departmental Science Development Program GU-2632 are gratefully acknowledged, (b) Paper presented in part at the Southwest

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Photochemistry in the Far Ultraviolet

Cited by 27 publications

References 97 publications

Unraveling the Photodissociation Branching and Pathways of Methane at 118 nm by Imaging the CH₃, CH₂, and CH Fragments

Unraveling the Photodissociation Branching and Pathways of Methane at 118 nm by Imaging the CH₃, CH₂, and CH Fragments

Evolution of OH reactivity in NO-free volatile organic compound photooxidation investigated by the fully explicit GECKO-A model

Exchange reaction of methane-d4 with hydrogen chloride

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Photochemistry in the Far Ultraviolet

Cited by 27 publications

References 97 publications

Unraveling the Photodissociation Branching and Pathways of Methane at 118 nm by Imaging the CH3, CH2, and CH Fragments

Unraveling the Photodissociation Branching and Pathways of Methane at 118 nm by Imaging the CH3, CH2, and CH Fragments

Evolution of OH reactivity in NO-free volatile organic compound photooxidation investigated by the fully explicit GECKO-A model

Exchange reaction of methane-d4 with hydrogen chloride

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Unraveling the Photodissociation Branching and Pathways of Methane at 118 nm by Imaging the CH₃, CH₂, and CH Fragments

Unraveling the Photodissociation Branching and Pathways of Methane at 118 nm by Imaging the CH₃, CH₂, and CH Fragments