arXiv subjects

Kevin M. Hickson

Publications and source records attributed to Kevin M. Hickson.

At least 19 recordsLinked to original sources

arXiv2026

Kinetic and H-atom product study of the C + C6H6 reaction at low temperatures

The reactions of atomic carbon in its ground electronic state configuration, C(3P), are potentially important processes in astrochemistry due to the large abundance of C(3P) atoms in the interstellar medium (ISM) and its high overall reactivity towards a wide range of molecules. Although benzene, C6H6, has not been detected in the dense ISM, its presence at high abundance levels is inferred through the detection of functionalized derivatives. Here we present a combined experimental and astrochemical modeling investigation of the gas-phase C(3P) + C6H6 reaction. Experimentally, rate constants were determined over the 50-296 K range using the Laval nozzle technique coupled with pulsed laser photolysis and laser induced fluorescence for C(3P) generation and detection respectively. Product yields of atomic hydrogen, H(2S) were also measured at 177 and 296 K to provide some information on the product channels of the reaction. The measured rate constants are very large, between 3.3 and 5.7 x 10-10 cm3 s-1 indicating the fast, barrierless nature of the reaction, while the H-atom yields are all below 10 % when compared to the C(3P) + C2H4 reference reaction. As the C(3P) + C6H6 reaction is not currently included in astrochemical databases, its influence on the simulated abundances of C6H6 and related species was tested using a gas-grain model of dense interstellar clouds. The C(3P) + C6H6 reaction is shown to be the main loss process for interstellar benzene, while different hypotheses regarding the product channels are discussed in the context of observations to shed light on the preferred formation pathways.

astro-ph.GA↗

arXiv2026

The Reaction between Atomic Carbon and Molecular Nitrogen as a Source of Cyanamide and Carbodiimide on Interstellar Ices

Reactions occurring on the ice-covered surfaces of interstellar dust grains are considered to be among the most important sources of complex species in the interstellar medium. Despite this, molecules such as cyanamide, NH2CN, are largely underpredicted by current astrochemical models suggesting that the network of reactions currently used to describe this species and its tautomer carbodiimide, HNCNH, are incomplete. Here, we performed a theoretical investigation of the reaction of ground state atomic carbon C(3P) with molecular nitrogen N2 in both the gas-phase and on the surface of amorphous solid water (ASW) clusters to examine its potential importance in the formation of NH2CN and HNCNH. We show that the reaction of gas-phase C-atoms with N2 molecules already present on the ASW surface results in the barrierless formation of CNN. Following exothermic hydrogenation reactions, the N-N bond of the C-N-N bearing intermediates is broken allowing the formation of molecules with N-C-N backbones through cyclic intermediates over low barriers. To test the importance of these processes to NH2CN and HNCNH formation, these reactions were included in a three-phase astrochemical model of low-mass protostellar evolution employing a reaction network that was updated to better describe the formation and destruction pathways of related small nitrogen bearing molecules. These simulations demonstrate that the ice surface reaction between C and N2 represents by far the dominant source of NH2CN and HNCNH in protostellar environments and in dense clouds.

astro-ph.GA↗

arXiv2025

A Gas-Phase Kinetic Study of the N(2D) + CH3CCH and N(2D) + CH3CN Reactions

The chemistry of planetary atmospheres containing molecular nitrogen as a major atmospheric component is strongly influenced by the reactions of atomic nitrogen. Although nitrogen atoms in their ground electronic state N(4S) are mostly unreactive towards stable molecules, electronically excited nitrogen atoms N(2D) are much more reactive and could play an important role in the formation of nitriles and other nitrogen bearing organic molecules in planetary atmospheres such as Titan. Despite this, few kinetic studies of N(2D) reactions have been performed over the appropriate low temperature range. Here, we report the results of an experimental study of the reactions N(2D) + methylacetylene, CH3CCH, and N(2D) + acetonitrile, CH3CN, using a supersonic flow reactor at selected temperatures between 50 K and 296 K. N(2D) atoms, which were generated indirectly as a product of the C(3P) + NO reaction, were subsequently detected by laser induced fluorescence in the vacuum ultraviolet wavelength region. The measured rate constants are significantly larger than the estimated values in current photochemical models and do not display large variations as a function of temperature. The new rate constants are included in a 1D coupled ion-neutral model of Titans atmosphere to test their influence on the simulated species abundances. In addition, the overall description of both reactions is improved by considering the results of recent experimental and theoretical work examining the product channels of these processes. These simulations indicate that while the N(2D) + CH3CCH reaction has only a limited overall influence on Titans atmospheric chemistry, the N(2D) + CH3CN reaction could lead to the formation of significant relative abundances of cyanomethamine, HNCHCN, in the upper atmosphere.

astro-ph.EP↗

arXiv2025

Rate constants and product yields for the C + CH3CHO reaction at low temperatures

Reactions involving atomic carbon in its ground electronic state, C(3P), play an important role in astrochemistry due to high C-atom abundance levels. Here we performed a kinetic investigation of the reaction between C(3P) and acetaldehyde, CH3CHO, determining rate constants for this process over the 50-296 K range. Measurements of the formation of atomic hydrogen, H(2S), were also performed to provide insight into product formation. Experiments were conducted using a supersonic flow reactor coupled with pulsed laser photolysis for C-atom generation and pulsed laser induced fluorescence in the vacuum ultraviolet range for the detection of both C(3P) and H(2S) atoms. Quantum chemical calculations of the ground triplet state potential energy surface of C3H4O were also performed to provide theoretical support for the measurements. The rate constants were large and temperature independent with an average value of 4.0 x 10-10 cm3 s-1. This result is consistent with the theoretical results which predict either very low barriers or none at all on the underlying potential energy surface. Although experimental difficulties prevented the quantitative determination of H-atom formation, qualitatively, H-atom yields were very low with CH3CH/C2H4 + CO as the major products based on the calculations. The influence of this reaction on interstellar chemistry was tested using a gas-grain model of dense interstellar clouds. These simulations predict that the C(3P) + CH3CHO reaction decreases gas-phase CH3CHO abundances by more than an order of magnitude at early and intermediate cloud ages, with a lower influence at typical dense cloud ages.

astro-ph.GA↗

arXiv2024

Kinetic Study of the Reactions of Ground State Atomic Carbon and Oxygen with Nitrogen Dioxide over the 50-296 K Temperature Range

The kinetics of the reactions of nitrogen dioxide, NO$_2$, with atomic oxygen and atomic carbon in their ground triplet states ($^3$P) have been studied at room temperature and below using a supersonic flow (Laval nozzle) reactor. O($^3$P) and C($^3$P) atoms (hereafter O and C respectively) were created in-situ by the pulsed laser photolysis of the precursor molecules NO$_2$ at 355 nm and CBr$_4$ at 266 nm respectively. While the progress of the O + NO$_2$ reaction was followed by detecting O atoms by a chemiluminescent tracer method, progress of the C + NO$_2$ reaction was followed by detecting C atoms directly by vacuum ultra violet laser induced fluorescence at 116 nm. The measured rate constants for the O + NO$_2$ reaction are found to be in excellent agreement with earlier work at higher temperatures and extend the available kinetic data for this process down to 50 K. The present work represents the first kinetics study of the C + NO$_2$ reaction. Although both reactions display rate constants that increase as the temperature falls, a more substantial rate increase is observed for the O + NO$_2$ reaction. The effects of these reactions on the simulated abundances of interstellar NO$_2$ and related compounds were tested using a gas-grain model of the dense interstellar medium, employing expressions for the rate constants of the form, $k(T) = α(T/300)^β$, with $α= 1 \times 10^{-11}$ cm$^3$ s$^{-1}$ and $β= -0.65$ for the O + NO$_2$ reaction and $α= 2 \times 10^{-10}$ cm$^3$ s$^{-1}$ and $β= -0.11$ for the C + NO$_2$ reaction. Although these simulations predict that gas-phase NO$_2$ abundances are low in dense interstellar clouds, NO$_2$ abundances on interstellar dust grains are predicted to reach reasonably high levels, indicating the potential for detection of this species in warmer regions.

astro-ph.GA↗

arXiv2024

Low temperature formation of pyridine and (iso)quinoline via neutral neutral reactions

Aromatic molecules represent fundamental building blocks in prebiotic chemistry and are contemplated as vital precursors to DNA and RNA nitrogen bases. However, despite the identification of some 300 molecules in extraterrestrial environments, the pathways to pyridine (C5H5N), pyridinyl (C5H4N), and (iso)quinoline (C9H7N) the simplest representative of mono and bicyclic aromatic molecule carrying nitrogen are elusive. Here, we afford compelling evidence on the gas phase formation of methylene amidogen (H2CN) and cyanomethyl (H2CCN) radicals via molecular beam studies and electronic structure calculations. The modeling of the chemistries of Taurus Molecular Cloud (TMC 1) and Titans atmosphere contemplates a complex chain of reactions synthesizing pyridine, pyridinyl, and (iso)quinoline from H2CN and H2CCN at levels of up to 75%. This study affords unique entry points to precursors of DNA and RNA nitrogen bases in hydrocarbon rich extraterrestrial environments thus changing the way we think about the origin of prebiotic molecules in our Galaxy.

physics.chem-ph↗

arXiv2024

A Low Temperature Kinetic Study of the C(3P) + CH3OCH3 Reaction. Rate constants, H-atom Product Yields and Astrochemical Implications

Atomic carbon in its ground electronic state, C(3P), is expected to be present at high abundances during the evolution of dense molecular clouds. Consequently, its reactions with other interstellar species could have a strong influence on the chemical composition of these regions. Here, we report the results of an investigation of the reaction between C(3P) and dimethylether, CH3OCH3, which was recently detected in dark cloud TMC-1. Experiments were performed to study the kinetics of this reaction using a continuous supersonic flow reactor employing pulsed laser photolysis and pulsed laser induced fluorescence for atomic radical generation and detection respectively. Rate constants for this process were measured between 50 K and 296 K, while additional measurements of the product atomic hydrogen yields were also performed over the 75-296 K range. To better understand the experimental results, statistical rate theory was used to calculate rate constants over the same temperature range and to provide insight on the major product channels. These simulations, based on quantum chemical calculations of the ground triplet state of the C3H6O molecule, allowed us to obtain the most important features of the underlying potential energy surface. The measured rate constant increases as the temperature falls, reaching a value of k_(C+CH_3 OCH_3 )= 7.5 x 10-11 cm3 s-1 at 50 K, while the low measured H-atom yields support the theoretical prediction that the major reaction products are CH3 + CH3 + CO. The effects of this reaction on the abundances of interstellar CH3OCH3 and related species were tested using a gas-grain dense cloud model, employing an expression for the rate constant, k(T) = alpha(T/300)^beta, with alpha = 1.27 x 10-11 and beta = -1.01. These simulations predict that the C(3P) + CH3OCH3 reaction decreases gas-phase CH3OCH3 abundances by more than an order of magnitude at early times.

astro-ph.GA↗

arXiv2023

Kinetic Study of the Gas-Phase Reaction between Atomic Carbon and Acetone. Low Temperature Rate Constants and Hydrogen Atom Product Yields

The reactions of ground state atomic carbon, C(3P), are likely to be important in astrochemistry due to the high abundance levels of these atoms in the dense interstellar medium. Here we present a study of the gas-phase reaction between C(3P) and acetone, CH3COCH3. Experimentally, rate constants were measured for this process over the 50 to 296 K range using a continuous-flow supersonic reactor, while secondary measurements of H(2S) atom formation were also performed over the 75 to 296 K range to elucidate the preferred product channels. C(3P) atoms were generated by In-situ pulsed photolysis of carbon tetrabromide, while both C(3P) and H(2S) atoms were detected by pulsed laser induced fluorescence. Theoretically, quantum chemical calculations were performed to obtain the various complexes, adducts and transition states involved in the C(3P) + CH3COCH3 reaction over the 3A'' potential energy surface, allowing us to better understand the reaction pathways and help to interpret the experimental results. The derived rate constants are large, (2-3) x 10-10 cm3 s-1 , displaying only weak temperature variations; a result that is consistent with the barrierless nature of the reaction. As this reaction is not present in current astrochemical networks, its influence on simulated interstellar acetone abundances is tested using a gas-grain dense interstellar cloud model. For interstellar modelling purposes, the use of a temperature independent value for the rate constant, k(C+CH3COCH3 )= 2.2 x 10-10 cm3 s-1, is recommended. The C(3P) + CH3COCH3 reaction decreases gas-phase CH3COCH3 abundances by as much as two orders of magnitude at early and intermediate cloud ages.

astro-ph.GA↗

arXiv2023

Low-Temperature Kinetics for the N + NO reaction: Experiment Guides the Way

The reaction N(4S) + NO -> O(3P) + N2 plays a pivotal role in the conversion of atomic to molecular nitrogen in dense interstellar clouds and in the atmosphere. Here we report a joint experimental and computational investigation of the N + NO reaction with the aim of providing improved constraints on its low temperature reactivity. Thermal rates were measured over the 50 to 296 K range in a continuous supersonic flow reactor coupled with pulsed laser photolysis and laser induced fluorescence for the production and detection of N(4S) atoms, respectively. With decreasing temperature, the experimentally measured reaction rate was found to monotonously increase up to a value of (6.6 +- 1.3) x 10-11 cm3 s-1 at 50 K. To confirm this finding, quasi-classical trajectory simulations were carried out on a previously validated, full-dimensional potential energy surface (PES). However, around 50 K the computed rates decreased which required re-evaluation of the reactive PES in the long-range part due to a small spurious barrier with height 40 K in the entrance channel. By exploring different correction schemes the measured thermal rates can be adequately reproduced, displaying a clear negative temperature dependence over the entire temperature range. The possible astrochemical implications of an increased reaction rate at low temperature are also discussed.

astro-ph.GA↗

arXiv2022

An Experimental and Theoretical Investigation of the Gas-Phase C(3P) + N2O Reaction. Low Temperature Rate Constants and Astrochemical Implications

The reaction between atomic carbon in its ground electronic state, C(3P), and nitrous oxide, N2O, has been studied below room temperature due to its potential importance for astrochemistry, with both species considered to be present at high abundance levels in a range of interstellar environments. On the experimental side, we measured rate constants for this reaction over the 50-296 K range using a continuous supersonic flow reactor. C(3P) atoms were generated by the pulsed photolysis of carbon tetrabromide at 266 nm and were detected by pulsed laser induced fluorescence at 115.8 nm. Additional measurements allowing the major product channels to be elucidated were also performed. On the theoretical side, statistical rate theory was used to calculate low temperature rate constants. These calculations employed the results of new electronic structure calculations of the 3A" potential energy surface of CNNO and provided a basis to extrapolate the measured rate constants to lower temperatures and pressures. The rate constant was found to increase monotonically as the temperature falls, reaching a value of k(C(3P)+N2O)(50 K) = (7.9 +- 0.8) x 10-11 cm3 s-1 at 50 K. As current astrochemical models do not include the C + N2O reaction, we tested the influence of this process on interstellar N2O and other related species using a gas-grain model of dense interstellar clouds. These simulations predict that N2O abundances decrease significantly at intermediate times (10^3 - 10^5 years) when gas-phase C(3P) abundances are high.

astro-ph.GA↗

arXiv2021

A kinetic study of the gas-phase C(3P) + CH3CN reaction at low temperature. Rate constants, H-atom product yields and astrochemical implications

Rate constants have been measured for the C(3P) + CH3CN reaction between 50 K and 296 K using a continuous-flow supersonic reactor. C(3P) atoms were created by the in-situ pulsed laser photolysis of CBr4 at 266 nm, while the kinetics of C(3P) atom loss were followed by direct vacuum ultra-violet laser induced fluorescence at 115.8 nm. Secondary measurements of product H(2S) atom formation were also made, allowing absolute H-atom yields to be obtained by comparison with those obtained for the C(3P) + C2H4 reference reaction. In parallel, quantum chemical calculations were performed to obtain the various complexes, adducts and transition states relevant to the title reaction over the triplet potential energy surface, allowing us to better understand the preferred reaction pathways. The reaction is seen to be very fast, with measured rate constants in the range (3-4) x 10-10 cm3 s-1 with little or no observed temperature dependence. As the C + CH3CN reaction is not considered in current astrochemical networks, we test its influence on interstellar methyl cyanide abundances using a gas-grain dense interstellar cloud model. Its inclusion leads to predicted CH3CN abundances that are significantly lower than the observed ones.

astro-ph.GA↗

arXiv2020

Chemical nitrogen fractionation in dense molecular clouds

Nitrogen-bearing molecules display variable isotopic fractionation levels in different astronomical environments such as in the interstellar medium or in the Solar System. Models of interstellar chemistry are unable to induce nitrogen fraction in cold molecular clouds as exchange reactions for 15N are mostly inefficient. Here, we developed a new gas-grain model for nitrogen fractionation including a thorough search for new nitrogen fractionation reactions and a realistic description of atom depletion onto interstellar dust particles. We show that, while dense molecular cloud gas-phase chemistry alone leads to very low fractionation, 14N atoms are preferentially depleted from the gas-phase due to a mass dependent grain surface sticking rate for atomic nitrogen. However, assuming an elementary 14N/15N ratio of 441 (equal to the solar wind value), our model leads to only low 15N enrichment for all N-containing species synthesized in the gas-phase with predicted 14N/15N ratios in the range 360-400. Higher enrichment levels can neither be explained by this mechanism, nor through chemistry, with two possible explanations. (I) The elementary 14N/15N ratio in the local ISM is smaller, as suggested by the recent work of Romano et al, with an hypothetic 15NNH+ and 15NNH+ depletion due to variation of the electronic recombination rate constant variation with the isotopes. (II) N2 photodissociation leads to variable nitrogen fractionation in diffuse molecular clouds where photons play an important role, which is conserved during dense molecular cloud formation as suggested by the work of Furuya & Aikawa.

astro-ph.GA↗

arXiv2020

Tunneling Enhancement of the Gas-Phase CH + CO2 Reaction at Low Temperature

The rates of numerous activated reactions between neutral species increase at low temperatures through quantum mechanical tunneling of light hydrogen atoms. Although tunneling processes involving molecules or heavy atoms are well known in the condensed phase, analogous gas-phase processes have never been demonstrated experimentally. Here, we studied the activated CH + CO2 -> HCO + CO reaction in a supersonic flow reactor, measuring rate constants that increase rapidly below 100 K. Mechanistically, tunneling is shown to occur by CH insertion into the C-O bond, with rate calculations accurately reproducing the experimental values. To exclude the possibility of H-atom tunneling, CD was used in additional experiments and calculations. Surprisingly, the equivalent CD + CO2 reaction accelerates at low temperature as zero point energy effects remove the barrier to product formation. In conclusion, heavy-particle tunneling effects might be responsible for the observed reactivity increase at lower temperatures for the CH + CO2 reaction, while the equivalent effect for the CD + CO2 reaction results instead from a submerged barrier with respect to reactants.

astro-ph.GA↗

arXiv2020

A Kinetic Study of the N(2D) + C2H4 Reaction at Low Temperature

Electronically excited nitrogen atoms N(2D) are important species in the photochemistry of N2 based planetary atmospheres such as Titan. Despite this, few N(2D) reactions have been studied over the appropriate low temperature range. During the present work, rate constants were measured for the N(2D) + ethene (C2H4) reaction using a supersonic flow reactor at temperatures between 50 K and 296 K. Here, a chemical reaction was used to generate N(2D) atoms, which were detected directly by laser induced fluorescence in the vacuum ultraviolet wavelength region. The measured rate constants displayed very little variation as a function of temperature, with substantially larger values than those obtained in previous work. Indeed, considering an average temperature of 170 K for the atmosphere of Titan leads to a rate constant that is almost seven times larger than the currently recommended value. In parallel, electronic structure calculations were performed to provide insight into the reactive process. While earlier theoretical work at a lower level predicted the presence of a barrier for the N(2D) + C2H4 reaction, the present calculations demonstrate that two of the five doublet potential energy surfaces correlating with reagents are likely to be attractive, presenting no barriers for the perpendicular approach of the N atom to the carbon double bond of ethene. The measured rate constants and new product channels taken from recent dynamical investigations of this process are included in a 1D coupled ion-neutral model of Titans atmosphere. These simulations indicate that the modeled abundances of numerous nitrogen bearing compounds are noticeably affected by these changes.

physics.chem-ph↗

arXiv2020

Experimental and Theoretical Studies of the N(2D) + H2 and D2 Reactions

This study reports the results of an experimental and theoretical investigation of the N(2D) + H2 and N(2D) + D2 reactions at room temperature and below. On the experimental side, a supersonic flow (Laval nozzle) reactor was employed to measure rate constants for these processes at temperatures as low as 127 K. N(2D) was produced indirectly by pulsed laser photolysis and these atoms were detected directly by pulsed laser induced fluorescence in the vacuum ultraviolet wavelength region. On the theoretical side, two different approaches were used to calculate rate constants for these reactions; a statistical quantum mechanical (SQM) method and a quasi-classical trajectory capture model including a semi-classical correction for tunneling (SC-Capture). This work is described in the context of previous studies, while the discrepancies between both experiment and theory, as well as between the theoretical results themselves are discussed.

physics.chem-ph↗

arXiv2020

Gas-grain model of carbon fractionation in dense molecular clouds

Carbon containing molecules in cold molecular clouds show various levels of isotopic fractionation through multiple observations. To understand such effects, we have developed a new gas-grain chemical model with updated 13C fractionation reactions (also including the corresponding reactions for 15N, 18O and 34S). For chemical ages typical of dense clouds, our nominal model leads to two 13C reservoirs: CO and the species that derive from CO, mainly s-CO and s-CH3OH, as well as C3 in the gas phase. The nominal model leads to strong enrichment in C3, c-C3H2 and C2H in contradiction with observations. When C3 reacts with oxygen atoms the global agreement between the various observations and the simulations is rather good showing variable 13C fractionation levels which are specific to each species. Alternatively, hydrogen atom reactions lead to notable relative 13C fractionation effects for the two non-equivalent isotopologues of C2H, c-C3H2 and C2S. As there are several important fractionation reactions, some carbon bearing species are enriched in 13C, particularly CO, depleting atomic 13C in the gas phase. This induces a 13C depletion in CH4 formed on grain surfaces, an effect that is not observed in the CH4 in the solar system, in particular on Titan. This seems to indicate a transformation of matter between the collapse of the molecular clouds, leading to the formation of the protostellar disc, and the formation of the planets. Or it means that the atomic carbon sticking to the grains reacts with the species already on the grains giving very little CH4.

astro-ph.GA↗

arXiv2019

Oxygen fractionation in dense molecular clouds

We have developed the first gas-grain chemical model for oxygen fractionation (also including sulphur fractionation) in dense molecular clouds, demonstrating that gas-phase chemistry generates variable oxygen fractionation levels, with a particularly strong effect for NO, SO, O2, and SO2. This large effect is due to the efficiency of the neutral 18O + NO, 18O + SO, and 18O + O2 exchange reactions. The modeling results were compared to new and existing observed isotopic ratios in a selection of cold cores. The good agreement between model and observations requires that the gas-phase abundance of neutral oxygen atoms is large in the observed regions. The S16O/S18O ratio is predicted to vary substantially over time showing that it can be used as a sensitive chemical proxy for matter evolution in dense molecular clouds.

astro-ph.GA↗

arXiv2016

An experimental and theoretical investigation of the C(1D) + D2 reaction

In a previous joint experimental and theoretical study of the barrierless chemical reaction C(1D) + H2 at low temperatures (300-50 K) [K. M. Hickson, J.-C. Loison, H. Guo, Y. V. Suleimanov, J. Phys. Chem. Lett., 2015, 6, 4194.], excellent agreement was found between experimental thermal rate constants and theoretical estimates based on ring polymer molecular dynamics (RPMD) over the two lowest singlet potential energy surfaces (PESs). Here, we extend this work to one of its deuterated counterparts, C(1D) + D2, over the same temperature range. Experimental and RPMD results are in very good agreement when contributions from both PESs to this chemical reaction are included in the RPMD simulations. The deviation between experiment and the RPMD calculations does not exceed 25 % and both results exhibit a slight negative temperature dependence. The first excited 1A" PES plays a more important role than the ground 1A' PES as the temperature is decreased, similar to our previous studies of the C(1D) + H2 reaction but with a more pronounced effect. The small differences in temperature dependence between the earlier and present experimental studies of C(1D) + H2/D2 reactions are discussed in terms of the use of non-equilibrium populations of ortho/para-H2/D2. We argue that RPMD provides a very convenient and reliable tool to study low-temperature chemical reactions.

physics.chem-ph↗