The photodissociation dynamics of the ethyl radical, C2H5, investigated by velocity map imaging

Photodissociation dynamics of the ethyl radical via the Ã2A'(3s) state: H-atom product channels and ethylene product vibrational state distribution

The photodissociation dynamics of jet-cooled ethyl radical (C2H5) via the Ã2A'(3s) states are studied in the wavelength region of 230-260 nm using the high-n Rydberg H-atom time-of-flight (TOF) technique. The H + C2H4 product channels are reexamined using the H-atom TOF spectra and photofragment translational spectroscopy. A prompt H + C2H4(X̃1Ag) product channel is characterized by a repulsive translational energy release, anisotropic product angular distribution, and partially resolved vibrational state distribution of the C2H4(X̃1Ag) product. This fast dissociation is initiated from the 3s Rydberg state and proceeds via a H-bridged configuration directly to the H + C2H4(X̃1Ag) products. A statistical-like H + C2H4(X̃1Ag) product channel via unimolecular dissociation of the hot electronic ground-state ethyl (X̃2A') after internal conversion from the 3s Rydberg state is also examined, showing a modest translational energy release and isotropic angular distribution. An adiabatic H + excited triplet C2H4(ã3B1u) product channel (a minor channel) is identified by energy-dependent product angular distribution, showing a small translational energy release, anisotropic angular distribution, and significant internal excitation in the C2H4(ã3B1u) product. The dissociation times of the different product channels are evaluated using energy-dependent product angular distribution and pump-probe delay measurements. The prompt H + C2H4(X̃1Ag) product channel has a dissociation time scale of <10 ps, and the upper bound of the dissociation time scale of the statistical-like H + C2H4(X̃1Ag) product channel is <5 ns.

Photodissociation of the trichloromethyl radical: photofragment imaging and femtosecond photoelectron spectroscopy

Halogen-containing radicals play a key role in catalytic reactions leading to stratospheric ozone destruction, thus their photochemistry is of considerable interest. Here we investigate the photodissociation dynamics of the trichloromethyl radical, CCl₃ after excitation in the ultraviolet. While the primary processes directly after light absorption are followed by femtosecond-time resolved photoionisation and photoelectron spectroscopy, the reaction products are monitored by photofragment imaging using nanosecond-lasers. The dominant reaction is loss of a Cl atom, associated with a CCl₂ fragment. However, the detection of Cl atoms is of limited value, because in the pyrolysis CCl₂ is formed as a side product, which in turn dissociates to CCl + Cl. We therefore additionally monitored the molecular fragments CCl₂ and CCl by photoionisation at 118.2 nm and disentangled the contributions from various processes. A comparison of the CCl images with control experiments on CCl₂ suggest that the dissociation to CCl + Cl₂ contributes to the photochemistry of CCl₃.

Ultraviolet Photodissociation Dynamics of the Cyclohexyl Radical: The H-Atom Product Channel

The ultraviolet (UV) photodissociation dynamics of the jet-cooled cyclohexyl (c-C6H11) radical is studied using the high-n Rydberg atom time-of-flight (HRTOF) technique. The cyclohexyl radical is produced by the 193 nm photodissociation of chlorocyclohexane and bromocyclohexane and is examined in the photolysis wavelength region of 232-262 nm. The H-atom photofragment yield (PFY) spectrum contains a broad peak centered at 250 nm, which is in good agreement with the UV absorption spectrum of the cyclohexyl radical and assigned to the 3p Rydberg states. The translational energy distributions of the H-atom loss product channel, P(ET)'s, are bimodal, with a slow (low ET) component peaking at ∼6 to 7 kcal/mol and a fast (high ET) component peaking at ∼44-48 kcal/mol. The fraction of the average translational energy in the total excess energy, ⟨fT⟩, is in the range of 0.16-0.25 in the photolysis wavelength region of 232-262 nm. The H-atom product angular distribution of the slow component is isotropic, while that of the fast component is anisotropic with an anisotropy parameter of β ≈ 0.5-0.7. The bimodal product translational energy and angular distributions indicate two dissociation pathways to the H + C6H10 products in cyclohexyl. The high-ET anisotropic component is from a repulsive, prompt dissociation on a repulsive potential energy surface coupling with the Rydberg excited states to produce H + cyclohexene. The low-ET isotropic component is consistent with the unimolecular dissociation of hot radical on the ground electronic state after internal conversion from the Rydberg states. The similarity of the photodissociation dynamics of the cyclohexyl radical to the previously studied small linear and branched alkyls expands on the understanding of the dissociation dynamics of alkyl radicals to include larger cyclic alkyl radicals.

Hierarchical Study of the Reactions of Hydrogen Atoms with Alkenes: A Theoretical Study of the Reactions of Hydrogen Atoms with C2-C4 Alkenes

The present study complements our previous studies of the reactions of hydrogen atoms with C₅ alkene species including 1- and 2-pentene and the branched isomers (2-methyl-1-butene, 2-methyl-2-butene, and 3-methyl-1-butene), by studying the reactions of hydrogen atoms with C₂–C₄ alkenes (ethylene, propene, 1- and 2-butene, and isobutene). The aim of the current work is to develop a hierarchical set of rate constants for Ḣ atom addition reactions to C₂–C₅ alkenes, both linear and branched, which can be used in the development of chemical kinetic models. High-pressure limiting and pressure-dependent rate constants are calculated using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory and a one-dimensional master equation (ME). Rate constant recommendations for Ḣ atom addition and abstraction reactions in addition to alkyl radical decomposition reactions are also proposed and provide a useful tool for use in mechanisms of larger alkenes for which calculations do not exist. Additionally, validation of our theoretical results with single-pulse shock-tube pyrolysis experiments is carried out. An improvement in species mole fraction predictions for alkene pyrolysis is observed, showing the relevance of the present study.

Photodissociation Dynamics of the Cyclohexyl Radical from the 3p Rydberg State at 248 nm

The photodissociation of jet-cooled cyclohexyl was studied by exciting the radicals to their 3p Rydberg state by using 248 nm laser light and detecting photoproducts by photofragment translational spectroscopy. Both H atom loss and dissociation to heavy fragment pairs are observed. The H atom loss channel exhibits a two-component translational energy distribution. The fast photoproduct component is attributed to impulsive cleavage directly from an excited state, likely the Rydberg 3s state, forming cyclohexene. The slow component is due to statistical decomposition of hot cyclohexyl radicals that internally convert to the ground electronic state prior to H atom loss. The fast and slow components are present in an ∼0.7:1 ratio, similar to findings in other alkyl radicals. Internal conversion to the ground state also leads to ring-opening followed by dissociation to 1-buten-4-yl + ethene in comparable yield to H-loss, with the C₄H₇ fragment containing enough internal energy to dissociate further to butadiene via H atom loss. A very minor ground-state C₅H₈ + CH₃ channel is observed, attributed predominantly to 1,3-pentadiene formation. The ground-state branching ratios agree well with RRKM calculations, which also predict C₄H₆ + C₂H₅ and C₃H₆ + C₃H₅ channels with similar yield to C₅H₈ + CH₃. If these channels were active, it was at levels too low to be observed.

Photodissociation of Benzoyl Chloride: A Velocity Map Imaging Study Using VUV Detection of Chlorine Atoms

UV photodissociation of benzoyl chloride, Ph-CO-Cl, is associated with the loss of a chlorine atom. Here we excite benzoyl chloride to the S₁, S₂, and S₃ excited states at 237, 253, 265, and 279.6 nm and detect the Cl photofragment by [1 + 1'] photoionization using 118.9 nm VUV radiation. The translational energy distribution of the Cl atom is measured by velocity map ion imaging. An isotropic image and a unimodal translational energy distribution are observed at all dissociation wavelengths, and a fraction of 18-20% of the excess energy is released into translation. The results indicate a dissociation that predominately proceeds from the vibrationally hot S₀ ground state, although the observed translational energy release deviates significantly from a prior distribution. However, the impulsive model does also not represent the translational energy release. As a Cl/Cl* branching ratio of 9:1 or more is observed in one-color experiments at 235 nm, we conclude that direct dissociation from excited electronic states contributes only to a minor extent.

Ultraviolet photodissociation dynamics of the n-butyl, s-butyl, and t-butyl radicals

Photodissociation dynamics of the jet-cooled n-butyl radical via the 3s Rydberg state and the s-butyl radical via the 3p Rydberg states in the ultraviolet region of 233 nm-258 nm, as well as the t-butyl radical via the 3d Rydberg states at 226 nm-244 nm, are studied using the high-n Rydberg atom time-of-flight technique. The H-atom photofragment yield spectra of the n-butyl, s-butyl, and t-butyl radicals show a broad feature centered around 247 nm, 244 nm, and 234 nm, respectively. The translational energy distributions of the H + C₄H₈ products, P(E_T)'s, of the three radicals are bimodal, with a slow (low E_T) component peaking at ∼6 kcal/mol and a fast (high E_T) component peaking at ∼52 kcal/mol-57 kcal/mol, ∼43 kcal/mol, and ∼37 kcal/mol for n-butyl, s-butyl, and t-butyl, respectively. The fraction of the products' translational energy in the available energy, ⟨ f_T⟩, is 0.31, 0.30, and 0.27 for n-butyl, s-butyl, and t-butyl, respectively. The H-atom product angular distributions of the slow component are isotropic for all three radicals, while those of the fast component are anisotropic for n-butyl and s-butyl with an anisotropy parameter β ∼ 0.7 and ∼ 0.3 and that of the fast component of t-butyl is nearly isotropic. The bimodal product translational energy and angular distributions indicate two dissociation pathways to the H + C₄H₈ products in these three radicals, a direct, prompt dissociation on the repulsive potential energy surface coupling with the Rydberg excited states, and a unimolecular dissociation of the hot radical on the ground electronic state after internal conversion from the Rydberg states.

Ultraviolet photochemistry of ethane: implications for the atmospheric chemistry of the gas giants

Chemical processing in the stratospheres of the gas giants is driven by incident vacuum ultraviolet (VUV) light. Ethane is an important constituent in the atmospheres of the gas giants in our solar system. The present work describes translational spectroscopy studies of the VUV photochemistry of ethane using tuneable radiation in the wavelength range 112 ≤ λ ≤ 126 nm from a free electron laser and event-triggered, fast-framing, multi-mass imaging detection methods. Contributions from at least five primary photofragmentation pathways yielding CH2, CH3 and/or H atom products are demonstrated and interpreted in terms of unimolecular decay following rapid non-adiabatic coupling to the ground state potential energy surface. These data serve to highlight parallels with methane photochemistry and limitations in contemporary models of the photoinduced stratospheric chemistry of the gas giants. The work identifies additional photochemical reactions that require incorporation into next generation extraterrestrial atmospheric chemistry models which should help rationalise hitherto unexplained aspects of the atmospheric ethane/acetylene ratios revealed by the Cassini-Huygens fly-by of Jupiter.

The 3s versus 3p Rydberg state photodissociation dynamics of the ethyl radical

The photodissociation dynamics of the ethyl radical following excitation into the 3s and 3p Rydberg states are revisited in a joint experimental and theoretical study. Two different methods to produce the ethyl radical, pyrolysis and in situ photolysis, are employed in order to modify the initial ro-vibrational energy distribution characterizing the ethyl radical beam. H-atom velocity map images following excitation of the radical at 243 nm and at 201 nm are presented and discussed along with ab initio potential energy curves focussing on the bridged C2v geometry. The reported results show that the dynamics following excitation to the 3s Rydberg state is insensitive to the initial internal energy of the parent radical, in contrast to the dynamics on the 3p Rydberg state, which is strongly modified. The role of the bridged C2v geometry on both photodynamics is highlighted and discussed.

Site-specific hydrogen-atom elimination in photoexcited ethyl radical

The photochemistry of the ethyl radical following excitation to the 3p Rydberg state is investigated in a joint experimental and theoretical study. Velocity map images for hydrogen atoms detected from photoexcited isotopologues CH3CH2, CH3CD2 and CD3CH2 at ∼201 nm, are discussed along with high-level ab initio electronic structure calculations of potential energy curves and non-adiabatic coupling matrix elements (NACME). A novel mechanism governed by a conical intersection allowing prompt site-specific hydrogen-atom elimination is presented and discussed. For this mechanism to occur, an initial ro-vibrational excitation is allocated to the radical permitting to access this reaction pathway and thus to control the ethyl photochemistry. While hydrogen-atom elimination from cold ethyl radicals occurs through internal conversion into lower electronic states followed by slow statistical dissociation, prompt site-specific Cα elimination into CH3CH + H, occurring through a fast non-adiabatic crossing to a valence bound state followed by dissociation through a conical intersection, is accessed by means of an initial ro-vibrational energy content into the radical. The role of a particularly effective vibrational promoting mode in this prompt photochemical reaction pathway is discussed.

Photoinduced C–H bond fission in prototypical organic molecules and radicals

We survey and assess current knowledge regarding the primary photochemistry of hydrocarbon molecules and radicals.

Ab initio studies on the photodissociation dynamics of the 1,1-difluoroethyl radical

Born-Oppenheimer molecular dynamics trajectory calculations at the HCTH147/6-31G** level of theory simulate the dissociation dynamics of photolytically excited 1,1-difluoroethyl radicals. EOMCCSD/AUG-cc-pVDZ calculations show that an excitation energy of 94.82 kcal/mol is necessary to initiate photodissociation reactions. In contrast to photodissociation dynamics of ethyl radicals where a large discrepancy between actual dissociation rates and rates that are predicted by statistical rate theories, we find reaction rates of 5.1 × 10¹¹ s^-1 for the dissociation of an H atom, which is in perfect accord with what is predicted by Rice-Ramsperger-Kassel-Marcus (RRKM) calculations and there is no indication of any nonstatistical effects. However, our trajectory calculations show a much larger fraction of C-C bond breakage reaction of 56% occurring than that expected by RRKM (only 16%).

Isomer Identification in Flames with Double-Imaging Photoelectron/Photoion Coincidence Spectroscopy (i2PEPICO) using Measured and Calculated Reference Photoelectron Spectra

Double-imaging photoelectron/photoion coincidence (i2PEPICO) spectroscopy using a multiplexing, time-efficient, fixed-photon-energy approach offers important opportunities of gas-phase analysis. Building on successful applications in combustion systems that have demonstrated the discriminative power of this technique, we attempt here to push the limits of its application further to more chemically complex combustion examples. The present investigation is devoted to identifying and potentially quantifying compounds featuring five heavy atoms in laminar, premixed low-pressure flames of hydrocarbon and oxygenated fuels and their mixtures. In these combustion examples from flames of cyclopentene, iso-pentane, iso-pentane blended with dimethyl ether (DME), and diethyl ether (DEE), we focus on the unambiguous assignment and quantitative detection of species with the sum formulae C5H6, C5H7, C5H8, C5H10, and C4H8O in the respective isomer mixtures, attempting to provide answers to specific chemical questions for each of these examples. To analyze the obtained i2PEPICO results from these combustion situations, photoelectron spectra (PES) from pure reference compounds, including several examples previously unavailable in the literature, were recorded with the same experimental setup as used in the flame measurements. In addition, PES of two species where reference spectra have not been obtained, namely 2-methyl-1-butene (C5H10) and the 2-cyclopentenyl radical (C5H7), were calculated on the basis of high-level ab initio calculations and Franck-Condon (FC) simulations. These reference measurements and quantum chemical calculations support the early fuel decomposition scheme in the cyclopentene flame towards 2-cyclopentenyl as the dominant fuel radical as well as the prevalence of branched intermediates in the early fuel destruction reactions in the iso-pentane flame, with only minor influences from DME addition. Furthermore, the presence of ethyl vinyl ether (EVE) in DEE flames that was predicted by a recent DEE combustion mechanism could be confirmed unambiguously. While combustion measurements using i2PEPICO can be readily obtained in isomer-rich situations, we wish to highlight the crucial need for high-quality reference information to assign and evaluate the obtained spectra.

The atmospheric oxidation of CH3OOH by the OH radical: the effect of water vapor

The relative humidity can enhance the atmospheric oxidation of CH₃OOH by OH into CH₃O₂ + H₂O up to 19% whereas the formation of H₂CO + OH + H₂O is enhanced up to 5% only under the same conditions.

Photodissociation dynamics of cyclopropenylidene, c-C3 H2

In this joint experimental and theoretical study we characterize the complete dynamical "life cycle" associated with the photoexcitation of the singlet carbene cyclopropenylidene to the lowest lying optically bright excited electronic state: from the initial creation of an excited-state wavepacket to the ultimate fragmentation of the molecule on the vibrationally hot ground electronic state. Cyclopropenylidene is prepared in this work using an improved synthetic pathway for the preparation of the precursor quadricyclane, thereby greatly simplifying the assignment of the molecular origin of the measured photofragments. The excitation process and subsequent non-adiabatic dynamics have been previously investigated employing time-resolved photoelectron spectroscopy and are now complemented with high-level ab initio trajectory simulations that elucidate the specific vibronic relaxation pathways. Lastly, the fragmentation channels accessed by the molecule following internal conversion are probed using velocity map imaging (VMI) so that the identity of the fragmentation products and their corresponding energy distributions can be definitively assigned.

Photodissociation dynamics of propargylene, HCCCH

We report a joint theoretical and experimental study on the photodissociation of the C₃H₂ isomer propargylene, HCCCH, combining velocity map imaging with nonadiabatic surface hopping calculations.

Roaming Dissociation of Ethyl Radicals

Previous studies on the photodissociation of C2H5 reported rate constants for H-atom formation several orders of magnitude smaller than that predicted by Rice-Ramsperger-Kassel-Marcus (RRKM) theory. This Letter provides a potential explanation for this anomaly, based on direct trajectory calculations of C2H5 dissociation. The trajectories reveal the existence of a roaming dissociation channel that leads to the formation of C2H3 and H2. This channel is found to proceed over the ridge between the transition state of H-atom elimination and that of bimolecular H-abstraction. The formed C2H3 radical can subsequently dissociate to C2H2 and a H atom; this secondary dissociation is suggested to be a potential reason for the unexpectedly slow H-atom formation observed in the photodissociation experiments.

A classical trajectory study of the dissociation and isomerization of C2H5

Motivated by photodissociation experiments in which non-RRKM nanosecond lifetimes of the ethyl radical were reported, we have performed a classical trajectory study of the dissociation and isomerization of C2H5 over the energy range 100-150 kcal/mol. We used a customized version of the AIREBO semiempirical potential (Stuart, S. J.; et al. J. Chem. Phys. 2000, 112, 6472-6486) to more accurately describe the gas-phase decomposition of C2H5. This study constitutes one of the first gas-phase applications of this potential form. At each energy, 10,000 trajectories were run and all underwent dissociation in less than 100 ps. The calculated dissociation rate constants are consistent with RRKM models; no evidence was found for nanosecond lifetimes. An analytic kinetics model of isomerization/dissociation competition was developed that incorporated incomplete mode mixing through a postulated divided phase space. The fits of the model to the trajectory data are good and represent the trajectory results in detail through repeated isomerizations at all energies. The model correctly displays single exponential decay at lower energies, but at higher energies, multiexponential decay due to incomplete mode mixing becomes more apparent. At both ends of the energy range, we carried out similar trajectory studies on CD2CH3 to examine isotopic scrambling. The results largely support the assumption that a H or a D atom is equally likely to dissociate from the mixed-isotope methyl end of the molecule. The calculated fraction of products that have the D atom dissociation is ∼20%, twice the experimental value available at one energy within our range. The calculated degree of isotopic scrambling is non-monotonic with respect to energy due to a non-monotonic ratio of the isomerization to dissociation rate constants.