Hot Hydrogen Atoms Research Articles

Hot-atom—laser-induced-fluorescence experiments on the reaction of H(2S) with CO2

The nascent rotational and fine-structure state distributions in OH(υ = 0) produced in the endothermic reaction H + CO2 → OH + CO have been measured with hot hydrogen atoms from the photodissociation of HBr at 193 nm. Considerable OH rotational excitation broadly peaked at aroundK = 11 is observed. The OH spin doublets are produced statistically (±10%). The partitioning between the λ doublets is non-statistical with a strong preference for the π+ component (σ(π+)/σ(π−) = 3.0 ± 1.0), consistent with a planar HOCO intermediate whose OCOH bond breaks in that plane.

Experimental determination of product quantum state distributions in the H+D2→HD+D reaction

Rotational and vibrational state distributions of HD formed in the reaction of translationally hot hydrogen atoms with D2 have been measured by CARS spectroscopy under nearly single-collisions conditions. The rotational and vibrational excitation of the HD is small; most of the energy available to the products appears in translation.

Formation and photochemistry of Methylamine in Jupiter's atmosphere

The formation of methylamine (CH 3NH 2) in the upper troposphere and lower stratosphere of Jupiter is investigated. Translationally hot hydrogen atoms are produced in the photolysis of ammonia, phosphine, and acetylene which react with methane to produce methyl (CH 3) radicals; the latter recombine with NH 2 to form CH 3NH 2. Also, methane is catalytically dissociated to CH 3 + H by the species C 2 and C 2H produced in the photolysis of acetylene. It is shown that the combined production of CH 3NH 2 and subsequent photolysis to HCN is unlikely to account for the HCN observed near Jupiter's tropopause. Recombination of NH 2 and C 2H 5N followed by photolysis to HCN is the preferred path. Production of C 2H 6 by these two processes is negligible in comparison to the downward flux of C 2H 6 from the Lyman α photolysis region of CH 4. An upper limit column density on CH 3PH 2 is estimated to be ∼10 13 cm −2 as compared to 10 15 cm −2 for CH 3NH 2. Hot H atoms account for a negligible fraction of the total ortho-para conversion by the reaction H + H 2

Dissociation of isomerization of excited C 3H 5 radicals in the gas phase

Highly excited C 3H 5 radicals were formed in the gas phase by the addition of hot hydrogen atoms ( E = 22.9 kcal mol −1) to either allene or propyne. The hydrogen atoms were generated by the action of UV radiation (253.7 nm) on H 2S. 2-propenyl radicals undergo mainly cleavage of the CH bond yielding allene or propyne; the latter decomposition channel predominates. The main reaction of 1-propenyl radicals is cleavage of the CC bond yielding methyl radicals and acetylene. Rice—Ramsperger—Kassel—Marcus calculations indicate that isomerization of the propenyl radicals into the allylic structure may be of importance. The calculations involving allyl radicals with an excess energy as high as about 81 kcal mol −1 demonstrate that dissociation yielding allene and a hydrogen atom is not effective; isomerization can occur prior to dissociation but the rate constants for both processes are markedly lower than that of the propenyl radical. The implications of these results with respect to the determination of the photolysis mechanism for gaseous olefins are briefly discussed.

Photochemistry of NH3, CH4 and PH3. Possible applications to the Jovian planets.

Photolysis of NH3 at 185 nm in the presence of a two-fold excess of CH4 results in the loss of about 0.25 mole of CH4 per mole of NH3 decomposed (delta CH4/delta NH3). The loss arises from the abstraction of hydrogen atoms from CH4 by photolytically generated hot hydrogen atoms, the presence of which is established by the constancy of delta CH4/delta NH3 between 298 and 156 K and by the quenching of the abstraction reaction when either H2 or SF6 is added. From the latter result, it can be concluded that NH3 photolysis in the H2-abundant atmosphere of Jupiter is not responsible for the presence of the carbon compounds observed there such as ethane, acetylene, and hydrogen cyanide, but may have had a role in the early atmosphere of Titan. Photolysis of PH3 with a 206 nm light source gives P2H4, which in turn is converted to a red-brown solid (P4?). The course of the photolysis is not changed appreciably when the temperatures is lowered to 157 K except that the concentration of P2H4 increases. The presence of H2 has no effect on the P2H4 yield. Photolysis of 9:1 NH3:PH3 gives a rate of decomposition of PH3 that is comparable with that observed by the direct photolysis of PH3. Comparable amounts of P2H4 and the red-brown solid are also observed. The mechanisms of these photochemical reactions together with their implications to the atmospheric chemistry of Jupiter are discussed. The structures of the compounds responsible for the wide array of colors e.g., brown, red and white, observed in the atmosphere of Jupiter have been the subject of extensive speculation. One theory suggests that these colors are due to organic materials formed by the action of either solar ultraviolet light or electric discharges on mixtures of CH4, NH3 and NH4HS in the Jovian atmosphere (Ponnamperuma, 1976; Khare et al., 1978). An alternative hypothesis is that the colors are due to inorganic compounds resulting from the photolysis of NH4HS and PH3 (Lewis and Prinn, 1970; Prinn and Lewis, 1975). In this paper we will summarize our experiments which were designed to test some of these hypotheses.

Irradiation of NH 3CH 4 mixtures as a model of photochemical processes in the Jovian planets and Titan

Photolysis of NH 3 in the presence of CH 4 with a 185-nm light source results in the generation of hot hydrogen atoms that abstract hydrogen from the CH 4 to produce CH 3·. Subsequent reactions of CH 3· and NH 2· give hydrocarbons, CH 3NH 2, and HCN. The extent of reaction of CH 4 was measured by the ratio of the moles of CH 4 reacted per mole of NH 3 decomposed ( Δ CH 4 Δ NH 3 ). This ratio increases with diminishing NH 3 pressure at constant CH 4 pressure but it remains constant if CH 4 NH 3 ⪰3 . The Δ CH 4 Δ NH 3 ratio is independent of temperature in the range 156–298° K, suggesting that hot hydrogen atoms were responsible for the reaction of CH 4. This postulate was confirmed by the observation that this reaction was quenched when H 2 or SF 6 was added to the reaction mixture.

Addition of hot hydrogen atoms to 1‐butene in the gas phase and dissociation of excited butyl radicals

AbstractThe reaction of hot hydrogen atoms originating from 253.7‐ and 228.8‐nm photolyses of hydrogen sulfide with 1‐butene was investigated. Of the hydrogen atoms undergoing addition a substantial part undergoes it in a first collision (37 and 48% at 253.7 and 228.8 nm, respectively) yielding highly excited butyl radicals. The ratio of nonterminal to terminal addition is 0.5 and practically does not depend on the energy of the hydrogen atoms over the range of 15–33 kcal/mol. Comparing the results of 229‐ and 254‐nm photolyses of hydrogen sulfide with those of 313‐ and 334‐nm photolyses of hydrogen iodide with the use of the decomposition rate constants of n‐butyl radicals calculated by the RRKM methods, the conclusion is reached that the hydrogen atom from H2S photodissociation has 90–95% of the available energy.

Semiempirical hot atom theory. 1. Initialization and application

A semiempirical approach to the modeling of the kinetics of reaction systems containing both hot and nonhot atoms is proposed. The approach is based on the probabilistic kinetic theory of hot-atom reactions formulated by Wolfgang (1963), with transmission probabilities estimated for a rectangular potential barrier for hot-atom and nonhot-atom reactions. A computational scheme for determining product concentrations following hot and nonhot reactions in a system containing photolytically produced hot atoms is then applied to the DBr + CH4 and HBr + CD4 hot hydrogen atom systems studied by Martin and Willard (1964), and good agreement is obtained between theoretical and experimental results.

The vacuum ultraviolet photolysis of hydrogen chloride. The role of the hot hydrogen atoms

Abstract The VuV photolysis of HCl has been performed in the presence of chlorine as an H atom scavenger. Nitrogen and argon were used as moderators. It has been shown that H atoms react mainly while hot even at the five-fold excess of the moderator. The ratio of the thermalization rate constants for nitrogen and argon has been determined to be 3.0 which agrees with the supposition that inelastic collisions play an important role when the molecular moderators are used. The relative rate constant for reaction of both hot and thermal hydrogen atoms with Cl2 and HCl as well as the effective energy loss per collision has been obtained. The results are well consistent with the Wolfgang's hot atoms theory.

Reaction of hot hydrogen atoms with chloromethane

The integral reaction probability for the reaction of hot hydrogen atoms wilh CH3Cl. H*+CH3Cl→-HCl+CH3, has been determined as a function of the initial energy of the H atoms. The apparent threshold energy for the reaction. which is expected to be higher than the true threshold, is 47 ± 14 kJ mol−1.

Reaction of photochemically generated hot hydrogen atoms with 1-butene

The reaction of hot hydrogen atoms with 1-butene was studied in the gas phase at pressures in the range 0.4–400 Torr (0.5–533 hPa). Hydrogen atoms were generated by exposing HI to the action of UV light (λ = 334 and 313 nm). Some of the hydrogen atoms add to the double bond in a first collision yielding highly excited sec-butyl and n-butyl radicals; the remainder undergo thermalization according to a step-ladder model. The evidence that hot hydrogen atom addition occurs is based on kinetic considerations—both experimental and calculated RRKM rate constants for decomposition of the excited butyl radicals are given—and is supported by the observation that the contribution of non-terminal addition of hot hydrogen atoms reaches a level of about 30%, whereas thermal hydrogen atoms add mainly (about 94%) to the terminal carbon atom. The hot hydrogen atom + olefin chemical activation technique provides an interesting tool for the investigation of highly excited radicals.

ESR studies of irradiated methane and ethane at 4.2 K and mechanism of pairwise trapping of radicals in irradiated alkanes

The spatial distributions of trapped hydrogen atoms and methyl radicals in CH4 and CD4 irradiated at 4.2 K have been studied by ESR. Most of the hydrogen atoms are trapped within ∼16 Å from each parent methyl radical forming distributed radical pairs. This indicates that the thermalization distance of hydrogen atoms produced by 4.2 K radiolysis is ≲16 Å in methane. Evidence has been obtained for the formation of a small amount of ĊH3...ĊH3 pairs, which are supposedly formed from hot hydrogen atom reactions. No evidence has been obtained for the formation of a pair of hydrogen atoms. There has been also obtained the experimental evidence that thermal hydrogen atoms unreactive with methane can abstract a hydrogen atom from ethane at 10–20 K. In the light of these results, the following conclusions are deduced: A part of hydrogen atoms produced in radiolysis of alkanes undergo hot abstraction to form very close alkyl radical pairs whereas the rest is thermalized and immobilized at the place close to each parent alkyl radical. The thermalized hydrogen atoms also abstract a hydrogen atom from the surrounding molecules to form distributed alkyl radical pairs, when the matrix molecule is reactive with thermal hydrogen atoms. On the other hand, the hydrogen atoms are mobile at elevated temperatures and the isolated alkyl radicals are mainly formed.

Photochemical and hot hydrogen atom reactions of acetaldehyde

Photolysis and hot hydrogen atom reactions of acetaldehyde have been studied simultaneously not only with a cadmium but also with a zinc resonance lamp as the light source. Contrary to previous results, new products (acetone, ethanol, 2-propanol, ethyl acetoacetate, and diacetone alcohol) are identified in the photolysis of acetaldehyde. On the other hand, the principal products in decreasing order from the hot atom reaction with acetaldehyde have been established to be hydrogen, biacetyl, methane, carbon monoxide, and ethanol, whereas acetone and ethylene existed as minor products. A study of product quantum yields at 214 nm vs. various (CH/sub 3/CHO)/(CH/sub 2/S) ratios are reported. The formation of several unexpected products in these studies strongly suggested that the aldehyde functional group of acetaldehyde has an unsaturated character relative to the addition of both hydrogen atoms and methyl radicals. This latter new observation and accompanying mechanisms are discussed. 2 figures, 6 tables.

Gas phase photolysis of 1-butene at 123.7 nm (10.0 eV)

The photolysis of 1-butene was carried out in a static system using the krypton resonance line at 123.7 nm (10.0 eV) at pressures in the range 15–500 Torr (2–66.5 kPa). The major products observed were ethylene, acetylene, 1,3-butadiene, allene, n-butane and propylene. Identification of the radical species was made by the use of scavengers such as oxygen and H 2S. Evidence is presented for the occurrence of nine primary processes to which quantum yields have been ascribed. The main processes are fragmentation giving C 3H 5 radicals with a yield φ of 0.29 and the formation of C 4H 6 hydrocarbons with a yield φ of 0.23. A hydrogen atom mechanism, involving the occurrence of hot hydrogen atoms that have an excess energy as high as 0.6 eV, was proposed to account for the pressure dependence of propylene. Dissociation of excited radicals contributes to the formation of ethylene.

Gas phase photolysis of 1-butene at 147 nm (8.4 eV)

The photolysis of 1-butene was carried out in a static system using the xenon resonance line at 147 nm (8.4 eV) at pressures in the range 15 – 500 Torr (2 – 66.5 kPa). The major products observed were acetylene, ethane, ethylene, allene, propyne, 1,3-butadiene and isopentane. The radical species were identified by using scavengers such as oxygen and H 2S. Evidence is presented for the occurrence of ten primary processes to which quantum yields have been ascribed. The main process is a β cleavage of the CC bond which occurs with a yield of φ = 0.51. A hydrogen atom mechanism involving the occurrence of hot hydrogen atoms (about 30% of them have an excess energy of about 0.26 eV) was proposed to account for the pressure dependence of propylene. Dissociation of excited radicals contributes to the formation of allene and propyne.

Gas phase photolysis of 1-pentene and 1-pentene-d10 at 7.1 and 7.6 eV. Kinetic considerations

The gas phase photolysis of n-pentene was carried out in a static system using nitrogen resonance lines at [Formula: see text] and the bromine line at [Formula: see text] The mechanism for the photolysis was proposed and compared to what was concluded at 8.4 eV (147 nm, the xenon resonance line). The kinetics of the decomposition of the excited C3H5* radicals formed in the primary photochemical process and the C5H11* radicals formed by the addition of hydrogen atoms to the parent molecules were discussed. The investigations were extended to the n-C5D10 photolytic System. The observed decomposition rate constants of the excited pentyl radicals as well as the secondary non-equilibrium isotope effects agree with the data published earlier. It is concluded from these experiments that, at least at 7.6 eV, hot hydrogen atoms are produced.Only a small fraction of the C3H5* radicals décompose and yield aliène. At the same time the combined primary–secondary non-equilibrium isotope effects are much less than those calculated for the 'pure' primary isotope effects. To account for these observations, it is assumed that the C3H5* radicals are formed with a wide spread in the internal energies. Since the threshold of the decomposition of the excited C3H5* radical lies above its mean excess energy (calculated on the statistical basis), an analogy in the energy-distribution functions on the radicals activated photochemically and thermally may be suggested. If so, an inverse secondary isotope effect may contribute to the gross effect involved in the C3H5* radical decomposition.

Doppler Temperature and Particle Confinement Time Determined from Measurements of Hα-Line on a Tokamak Plasma in the JFT-2a Device

Radiation from the Hα-line has been investigated in the hydrogen tokamak plasma obtained under a typical JFT-2a discharge. The profiles of the Hα-line were measured along both perpendicular and tangential paths to the toroidal magnetic field. The measured profiles can be interpreted in terms of Doppler broadening at the line wings due to hot hydrogen atoms originating from the resonance charge-exchange process and at the line center due to cold particles having energies of a few to several eV. The peak value of the Doppler temperature for the hot particle was approximately 50 eV. Equal temperatures were obtained from measurements along perpendicular and tangential directions to the toroidal magnatic field. Distributions in the minor cross-section of photon numbers of the Hα-line were also measured, from which the confinement time of charged particles was found to be 0.5 to 2.5 msec.

Threshold energy for the D-Abstraction reaction between H and CDCl3

The reaction of hot hydrogen atoms with CDCl3 has been examined. Atoms of known initial energy were generated by photolysis of HI or HBr in the presence of CDCl3, and the probability of the reaction H* + CDCl3 å HD + CCl3 relative to thermalisation determined. The phenomenological threshold energy for the reaction, which is expected to be slightly higher than the true threshold is 34 ± 4 kJ mol−1.

Moderation of photochemically generated hot hydrogen atoms

Hot hydrogen atoms generated by photolysis of gaseous HI react with OCS to produce CO by eqn (1). H*+ OCS → CO + SH (1). This reaction has been used to examine the moderation of the hot atoms by added C2H6, C3H8, n-C4H10, CH3F, C2F6, C3F8 and SF6. In each case the value of the energy-loss parameter α, relative to that for argon, is greater than that calculated assuming elastic collisions between the hot atom and moderator. Alkanes are the most efficient of the moderators examined, with relatively minor differences in α for C2H6, C3H8 and n-C4H10.

Gas phase photolysis of 1-butene at 147 nm (8.4 eV)

The photolysis of 1-butene was carried out in a static system using the xenon resonance line at 147 nm (8.4 eV) at pressures in the range 15 – 500 Torr (2 – 66.5 kPa). The major products observed were acetylene, ethane, ethylene, allene, propyne, 1,3-butadiene and isopentane. The radical species were identified by using scavengers such as oxygen and H 2S. Evidence is presented for the occurrence of ten primary processes to which quantum yields have been ascribed. The main process is a β cleavage of the C-C bond which occurs with a yield of φ = 0.51. A hydrogen atom mechanism involving the occurrence of hot hydrogen atoms (about 30% of them have an excess energy of about 0.26 eV) was proposed to account for the pressure dependence of propylene. Dissociation of excited radicals contributes to the formation of allene and propyne.