Hot Atom Reactions Research Articles

Gas phase synthesis of 4d transition metal carbonyl complexes with thermalized fission fragments in single-atom reactions

Abstract The formation of carbonyl complexes using atom-at-a-time quantities of short-lived transition metals from fusion and fission reactions was reported in 2012. Numerous studies focussing on this chemical system, which is also applicable for the superheavy elements followed. We report on a novel two-chamber approach for the synthesis of such complexes that allows spatial decoupling of thermalization and gas-phase carbonyl complex synthesis. Neutron induced fission on 235U and spontaneous fission of 248Cm were employed for the production of the fission products. These were stopped inside a gas volume behind the target and flushed with an inert-gas flow into a second chamber. This was flushed with carbon monoxide to allow the gas-phase synthesis of carbonyl complexes. Parameter studies of the transfer from the first into the second chamber as well as on the carbonyl complex formation and transport processes have been performed. High overall efficiencies of more than 50% were reached rendering this approach interesting for studies of superheavy elements. Our results show that carbonyl complex formation of thermalized fission products is a single-atom reaction, and not a hot-atom reaction.

Mechanisms of Formaldehyde and C2 Formation from Methylene Reacting with CO2 Adsorbed on Ni(110)

Methylene (CH2) is thought to play a significant role as a reaction intermediate in the catalysis of methane dry reforming as well as in converting synthesis gas to light olefins via Fischer–Tropsch synthesis. Here, we report high quality Born–Oppenheimer molecular dynamics (BOMD) simulations of the reaction mechanisms associated with CH2 impinging on a Ni(110) surface with CO2 adsorbed at 0.33 ML coverage. The results show the formation of formaldehyde, carbon monoxide, C2 species such as H2C–CO2, and others. Furthermore, we provide real-time demonstration of both Eley–Rideal (ER) and hot atom (HA) reaction mechanisms. The ER mechanism mostly happens when CH2 directly collides with an oxygen of CO2, while CH2 attacks the carbon of CO2, dominantly following the HA mechanism. If CH2 reaches the Ni surface, it can easily break one C–H bond to form CH and H on the surface. The mechanistic details of H2CO, H/CO, C2, and H/CH formation are illuminated through the study of bond breaking/formation, charge transfer, and spin density of the reactants and catalytic surface. This illuminates the key contribution of geometry and electronic structure of catalytic surface to the reaction selectivity. Moreover, we find that 3CH2 switches to surfaces of 1CH2 character as soon as the methylene and nickel/CO2 orbitals show significant interaction, and as a result the reactivity is dominated by low barrier mechanisms. Overall, the BOMD simulations provide dynamical information that allows us to monitor details of the reaction mechanisms, confirming and extending current understanding of CH2 radical chemistry in the dry reforming of methane and Fischer–Tropsch synthesis.

Formation of lithium-tritide by hot atom reactions of tritium produced in Pb-16Li

The release behaviors of hydrogen isotopes in Pb-16Li introduced by thermal gas exposure or produced by thermal neutron irradiation were compared to investigate hot atom reactions of tritium. The solubility of hydrogen isotope was evaluated to be S=6.56×10−7 exp(−0.11 [eV]/kT) [at. fr, Pa0.5], which is consistent with the literature values. The tritium TDS spectra for Pb-16Li with thermal neutron irradiation showed that about 5% of tritium was released at single release stage around 600K, which was higher than the melting point of Pb-16Li, although no release peak was found at around 600K for the hydrogen isotope-doped Pb-16Li. The kinetic analysis indicated that the tritium release would be associated with the dissociation of Li-T bond, indicating that LiT was formed in Pb-16Li by thermal neutron irradiation, suggesting that the formation of Li-T bonds should be considered to estimate tritium retention in Pb-16Li eutectic blanket systems in fusion reactors.

Vibrational temperature of the adlayer in the ‘hot-atom’ reaction mechanism

The hot-atom reaction mechanism brings about reaction rates several orders of magnitude higher than those expected in the case of adatoms which have thermalized with the surface. This paper addresses the issue of a possible thermodynamic characterization of the adlayer under reactive conditions and at the steady state. In turn, this implies having to determine the temperature of the adatoms. This is done by means of a nonequilibrium statistical thermodynamic approach, by exploiting a suitable definition of the entropy. The interplay between reaction rate, vibrational temperature of the adatoms and adsorbed quantities is highlighted. This paper shows that the vibrational temperature depends on reaction rate, logarithmically and exhibits a non-linear scaling on physical quantities linked to the energetics of the reaction, namely the adsorption energy and the binding energy of the molecule. The present modeling is also discussed in connection with response equations of nonequilibrium thermodynamics.

Quasiclassical studies of Eley–Rideal and hot-atom reactions on a surface at 94 K: H(D) → D(H) + Cu(1 1 1)

Reactions and reaction dynamics of gas-phase H(or D) atoms with D(or H) atoms adsorbed onto a Cu(1 1 1) surface have been investigated by the quasi-classical molecular dynamics method. To simulate the H(D) → D(H) + Cu(1 1 1) system at a 94 K surface temperature, D(or H) adsorbates were disseminated arbitrarily on the surface of Cu(1 1 1) to form 0.50, 0.28 and 0.18 ML of coverages. The interaction of hydrogen atoms and the surface system is worked out by an LEPS function. LEPS parameters have been determined by using the total energy values which were calculated by a density functional theory (DFT) method and the generalized gradient approximation (GGA) for the exchange-correlation energy for various configurations of one and two hydrogen atoms on the Cu(1 1 1) surface. The Cu(1 1 1) surface, imitated by an embedded-atom method which is a many-body potential parameterized by Voter-Chen, is formed as a multilayer slab. The slab atoms are permitted to move. Various processes, trapping onto the surface, inelastic reflection of the incident projectile and penetration of the adsorbate or projectile atom into the slab, are examined. The dependence of these mechanisms on isotopic replacement has also been analyzed. Considerable contributions of the hot-atom pathways for the product formations are consequently observed. The rate of subsurface penetrations is obtained to be larger than the sticking rate onto the surface.

Nonequilibrium processes in reactions of hot tritium atoms with cooled solid targets. Attenuation of the flow of tritium atoms in adsorption layers of alkyltrimethylammonium bromides

The permeability for atomic tritium of adsorption layers of aqueous solutio ns of alkyltrimethylammonium bromides RN(CH3)3Br differing in the length of the hydrocarbon radical R (C12H25, C14H29, C16H33) was studied. The change in the radioactivity of alanine present in the examined solutions was used as the measure of the attenuation of the flow of reactive tritium atoms. The formation of an adsorption monolayer of surfactant molecules on the liquid-gas phase boundary caused the radioactivity of alanine to decrease by a factor of 6.5 (R = C12H25) to 8 (R = C16H33) relative to the experiment with a straight alanine solution. A correlation between the structure of the adsorbed layers of alkyltrimethylammonium bromides and their permeability to atomic tritium is discussed.

Nonequilibrium processes in reactions of hot tritium atoms with cooled solid targets. Influence of the atomizer temperature on formation of labeled substances

A system consisting of a cold target and “hot” atoms generated by dissociation of tritium on a tungsten wire was studied with the aim to determine conditions for preparing tritium-labeled organic compounds with the maximal radiochemical yield. The influence of the atomizer temperature on the result of the reaction of tritium atoms with amino acids and tetraalkylammonium bromides was studied; homological series of the substrates were examined with the aim to evaluate the contributions of functional groups and hydrocarbon tail to the processes occurring in the target. The dependence of the yield of the labeled parent compound on the atomizer temperature varied in the range 1600–2000 K was determined. The rates of decarboxylation and deamination sharply grew with increasing temperature of the tungsten wire. The highest yield of labeled amino acids was attained at an atomizer temperature of 1800–1900 K, and at higher temperature their yield decreased. The difference between the activation energies of the elimination of the carboxy and amino groups and of the isotope exchange of hydrogen for tritium in the C-H bond appeared to be 93 and 59 kJ mol−1, respectively. For alkyltrimethylammonium bromides with the alkyl radicals C12H25, C14H29, and C16H33, the yield of the labeled parent compound reached 80–90% and was virtually independent of the atomizer temperature. The capability of tritium atoms to penetrate into the targets was evaluated. For the exponential model of the attenuation of the flow of tritium atoms inside the target, the attenuation factor for freeze-dried amino acids and alkyltrimethylammonium bromides as targets was 1.8 nm−1.

Quasiclassical study of Eley–Rideal and hot atom reactions of H atoms with Cl adsorbed on a Au(111) surface

Using quasiclassical methods and a potential energy surface based on total energy calculations, we have found that H atoms react with Cl atoms adsorbed onto a Au(111) surface to produce HCl via Eley-Rideal (ER), hot atom (HA), and Langmuir-Hinschelwood (LH) pathways. We observe two ER mechanisms. At small normal incidence energies reaction results from a more or less direct collision with Cl, leading to a large amount of product vibration (nu=8), and relatively cold rotation and translation. In the second mechanism, more dominant at near-normal incidence and/or large incident energies, the H atom passes near Cl, recoils from the metal, and is pulled into orbit about Cl. This leads to broader product state distributions, and a more even distribution of the 3.0 eV of available energy among the product degrees of freedom, similar to products formed via the HA pathway. Overall, ER processes tend to contribute less than 10% to the reactivity, and most of the HCl is formed via HA processes. There is an increase in HCl formation with surface temperature for both the ER and HA mechanisms, but this increase is relatively weak. We observe typically about 12% H atom sticking, which would lead to HCl formation via a LH process in the experiments, above 140 K. We observe a weak forward scattering due to the direct ER component, as in the experiments. However, unlike the experiments, we observe a dip in our product angular distributions about thetaf=0 degrees, which we ascribe to our quasiclassical approximation. While we tend to see more energy in the hot products than in the experiments, our product translational, rotational, and vibrational distributions are in relatively reasonable agreement with those measured. One major disagreement with experiment is that there is apparently a significant sticking of the H atom at low temperatures, leading to a large LH component. In addition, the ER and HA components increase much more strongly with temperature than in the calculations. It is possible that electon-hole pair excitations in the metal strongly relax both the H atom and the excited HCl molecules formed.

Water formation on Pd(111) by reaction of oxygen with atomic and molecular hydrogen.

In this work we have studied the steady-state reaction of molecular and atomic hydrogen with oxygen on a Pd(111) surface at a low total pressure (<10(-7) mbar) and at sample temperatures ranging from 100 to 1100 K. Characteristic features of the water formation rate Phi(pH2; pO2; TPd) are presented and discussed, including effects that are due to the use of gas-phase atomic hydrogen for exposure. Optimum impingement ratios (OIR) for hydrogen and oxygen for water formation and their dependence on the sample temperature have been determined. The occurring shift in the OIR could be ascribed to the temperature dependence of the sticking coefficients for hydrogen (SH2) and oxygen (SO2) on Pd(111). Using gas-phase atomic hydrogen for water formation leads to an increase of the OIR, suggesting that hydrogen abstraction via hot-atom reactions competes with H2O formation. The velocity distributions of the desorbing water molecules formed on the Pd(111) surface have been measured by time-of-flight spectroscopy under various conditions, using either gas-phase H atoms or molecular H2 as reactants. In all cases, the desorbing water flux could be represented by a Maxwellian distribution corresponding to the surface temperature, thus giving direct evidence for a Langmuir-Hinshelwood mechanism for water formation on Pd(111).

Abstraction of D on Ag( [formula omitted]) and Ag( [formula omitted]) surfaces by gaseous H atoms : The role of electron–hole excitations in hot atom reactions and the transition to Eley–Rideal kinetics

H and D were adsorbed on Ag(1 0 0) and Ag(1 1 1) surfaces and characterized by thermal desorption spectroscopy. On Ag(1 0 0) surfaces hydrogen desorption from surface sites at 150 K and subsurface sites around 110 K was observed. Similarly, desorption of deuterium desorption around 150 and 110 K indicated surface and subsurface bound D. On Ag(1 1 1) surfaces only adsorption related desorption near 160 K was monitored. Abstraction of adsorbed D by gaseous H on Ag(1 0 0) is affected by reconstruction of the surface and the HD kinetics exhibits a H fluence (coverage) dependant cross-section. On Ag(1 1 1) adsorbed D is abstracted by gaseous H with a HD kinetics strictly according to the Eley–Rideal phenomenology. These features are close analogues of those observed on Cu(1 0 0) and Cu(1 1 1) surfaces. A comparison of the abstraction kinetics on various transition metals suggests that sticking of hot atoms, the reacting species on the surface, is controlled by electron–hole excitations. By this effect, the HD kinetics in abstraction on noble d-metals like Cu and Ag with a small density of states at the Fermi level and a small probability for e–h excitation exhibit Eley–Rideal phenomenology. Due to the small Ag–D bond energy on Ag(1 1 1) the attraction between incoming H and adsorbed D causes an increase of the abstraction cross-section at low D coverage, as was recently predicted by theory and verified by experiments on graphite surfaces.

Abstraction of D chemisorbed on graphite (0001) with gaseous H atoms

The kinetics of HD formation during admission of H atoms to D covered HOPG surfaces was measured at 150 K as a function of the initial D coverage. The phenomenology of the HD kinetics is as expected for an Eley-Rideal reaction or hot-atom reaction scenario with preference for reaction over sticking for hot atoms. It can be described by an abstraction cross-section, which varies between 17 (low coverage) and 4 Å 2 (high coverage). The large abstraction cross-section at low coverages is in accordance with a steering effect of the adsorbed D on the incoming H as predicted by theory.

The Effects of Lattice Motion on Eley-Rideal and Hot Atom Reactions: Quasiclassical Studies of Hydrogen Recombination on Ni(100)

Quasiclassical methods are used to simulate the interactions of H or D atom beams with D- or H-covered Ni(100) surfaces. The Ni substrate is treated as a multilayer slab, and the Ni atoms are allowed to move. The model potential energy surface is fit to the results of detailed total-energy calculations based on density functional theory. Most of the incident atoms trap to form hot atoms, which can eventually react with an adsorbate, or dissipate their energy and stick. The incident atom is found to lose several tenths of an eV of energy into the metal, upon initially colliding with the surface. This limits reflection to a few percent, at all coverages, and secondary reactions between adsorbates are significantly lowered. Long time hot atom reactions are also found to be damped out by the inclusion of lattice motion, leading to increased sticking, even at high coverages where dissipation into the adsorbates should be the primary energy loss mechanism. Overall, the inclusion of lattice motion is found to improve agreement with experiment.

Kinetic model for Eley–Rideal and hot atom reactions between H atoms on metal surfaces

A simple kinetic model is used to describe the interaction of H and D atomic beams with H- and D-covered metal surfaces. The atoms incident from the gas phase can have a direct Eley–Rideal reaction with an adsorbate, reflect, penetrate into the bulk, knock an adsorbate out of its binding site, or trap to form a hot atom. These hot mobile atoms can go on to react with other adsorbates, or eventually relax and stick. A coarse-graining approach, which takes advantage of the large difference between the time scales for the kinetics experiments and the reaction dynamics, allows us to derive relatively simple kinetic equations for reaction rates and coverages. The approach is similar to a kinetic random walk model developed by Küppers and co-workers [J. Phys. Chem. 109, 4071 (1998)] except that our equations can be used to derive analytical expressions for saturation coverages, rates, and yields. The model is applied to the case of H atom reactions on a Ni(100) surface, and a detailed comparison is made with both experimental and quasiclassical studies.

Eley–Rideal and hot atom reactions between hydrogen atoms on Ni(100): Electronic structure and quasiclassical studies

The reactions of gas-phase H (or D) atoms with D (or H) atoms adsorbed onto a Ni(100) surface are studied. Electronic structure calculations based on density functional theory are used to examine the interaction of H atoms with the Ni(100) surface, as well as the interactions between two H atoms near the metal surface. A model potential-energy surface based on ideas from effective medium theory is fit to the results of these electronic structure calculations. Quasiclassical trajectory methods are used to simulate the interaction of low energy H and D atom beams with H and D-covered Ni(100) surfaces. It is found that hot-atom processes dominate the formation of molecular hydrogen. The distribution of energy in the product molecules is examined with regard to the various pathways available for reaction. The initial adsorbate coverage is varied and is shown to control the relative amounts of reflection, reaction, sticking, and subsurface penetration. Our results are compared with those from similar studies on Cu(111) and available experimental data for Ni(100).

Channeling of Products in the Hot Atom Reaction H + (CN)2 → HCN/HNC + CN and in the Reaction of CN with CH3SH

Infrared transient absorption spectroscopy was used to determine the total product branching fractions for the gas-phase hot atom reaction H + (CN){sub 2} {yields} HCN/HNC + CN (a) and the reaction CN + CH{sub 3}SH {yields} HCN/HNC + CH{sub 3}S/CH{sub 2}SH (b) at 293 K. The reactive H atoms had an initial mean translational energy of 92 kJ mol-1, with a 38 kJ mol{sup -1} fwhm Gaussian energy distribution. The branching fractions determined for the product channels forming HCN and HNC, respectively, are 0.88 and 0.12 ({+-}0.05) for reaction (a) and 0.81 and 0.19 ({+-}0.08) for reaction (b). The bimolecular rate constant for reaction (b) was measured to be (2.7 {+-} 0.3) x 10{sup -10} cm{sup 3} molec{sup -1} s{sup -1} at 293 K. The observed product branching fractions for reaction (a) are consistent with the assumption that the average reactive cross sections for the two product channels are approximately equal above their respective energy thresholds. The results for reaction (a) are compared with the related H + XCN (X = Br, Cl) reactions. The large rate coefficient for reaction (b) suggests an interaction via a long-range intermolecular potential, which is facilitated by the small ionization energy of CH{sub 3}SHmore » and large electron affinity of CN. The results for reaction (b) are compared with the related reactions of Cl and OH with CH{sub 3}SH.« less

Interaction of hydrogen atoms with Si(111) surfaces: Adsorption, abstraction, and etching

The interaction of H atoms with Si(111) surfaces with respect to adsorption, abstraction, and etching was investigated using thermal desorption and product detection techniques. The study covers a wide range of coverages and the temperature range 100–1000 K. After H admission to Si(111) at 100 K in H2 desorption spectra decomposition of trihydride (t), dihydride (d), and monohydride (m) was observed around 455, 700, and 820 K, respectively. Adsorption of H at 380 K leads to desorption from d and m, and after admission of H at 680 K desorption from m was observed. The kinetics of m, d, and t desorption is according to first-order kinetics, only the m peak exhibits at small coverages second-order phenomenology. H exposure above 400 K leads to desorption of subsurface α-hydrogen at 920 K in thermal desorption spectra. Nonstationary etching via silane formation was monitored around 630 K. The nonstationary silane etch peak occurs through a quasi-first-order process in the admission temperature range 100–500 K and assumes a second-order phenomenology at admission temperatures between 500 and 600 K. This silane is formed through the recombination of surface silyl (t) and H in silylene (d) groups. Its yield decreases with the temperature at which H was admitted and is negligible after admission above 620 K since silyl groups are no longer available on the surface. Stationary etching during subjecting the surface with a continuous H flux occurs via a direct reaction step between the incoming H and surface silyl groups. The stationary etch yield decreases from 200 to 600 K due to depletion of surface silyl groups. In parallel to stationary etching, H abstraction proceeds with much higher probability. The kinetics of D abstraction by H from the monodeuteride phase at 680 K, measured through the HD product rate, as well as the formation of homonuclear D2 products contradict the operation of an Eley–Rideal (ER) mechanism, but are in excellent agreement with the solutions of a hot-atom (HA) reaction kinetic model which was recently successfully applied to abstraction on metal surfaces. This model is based solely on hot-atom processes and includes competition of reaction and sticking of hot atoms. Four parameters are needed to reproduce the measured HD rate data. At 680 K the abstraction cross section is 3.2 Å2 and about 5% of the adsorbed D occurs in D2 products. Subsurface α-D is abstracted at 680 K or higher temperatures with a cross section of 1.2 Å2. Abstraction at lower temperatures, either from monodeuteride surfaces or from surfaces saturated with di- and trideuteride proceeds with a smaller cross section and a reduced D2 product yield. At 100 K the HD cross section is only 2.2 Å2 (monodeuteride) or 1.4 Å2 (saturated surface), the HD kinetics is phenomenologically like that required by the ER mechanism, and a negligible quantity of D2 is formed. The HA reaction model allows one to reproduce these features by adjusting the model parameters accordingly.

Kinetics, mechanism, and dynamics of the gas-phase H(D) atom reaction with adsorbed D(H) atom on Pt(111)

We have investigated the kinetics of the abstraction reaction H(D)+Dad(Had)/Pt(111) at 100 K and saturation coverage (θsat=0.95±0.06 ML) using an H(D) atom beam, in which the angle-integrated and angle-resolved product desorption rates were simultaneously monitored with two mass spectrometers. HD molecules are formed by the abstraction reaction as well as by the secondary hot atom (s-HA) reactions, Ds*(Hs*)+Had(Dad)→HD, where Ds*(Hs*) is a collisionally excited surface D(H) atom. The two reaction components of HD show quite distinct angular distributions; while the former component is sharply forward-peaked to be represented by cos12(θf−3°), the latter component preferentially desorbs at large desorption angles centered at θf∼45°. The two HD formation reactions also exhibit distinct kinetics, which could be separately identified by properly selecting the desorption angle. Concurrent desorption of D2(H2) formed by a homonuclear s-HA reaction was also observed with a relatively large yield amounting to 37%(31%) of the initially adsorbed D(H) atoms. The angular distribution of D2 is very similar to that of HD formed by the s-HA reactions. Varying the beam incidence angle has no effect on the reaction rate constants and the product branching ratio. From a kinetic analysis, we estimate a cross section σabst=1.30±0.07(1.49±0.11) Å2 for HD formation by abstraction in H(D)-on-Dad(Had) reaction. For D2(H2) formation, an effective cross section for generating reactive Ds*(Hs*) atom is estimated as σ*=1.87±0.08(1.61±0.24) Å2. These values can be translated into the probabilities Pabst=0.19(0.21), Pex=0.27(0.23), Pads=0.73(0.67), and Pscatt=0.08(0.12) for abstraction reaction, s-HA generation, adsorption, and scattering of an incident H(D) atom, respectively. The isotope effects are small and the corresponding cross sections differ at most by 15%. The mechanism-dependent product angular distributions are discussed in terms of the different reaction dynamics from the view points of the surface potential corrugation experienced by the energetic hydrogen atoms (incident, primary, and secondary hot atoms) and the extent of the parallel momentum conservation in their reactions.

A hot-atom reaction kinetic model for H abstraction from solid surfaces

Measurements of the abstraction reaction kinetics in the interaction of gaseous H atoms with D adsorbed on metal and semiconductor surfaces, H(g)+D(ad)/S→ products, have shown that the kinetics of the HD products are at variance with the expectations drawn from the operation of Eley–Rideal mechanisms. Furthermore, in addition to HD product molecules, D 2 products were observed which are not expected in an Eley–Rideal scenario. Products and kinetics of abstraction reactions on Ni(100), Pt(111), and Cu(111) surfaces were recently explained by a random-walk model based solely on the operation of hot-atom mechanistic steps. Based on the same reaction scenario, the present work provides numerical solutions of the appropriate kinetic equations in the limit of the steady-state approximation for hot-atom species. It is shown that the HD and D 2 product kinetics derived from global kinetic rate constants are the same as those obtained from local probabilities in the random walk model. The rate constants of the hot-atom kinetics provide a background for the interpretation of measured data, which was missing up to now. Assuming that reconstruction affects the competition between hot-atom sticking and hot-atom reaction, the application of the present model at D abstraction from Cu(100) surfaces reproduces the essential characteristics of the experimentally determined kinetics.

Hot atom reaction yields in Mu*+H2 and T*+H2 from quasiclassical trajectory cross sections on the Liu–Siegbahn–Truhlar–Horowitz surface

In order to provide an assessment of the “global” accuracy of the Liu–Siegbahn–Truhlar–Horowitz (LSTH) potential surface for H3, hot atom reaction yields, which are determined from collision processes over an energy range much wider than that of single-collision experiments, have been calculated for the Mu*+H2 and T*+H2 systems. The isotopic comparison of muonium (Mu=μ+e−), an ultralight isotope of hydrogen (mMu/mH≈1/9), with the heaviest H-atom isotope, tritium, is a novel approach in testing the global accuracy of the H3 surface. These reaction yields have been calculated using a formalism developed for (μ+) charge exchange, with input cross sections for elastic, inelastic (rovibrational excitation) and reactive collisions determined from quasi classical trajectories on the LSTH surface, in the center-of-mass energy range 0.5–11 eV. The rate of energy loss of the hot atom (Mu* or T*) due to elastic and inelastic collisions with the moderator (H2) drastically affects the hot atom reaction yield. In particular, the forwardness of the angular differential cross section for the elastic process plays a crucial role in determining the stopping power for hot atoms. Good agreement is obtained in the absolute yields for both Mu*+H2 and T*+H2, for the first time from microscopic cross sections, demonstrating that the LSTH surface remains surprisingly accurate over a wide range of energy and isotopic mass.

Collisional removal of atomic carbon, C[2p 2( 3P J)], by aldehydes and ketones, investigated by time-resolved atomic resonance absorption spectroscopy in the vacuum ultra-violet

A kinetic study is presented of the collisional removal of ground state atomic carbon, C[2p 2( 3P J )], with various aldehydes and ketones in the gas phase following pulsed irradiation. The atomic carbon was generated by the photolysis of C 3O 2 ( λ > ca. 160 nm) in the presence of excess helium buffer gas and the added reactant gases in a slow flow system, kinetically equivalent to a static system, and monitored photoelectrically by time-resolved atomic resonance absorption spectroscopy in the vacuum ultra-violet at λ = 166 nm (3 3P J ← 2 3P J ) using signal averaging techniques. Absolute second-order rate constants ( k R/cm 3 molecule −1 s −1, 300 K) for the removal of C(2 3P J ) with these reactants were found to be as follows: formaldehyde 6.2 ± 0.3 × 10 −10; acetaldehyde 5.4 ± 0.3 × 10 −10; propionaldehyde 4.1 ± 0.3 × 10 −10; n-butyraldehyde 6.6 ± 0.3 × 10 −10; pentanal 4.6 ± 0.2 × 10 −10; hexanal 5.3 ± 0.4 × 10 −10; acetone 5.9 ± 0.3 × 10 −10; butanone 5.1 ± 0.2 × 10 −10; 2-pentanone 3.8 ± 0.2 × 10 −10; and 3-pentanone 4.6 ± 0.1 × 10 −10. No significant monotonic variation is thus observed in the rate data within these series of collisional processes where, from the similarity in the observed results, it is concluded that reaction is dominated by attack on the carbonyl group. The large values of these rate constants indicate that reactions of C(2 3P J ) with aldehydes and ketones, some of which have been observed by radio frequency spectroscopy in interstellar clouds and considered to be generated initially by hot atom reactions, are sufficiently rapid to be included in modelling of the interstellar medium.