Production Of Hydrogen Atoms Research Articles

Kinetic and mechanistic study of the reaction of O(1D) with CF2HBr

AbstractA laser flash photolysis–resonance fluorescence technique has been employed to investigate the kinetics and mechanism of the reaction of electronically excited oxygen atoms, O(1D), with CF2HBr. Absolute rate coefficients (k1) for the deactivation of O(1D) by CF2HBr have been measured as a function of temperature over the range 211–425 K. The results are well described by the Arrhenius expression k1(T) = 1.72 × 10−10 exp(+72/T) cm3molecule−1 s−1; the accuracy of each reported rate coefficient is estimated to be ±15% (2σ). The branching ratio for nonreactive quenching of O(1D) to the ground state, O(3P), is found to be 0.39 ± 0.06 independent of temperature, while the branching ratio for production of hydrogen atoms at 298 K is found to be 0.02−0.02+0.01. The above results are considered in conjunction with other published information to examine reactivity trends in O(1D) + CF2XY reactions (X,Y = H, F, Cl, Br). © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 262–270, 2001

Quasiclassical Kinetics of the H2 + H2 Reaction and Dissociation

We have investigated the elementary processes occurring in H2 + H2 collisions by integrating large batches of quasiclassical trajectories. This has allowed us to calculate the vibrational state-selected rate coefficients as well as their aggregations in classes leading to the same products and their dependence on temperature. The contribution of the various elementary and aggregated processes both to the allocation of energy among the various vibrational states of the products and to the production of hydrogen atoms is analyzed. Some correlations of these contributions with the value of the impact parameter and with some characteristics of the reactive mechanism are also discussed. The key importance of hydrogen exchange processes is also singled out.

Reaction of carbon atoms, C (2p2, 3P) with C2H2, C2H4 and C6H6: Overall rate constant and relative atomic hydrogen production

The multichannel C + C2H2, C2H4 and C6H6 reactions were studied, at room temperature, in a low-pressure fast-flow reactor. Carbon atoms were obtained from the reaction of CBr4 with potassium atoms. The overall rate constants were found to be (2.4 ± 0.5) × 10−10, (2.1 ± 0.4) × 10−10 and (2.8 ± 0.8) × 10−10 cm3 molecule−1 s−1, respectively. The relative atomic hydrogen productions were determined by resonance fluorescence in the vacuum ultraviolet: H (C + C2H2/C + C2H4) = 0.58 ± 0.04, H (C + C6H6/C + C2H4) = 0.17 ± 0.05, the H production from the C + C2H4 reaction being the reference. The absolute H branching ratio of the C + C2H4 was estimated to be 0.92 ± 0.04.

Photolysis of HI in solid Xe: production and distribution of hydrogen atoms

The production and distribution of H atoms under UV photolysis of HI in a Xe matrix is studied. Our experiments indicate that H atoms do not migrate extensive distances during primary photolysis and the resulting spatial distribution is quite local. After the primary photolysis, H atoms can be further distributed to larger distances via radiation-induced secondary (hidden) processes, HXeI and Xe 2H + being possible intermediates in these processes. The amount of H atoms produced in the photolysis is comparable to the amount of I atoms, and the loss channels are minor.

Interaction of H 2O and O 2 with Ni 3Fe and their effects on ductility

Possible reasons for the high ductility of Ni 3Fe and its insensitivity toward the testing environment have been investigated. Thermal desorption experiments have shown that water dissociates on clean Ni 3Fe surfaces to produce atomic hydrogen. Ductility measurements of cast and cold-rolled polycrystalline Ni 3Fe demonstrated that the reduced ductility was obtained only when the testing was performed with oxygen carefully removed. X-ray photoemission studies indicate that oxygen interacts with water to form hydroxyls, thereby suppressing the production of atomic hydrogen. Hydrogen desorption from the Ni 3Fe surface requires a lower activation energy, resulting in a smaller surface hydrogen concentration at a given temperature. Hence it is possible that there is insufficient hydrogen to cause the nucleation and growth of brittle cracks for severe embrittlement.

Multiple Scattering of Hydrogen Lyα Radiation in the Coma of Comet Hyakutake (C/1996 B2)

The Goddard High Resolution Spectrograph on board the Hubble Space Telescope measured hydrogen Lyα (H Lyα) line profiles at different locations around the coma of comet Hyakutake (C/1996 B2) in 1996 April. The spectral resolution of about 4 km s-1 (Doppler velocity, FWHM) was significantly better (by a factor of 2-3) than any previous measurement and is sufficient to constrain models of atomic hydrogen production processes and inner coma thermodynamics. In a recent paper, we reported the line profile measurements and the results of an explicit model of the optically thin region of the coma. The spectrally integrated emission rates in all but the nucleus-centered spectrum were computed, but the inner coma line profiles were only discussed qualitatively. In the present paper, we investigate the details of the line profiles using a first-principles numerical model of the H coma and a new radiative transfer model based on the Monte Carlo method to account for the line radiation transport throughout the coma. This multiple scattering model uses number densities and velocity distributions of H atoms produced by the water dissociation processes, along with angle-dependent frequency redistribution to describe each scattering process. The computed spectra, when convolved with the instrument function, are in excellent agreement with the measured spectra. The model is able to reproduce features, such as the saturation of the line profiles in the optically deep regions, and the strong asymmetry of the isophotes due to shadowing of the nightside by H atoms of the inner coma. For the first time, a consistent and detailed radiative transfer treatment of a physically realistic cometary hydrogen density and velocity distribution confirms our overall understanding of water dissociation processes and partial thermalization in the coma.

Absolute measurements of electron capture cross sections of C3+ from atomic and molecular hydrogen

Absolute measurements of single- and double-electron-capture cross sections by C3+ projectiles on atomic and molecular hydrogen targets were performed for projectile energies between 1.0 and 3.5 MeV for the single- and 1.0 and 2.0 MeV for the double-capture processes. The H/H2 cross section ratios were measured using an absolutely calibrated tungsten-tube furnace for the production of atomic hydrogen. The single-capture data are compared with calculations based on the boundary-corrected first Born approximation, the eikonal approximation and a semiclassical model, presenting a good overall agreement. Calculations for the double capture using an analytical expression, obtained within the independent electron approximation and based on the same semiclassical model, give a reasonable qualitative description of the data.

Thermionic emission of hydrogen isotopes (Mu, H, D and T) from W and interpretation of a role of the hot W as an atomic hydrogen source

Resonant ionization was applied to learn about the production mechanism of thermal neutral hydrogen isotopes including muonium (Mu; μ +e − ) from a hot tungsten surface as reaction products with pulsed 500 MeV protons. The activation energy E a obtained in the temperature dependence of production of neutral muonium (1.72 (5) eV) as well as that of T atom (1.89 (1) eV), is consistent with the emission of the atoms directly from bulk sites in the solid rather than from the surface for which one should expect E a≈3 eV. Since hydrogen molecules are easily dissociated into hydrogen atoms interacting with a surface of the hot tungsten, even residual H 2 in the ultra high vacuum chamber of 3×10 −10 mbar at 1200 K was found to be easily detected and extracted by this method. From the results obtained through the temperature and pressure dependence measurements for D 2 and H 2, we propose a view of the role of hot W as a catalyzer to dissociate H 2 molecules and producing hydrogen atoms. After dissociation on the surface, the hydrogen atoms are dissolved into the bulk solid, and are thermionically emitted from the bulk to the vacuum at high temperatures.

Reactive quenching of electronically excited OH radicals in collisions with molecular hydrogen

The hydrogen atom products of the OH A 2Σ+ (v=0)+H2→H+H2O quenching reaction have been characterized by Doppler spectroscopy. The translational energy distribution of the products is bimodal, with the two components accounting for approximately 3% and 40% of the 4.72 eV of available energy.

Venus Lyα: A Morphological and Radiative Transfer Analysis

The Venus Lya corona is caused by resonance scattering of the solar 1215.67 A Lya line by hydrogen atoms in the Venus upper atmosphere. The atmospheric atomic hydrogen content is probed remotely via Lya observations. On 1990 February 10 the Galileo spacecraft flew by Venus, obtaining a series of Venus scans with the Ultraviolet Spectrometer Experiment. The Pioneer Venus (PV) Orbiter Ultraviolet Spectrometer obtained Venus Lya images approximately weekly throughout its 14 year mission (1978–1992), spanning the 11 year solar cycle. I develop a morphology of the behavior of the Venus Lya signal with respect to location on Venus and solar activity level. The predawn hydrogen density enhancement extends to high latitudes (160 ). I find an equatorial minimum of hydrogen and evidence for a polar hood of enhanced hydrogen abundance. This hood may be produced by a hydrogen influx from the predawn region that is not removed as efficiently as hydrogen in the subsolar region. All features examined persist throughout the solar cycle and increase in hydrogen abundance with solar activity. A search for small-scale (1000 km) features produced a null result. However, the timescale for ballistic transport of atomic hydrogen in the exosphere is ∼8 minutes, too short for the detection of nonequilibrium density features. I analyze the data using a two-dimensional nonisothermal complete-frequency-redistribution multiple-scattering code modified from the LYAB code provided by James Bishop for the geocorona. I employ the VTS3 neutral thermosphere model (A. Hedin, H. Niemann, W. Kasprzak, & A. Seiff, J. Geophys. Res., 88, 73 [1983]) and calculate diffusive profiles for the vertical distribution of atomic hydrogen, characterized by the hydrogen number density and vertical flux at the exobase n f 0 0 (L. Paxton, D. Anderson, & A. Stewart, J. Geophys. Res., 93, 1766 [1988]). The flux parameter regulates the amount of hydrogen in the lower thermosphere, and the exobase density controls the amount in the upper thermosphere and exosphere. The almost linear relationship between the PV Langmuir probe photoelectron current and the observed solar Lya output provides a daily proxy for solar illumination of Venus hydrogen. I determine the parameter values that best fit the data for selected segments of the sunlit disk and examine their behavior with respect to location and solar activity. Numerical results are limited to the region of spatially uniform temperature in the exosphere and thermosphere. For high solar activity ( ), subsolar values are F ∼ 200 n (5.6 10.7 0 cm 3 and cm 2 s , cor4 7 0.8) # 10 f (5.8 0.9) # 10 0 responding to hydrogen column densities of (2.0 0.3) # cm 2 above 200 km and between 135 12 13 10 (1.0 0.2) # 10 km and the CO2 Lya absorbing layer at 112 km. These values agree with the subsolar work of Paxton et al. and with densities derived from in situ measurements by H. Brinton, H. Taylor, H. Niemann, H. Mayr, A. Nagy, T. Cravens, & D. Strobel (Geophys. Res. Lett., 7, 865 [1980]). Exobase density, , den0 creases from a local solar time (LST) of 10 hr to 15 hr by to cm . At 12 hr LST, increases from the 4 4 7 # 10 4 # 10 n0 equator to 60 latitude by to cm . This exo4 4 4 # 10 7 # 10 base density minimum at low latitudes on the dayside is consistent with solar EUV heating driving the transport removal of hydrogen atoms from the dayside upper atmosphere. The hydrogen abundance in the lower thermosphere, , does not f0 vary significantly. However, because of the opacity of the overlying upper thermosphere and exosphere, the variation of hydrogen column abundance for the values observed would n0 not be detectable in . Both and increase with solar f n f 0 0 0 activity, and there is evidence suggesting solar cycle phase dependence. A detailed interpretation of these quantitative results requires a photochemical model of the Venus upper atmosphere. The details of suprathermal hydrogen atom production are essential to understanding the transport behavior of hydrogen in the upper thermosphere and exosphere.

Effect of strength on environment-assisted cracking of Ti–8V–6Cr–4Mo–4Zr–3Al in aqueous NaCl: Part II: Crack tip strain rate

The objective of this study is to critically examine the intrinsic environment-assisted cracking (EAC) resistances of three microstructures of Ti–8V–6Cr–4Mo–4Zr–3Al (Beta-C) in aqueous NaCl through variation in crack tip strain rate. The single-phase β microstructure of Beta-C cold worked to high strength, as well as a β/ α structure from short-term aging at 500°C, are immune to EAC in aqueous NaCl for a range of crack tip strain rates that should allow varying levels of atomic hydrogen production and accumulation. Intergranular EAC occurs in underaged Beta-C, but only if driven by high crack tip strain rate from rapid loading or stable crack growth. Increased process-zone hydrogen from increased crack tip strain rate promotes this embrittlement. The severity of EAC in ST/A Beta-C increases with aging time for constant crack tip strain rate. These results strengthen the Part I finding that increased strength does not uniquely promote intergranular EAC; rather, crack tip strain rate as well as grain boundary characteristics that change during α-precipitation hardening govern the EAC severity of β-Ti alloys.

One-Color Molecular Photodissociation and Detection of Hydrogen Atoms

The wavelength 205.14 nm is absorbed by many hydrogen-containing molecules, which then dissociate to form hydrogen atoms. These in turn can absorb two more 205.14 nm photons and reach the 3s or the 3d state. They can absorb a third photon and form hydrogen ions or decay to the ground state by stepwise fluorescence, first emitting the Balmer α line at 656.2 nm (3s or 3d→2p)and then the Lyman α line (2p→1s)at 121.6 nm. Thus the hydrogen atom kinetic energy can be probed in three different ways. This method broadens greatly the possibilities of investigating photodissociations leading to hydrogen atom products. It has the advantage of simplicity and the disadvantages of a single-color experiment. The method is tested with a molecule that has been extensively investigated, H2S, and then applied to three other molecules, formic acid (HCO2H), vinyl radical (C2H3), and allyl radical (C3H5). H2S has a perpendicular transition with a large release of kinetic energy. Studies of the latter molecules lead to the conclusion that formyl, carboxyl, vinyl, and allyl radicals absorb 205.1 nm light and release hydrogen atoms with a large fraction of the available energy. The dissociation pathways of formic acid are clarified.

On self-limitation of UV photolysis in rare-gas solids and some of its consequences for matrix studies

UV photolysis of small molecules embedded in rare-gas matrices is examined. We demonstrate that photolysis can be self-limited when products absorb the photolysing radiation. As a result of the rising absorption, in-situ detected luminescence of the photolysis product saturates faster than its concentration. In particular, the present study supports the conclusion that 193 nm photolysis of hydrogen-containing species in Xe matrices produces hydrogen atoms in amounts comparable with the other dissociating part of the precursor. Also, we show that 193 nm radiation activates mobility of hydrogen atoms in annealing, accelerating photochemical processes related to hydrogen mobility.

Evidence of enhanced atomic hydrogen production with halogens in diamond MWPACVD

Abstract Optical emission spectroscopy measurements and exhaust gas mass spectrometry were used for diagnostics in a microwave plasma assisted chemical vapor deposition reactor for diamond growth. An actinometry technique was used to study the atomic hydrogen generation with additions of CF4 and CCl2F2 in the feed gas. The relative emission intensity of the atomic hydrogen line Hα (656.3 nm) was measured with CF4 and CCl2F2 concentrations in the range of 0–5%. The results show an increase in atomic hydrogen concentration up to 80 and 200% when CF4 and CCl2F2 are added in the mixture, respectively. The -ideal actinometry conditions and the validity of the observed increase of atomic hydrogen with an change in electron temperature and density in the plasma was investigated. The dissociation process of CF4 and CCl2F2 associated with HF and HCl formation were discussed by measurements of stable species by mass spectrometry of the exhaust gas. The increase of the atomic hydrogen concentration is correlated with the easy HCl and/or HF dissociation by electron-impact.

Mechanism of boron-induced embrittlement of Ni 3Al in hydrogen

Materials that have been exposed to H{sub 2} or hydrogen-containing gases often show embrittlement due to atomic hydrogen. It is now recognized that hydrogen embrittlement is the source of brittle behavior of a number of intermetallics that were previously thought to be intrinsically brittle. A well-known example is polycrystalline Ni{sub 3}Al, which is susceptible to environmental embrittlement in atmospheres containing water vapor, and the mechanism of H production is the dissociation of H{sub 2}O at active Al sites. Further investigations to identify how B additions affect the fracture susceptibility of Ni{sub 3}Al in H{sub 2} atmospheres discovered a surprising phenomenon: boron-induced embrittlement. Insufficient data are available to build more than a speculative atomic-level model to explain this counterintuitive connection of boron with embrittlement. One explanation has been proposed in terms of the chemical participation of boron in the H{sub 2} dissociation step. However, the chemistry of boron does not support a direct dissociation step. And simple electronic structure considerations discussed later suggest boron would not act as a promoter in an indirect dissociation of H{sub 2}. The alternate mechanism proposed here, although tentative until further evidence is attained, is based on well-established atomic-level properties of boron in Ni{sub 3}Al. Inmore » the following, the concept of the activity of interfaces with regard to H{sub 2} molecular dissociation in the region of intersection of the grain boundary with the gas/solid interface is used to explain how boron can affect atomic hydrogen production. Activity will refer exclusively to the H{sub 2} molecular dissociation reaction unless noted otherwise.« less

Characterization of an argon-hydrogen microwave discharge used as an atomic hydrogen source. Effect of hydrogen dilution on the atomic hydrogen production

This work is devoted to the study of an argon-hydrogen microwave plasma used as an atomic hydrogen source. Our attention has focused on the effect of the hydrogen dilution in argon on atomic hydrogen production. Diagnostics are performed either in the discharge or in the post-discharge using emission spectroscopy (actinometry) and mass spectrometry. The agreement between actinometry and mass spectrometry diagnostics proves that actinometry on the Ha(656.3 nm) and Hβ(486.1 nm) hydrogen Balmer lines can be used to measure the relative atomic hydrogen density within the microwave discharge. Results show that the atomic hydrogen density is maximum for a gas mixture corresponding to the partial pressure ratioP H 2/P Ar range between 1.5 and 2. The variation of atomic hydrogen density can be explained by a change of the dominant reactive mechanisms. At a low hydrogen partial pressure the dominant processes are the charge transfers with recombinations between Ar+ and H2 which lead to ArH+ and H 2 + ion formation. Both ions are dissociated in dissociative electron attachment processes. At a low argon partial pressure the electron temperature and the electron density decrease with increasing partial pressure ratio. The dominant mechanisms become direct reactions between charged particles (e, H+, H 2 + , and H 3 + ) or excited species H(n=2) with H2 producing H atoms.

Production of atomic hydrogen in an rf gas discharge and mass spectrometer diagnostics of the process

84 investigated in the present study as part of the preparation for a series of experiments on the influence of variations in the electron environment on the lifetime of the b-active tritium nucleus. In the first experiment of the series it is proposed to investigate the chemical shift of the half-life for an atomicmolecular tritium pair. Since the energy of dissociation of molecular tritium is close to that of molecular protium, and since atomic tritium, because of its higher mass and lower mobility, recombines more slowly than atomic protium, the procedure used to obtain atomic tritium can be worked out in experiments with protium ~called hydrogen from now on!. Atomic hydrogen is produced by thermal dissociation in a tungsten furnace, by low-frequency and radio-frequency gas discharges, or by means of a plasma arc. In the present study we have used the rf gas discharge method to obtain atomic hydrogen. The high efficiency of this method helps to minimize the heating of the discharge cell and can be implemented without the intrusion of electrodes in the discharge volume, thereby ensuring high purity of the atomic hydrogen product. The problems associated with the stabilization and detection of atomic hydrogen stem from its high chemical activity, by virtue of which atoms rapidly recombine at the walls of the discharge volume and in mutual collisions. For this reason, most diagnostic techniques applied to atomic hydrogen are based on measurements of secondary effects attributable to the presence of atomic hydrogen. For example, a chemical target of molybdenum oxide has been used as the atomic hydrogen detector in some studies. When hydrogen atoms are incident on such a target, the oxide undergoes reduction, and the target changes color. However, the small range of measurement of the flux density (10–10 atoms/s •cm) and low accuracy (630% error! limit the possibilities of this method. Bass and Broida have measured the optical spectra of hydrogen atoms in the condensed and gaseous phases, but did not establish any relationship between the concentration of hydrogen atoms and the line intensities of the optical spectra. One approach that looks promising is the determination of hydrogen atoms by electron spin resonance ~ESR!. This method utilizes the presence of an unpaired electron due to the paramagnetism of the hydrogen atom, a property that can be exploited for detection by ESR. Only papers in which ESR has been used to measure atomic hydrogen in the condensed phase are known in the literature. Here we report mass spectrometer measurements of atomic hydrogen produced in an rf gas discharge. The experimental apparatus ~Fig. 1! consists of an rf oscillator 1 ~1 MHz, 300 W!, a discharge tube 2, an MI 1201 mass spec-

Mass spectrometry and diamond growth from gas mixtures

Abstract In this work we study diamond growth with gas mixtures containing carbon tetrachloride in a hot-filament assisted chemical vapor deposition (HFCVD) reactor. We study the growth dependence on filament temperature at a fixed substrate temperature (850°C). Mass spectrometry of the exhaust gas has also been performed as a function of filament temperature and CCl 4 concentration in the feed gas. The kinetics of CCl 4 dissociation is interpreted and the mechanisms for atomic hydrogen production from this kinetic process are discussed. Scanning electron microscopy and Raman spectroscopy have been used to characterize the diamond films.

Theoretical study of vibronic spectra and photodissociation pathways of methane

Potential energy surfaces for the first singlet and triplet excited states of methane have been studied using multireference configuration interaction (MRCI) and equation-of-motion coupled cluster (EOM-CCSD) ab initio molecular orbital calculations. The vertical excitation energies for the 1T2 and 3T2 states are computed to be 10.64–10.66 and 10.25–10.30 eV, respectively. Two minima are found on the first excited singlet surface, 1 (∼C3v) and 2 (C2v), with adiabatic excitation energies of 9.16–9.25 and 8.39–8.52 eV, respectively. No minima is located on the triplet surface. Vibronic spectra, calculated based on the geometries, vibrational frequencies, and normal modes of the ground and excited states, reproduce well the experimental results. The spectra due to the 3s(C2v)←1t2 transition start at ∼8.63 eV and form a broad underlying continuum. The 3s(C3v)←1t2 transition is shown to be responsible for the minor fine structure observed in the experimental absorption spectra between 9.5 and 10.6 eV. Dissociation pathways leading to various photofragmentation products are discussed on the basis of the calculated minimal energy pathways of H and H2 elimination. Production of CH3(2A2″) and fast hydrogen atoms, the major channel observed experimentally, is speculated to occur either via the S0←S1 internal conversion or, more likely, via the S1(1A″)→T1(3A) intersystem crossing followed by fast dissociation in the triplet state. Spin–orbit coupling between S1 and T1 has been calculated to be about 45 cm−1.

Quantum yield for H atom formation in the methane dissociation after photoexcitation at the Lyman-α (121.6 nm) wavelength

The gas-phase photodissociation dynamics of methane after excitation at the Lyman-α wavelength (121.6 nm) was investigated under collision-free conditions at room temperature. Narrow-band tunable Lyman-α laser radiation was generated by the resonant third-order sum-difference frequency conversion of pulsed-dye laser radiation and used both to photodissociate the parent molecules and to detect the photolytically produced hydrogen atoms via (2p 2P → 1s 2S) laser-induced fluorescence. H atom Doppler profiels were recorded and the absolute quantum yield for H atom formation, Φ H = (0.47 ± 0.11), was determined by means of photolytic calibration method where the Lyman-α photolysis of H 2O was used as a reference source of well-defined H atom concentrations. This value will be used in combination with previous results from H 2 and CH yield measurements to estimate the relative importance of the different product pathways in the CH 4 photodissociation.