Dissociation Of Molecular Hydrogen Research Articles

Proton production in one- and two-color laser ionization and dissociation of molecular hydrogen

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTProton production in one- and two-color laser ionization and dissociation of molecular hydrogenJesse D. Buck, David H. Parker, and David W. ChandlerCite this: J. Phys. Chem. 1988, 92, 13, 3701–3705Publication Date (Print):June 1, 1988Publication History Published online1 May 2002Published inissue 1 June 1988https://doi.org/10.1021/j100324a004RIGHTS & PERMISSIONSArticle Views86Altmetric-Citations17LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (583 KB) Get e-Alerts

Excitation and dissociation of molecular hydrogen in shock waves at interstellar densities

The rate constant (kD) for the thermal dissociation of molecular parahydrogen by collisions with helium at kinetic temperatures of 2000-10,000 K is determined for the entire range of densities appropriate to interstellar molecular clouds. Included in a solution of the master equation for the dissociation reaction are collisional transitions among the rotation-vibration levels, collision induced dissociation out of all levels, and infrared quadrupole emission. The rate constant decreases strongly over the density range of 1 million-10,000/cu cm due to the competition between radiative and collisional processes. At low densities, kD approaches a constant value corresponding to direct collisional dissociation out of the (v = 0, J = 0) level. It is noted that the population distribution among the rotation-vibration levels can show large deviations from a Boltzmann distribution and that, under certain circumstances, there are even population inversions. 74 references.

The rate of dissociation of molecular hydrogen by hydrogen atoms at very low densities

view Abstract Citations (52) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Rate of Dissociation of Molecular Hydrogen by Hydrogen Atoms at Very Low Densities Dove, John E. ; Mandy, Margot E. Abstract The collision-induced dissociation of H2 in the (υ = 0, J = 0) internal state by H atoms was studied by three-dimensional quasi-classical trajectories on the Liu-Siegbahn-Truhlar-Horowitz ab initio potential energy surface. The dynamic threshold for dissociation was found to be essentially coincident with the energetic threshold. The dissociation cross sections were fitted to an excitation function and used to predict the rate constant for dissociation of H2 by H atoms by shock waves at very low interstellar densities. Publication: The Astrophysical Journal Pub Date: December 1986 DOI: 10.1086/184805 Bibcode: 1986ApJ...311L..93D Keywords: Gas Density; Gas Dissociation; Hydrogen Atoms; Interstellar Chemistry; Molecular Interactions; Shock Heating; Atomic Recombination; Computational Astrophysics; Computational Chemistry; Shock Wave Interaction; Star Formation; Astrophysics; INTERSTELLAR: MOLECULES; MOLECULAR PROCESSES; SHOCK WAVES full text sources ADS |

The microstructure of bimetallic [formula omitted] catalysts: A chemisorption and analytical electron microscopy study

Supported bimetallic RuCu SiO 2 catalysts are characterized by transmission electron microscopy, energy dispersive X-ray spectroscopy, electron microdiffraction, and chemisorption. Metal particles up to 4 nm in diameter are bimetallic, while particles larger than 4 nm contain only Cu. Considerable compositional nonuniformity is observed from one individual metal particle to the next. Microdiffraction patterns obtained from individual particles can be attributed to either Ru or Cu suggesting no significant modification in crystallographic structure of either metal component. Addition of Cu to Ru results in a drastic suppression of H 2 chemisorption while the extent of O 2 chemisorption is not as strongly affected. The suppressed H 2 chemisorption capability of Ru in the bimetallic catalysts is an indication of atomic interdispersion of Ru and Cu on the surface of the bimetallic clusters, leading to the break-up of the Ru ensembles which would be necessary for dissociation of molecular hydrogen. The influence of catalyst preparation techniques on the relative interdispersion of Ru and Cu and consequent discrepancies in the RuCu literature are discussed.

Ultraviolet Spectrometer Observations of Uranus

Data from solar and stellar occultations of Uranus indicate a temperature of about 750 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane and acetylene in the lower levels. The ultraviolet spectrum of the sunlit hemisphere is dominated by emissions from atomic and molecular hydrogen, which are kmown as electroglow emissions. The energy source for these emissions is unknown, but the spectrum implies excitation by low-energy electrons (modeled with a 3-electron-volt Maxwellian energy distribution). The major energy sink for the electrons is dissociation of molecular hydrogen, producing hydrogen atoms at a rate of 10(29) per second. Approximately half the atoms have energies higher than the escape energy. The high temperature of the atmosphere, the small size of Uranus, and the number density of hydrogen atoms in the thermosphere imply an extensive thermal hydrogen corona that reduces the orbital lifetime of ring particles and biases the size distribution toward larger particles. This corona is augmented by the nonthermal hydrogen atoms associated with the electroglow. An aurora near the magnetic pole in the dark hemisphere arises from excitation of molecular hydrogen at the level where its vertical column abundance is about 10(20) per square centimeter with input power comparable to that of the sunlit electroglow (approximately 2x10(11) watts). An initial estimate of the acetylene volume mixing ratio, as judged from measurements of the far ultraviolet albedo, is about 2 x 10(-7) at a vertical column abundance of molecular hydrogen of 10(23) per square centimeter (pressure, approximately 0.3 millibar). Carbon emissions from the Uranian atmosphere were also detected.

EARLY GALACTIC EVOLUTION AND THE NATURE OF THE FIRST STARS

ABSTRACT The observational data relating to the early evolution of the Galaxy are reviewed in order to assess the plausibility of a number of models that have been proposed for the first stars. On the basis of standard fragmentation models, it is argued that primordial stars were very similar to normal stars, but that in some circumstances the formation of low-mass primordial stars may be suppressed through the dissociation of molecular hydrogen by UV radiation. The existence of these conditions at the time of formation of the Galaxy could explain the absence of zero-metal stars. It is likely that primordial stars were neither highly peculiar nor very numerous, nor do they or their remnants make any substantial contribution to the Galaxy today.

A model for atomic hydrogen-metal interactions — application to recycling, recombination and permeation

A model for the interaction of hydrogen with metals is discussed which includes the surface concentration and the surface potential and is based on the observation that molecular hydrogen is dissociated before it is chemisorbed at a metal surface. It is shown that an activation barrier for dissociation of molecular hydrogen at the metal surface leads to an enhanced solubility and increased permeation rate of hydrogen in a metal exposed to atomic hydrogen as compared to molecular hydrogen. The sticking probability of molecular hydrogen on clean transition metals has been measured in the past and shown to be of the order of unity. As no activation barrier is present no difference is expected between atomic and molecular hydrogen. Whenever large differences have been observed in the past they must be attributed to an impurity contamination of the metal surface. Substantial reductions of the sticking probability due to dissociation barriers have been found for metals contaminated with less than one monolayer of electronegative impurities. We compare our model with earlier descriptions of hydrogen metal interactions. A theoretical expression is derived for the “recombination rate constant”. Furthermore we discuss implications of our model on hydrogen permeation (“super permeation”) as well as hydrogen pumping and pumping panels in tokamaks.

Preparation of hydrogenated amorphous germanium by reactive thermal evaporation

Hydrogenated amorphous germanium was prepared by the thermal evaporation of high purity polycrystalline germanium in an atmosphere of hydrogen plasma produced by high voltage AC discharge of molecular hydrogen. The addition of hydrogen during the thermal evaporation of germanium is shown to improve the electrical properties of the resulting amorphous germanium films considerably by saturation of dangling bonds, if the dissociation of molecular hydrogen takes place. Hydrogenated sample deposited at 200°C has shown a high resistivity and an activation type conduction (with an activation energy of 0.38–0.39 eV) in measuring temperature range (above room temperature).

Broadening and shift parameters of the sodium D lines perturbed by atomic hydrogen

The broadening and shift parameters of the sodium D lines perturbed by atomic hydrogen have been measured in the plasma produced behind the reflected shock in a combustion-driven shock tube. The incident shock wave is propagated at a Mach number typically of M=11 in hydrogen with an admixture of Ar (10 to 40% in volume) at an initial pressure of a few Torr. The conditions created in the reflected shock region are the following: temperatures in the range of 4500 to 600K, molecular hydrogen dissociation from 90 to 99% and atomic hydrogen number densities 1017<NH<1019 cm-3. The electron number density is negligible. The line profiles are recorded with an intensified Reticon photodiode array coupled to a high-resolution spectrograph. The instrumental width is about 125 mAA. By varying the gas mixture composition, the initial pressure and the shock strength, and assuming the validity of the impact approximation, the damping constants of the three species Ar, H, H2 can be separated. The values obtained for Ar and H2 agree with previous measurements made in different experimental conditions; for atomic hydrogen, our new, more accurate values differ significantly from the available theoretical results.

The kinetic theory of H2 dissociation

view Abstract Citations (214) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The kinetic theory of H2 dissociation Lepp, S. ; Shull, J. M. Abstract Collisional dissociation of interstellar molecular hydrogen may be suppressed at low densities by radiative stabilization of high vibrational level populations. The rate coefficient for H2 dissociation is computed with a master equation eigenvalue approach, including collisional excitation and dissociation from all 15 vibrational levels. The rates are sensitive to vibrational 'ladder climbing', multiquantum vibrational excitations, and collisional dissociation from low vibrational levels. Including these effects, it is found that the low-density dissociation rate at 4500 K is only suppressed by a factor of a thousand from the high-density limit. Analytic fits to the dissociation and radiative cooling rates are provided over a range of densities and temperatures. Publication: The Astrophysical Journal Pub Date: July 1983 DOI: 10.1086/161149 Bibcode: 1983ApJ...270..578L Keywords: Gas Dissociation; Hydrogen Clouds; Interstellar Matter; Kinetic Theory; Molecular Clouds; Molecular Collisions; Molecular Excitation; Astrophysics full text sources ADS |

Reaction of molecular hydrogen with the 100 face of MoO 3: II. Kinetics initiated by atomic hydrogen and characterization of the surface electronic state

In order to provide a physical background to the model proposed in Part I, the kinetics of the reaction between the 100 face of a MoO 3 single crystal and, first a mixture of 4% atomic hydrogen in H 2, and later molecular hydrogen only, has been studied. The rate processes of the activation step and of the stationary step are entirely comparable with those observed for one single crystal surface loaded with platinum particles. Thus atomic hydrogen in the gas phase as well as atomic hydrogen produced by the dissociation of molecular hydrogen on a Pt particle may prepare the favourable surface state able to dissociatively chemisorb molecular hydrogen, ruling out — once again — the classical model of the hydrogen spillover process. This “favourable” state has a Fermi level which is 0.25 eV lower than that of initial MoO 3, as shown by measuring the work function with a Kelvin probe. This lowering is in good agreement with the variation of the free energy between MoO 3 and H 1.6MoO 3, measured electrochemically by others. This suggests that the protons inserted into the surface layers transform the initial MoO 3 layers into layers with composition H 1.6MoO 3. The starting material is thus transformed into a biphasic system, the diffusion of the reaction boundary between the two types of layers being the overall rate limiting process. The Fermi energy of H 1.6MoO 3 being known, it is possible to show that in transformed surface layers the conduction and the valence bands overlap, in agreement with the approximate profile of this band observed by XPS for a reacted single crystal surface. The d character of this band would explain why molecular hydrogen can be dissociatively chemisorbed when this favourable surface state is obtained. The fast electron delocalization within the Mo-O-Mo bonds yields fast oscillations in the oxidation states of the molybdenum atom in the surface layer, accounting for the presence of “oxidized” and “reduced” sites. The formal equation observed in Part I for the rate of stationary step is therefore explained, the impinging H 2 molecules reducing temporarily the oxidized fraction of the surface.

The acceleration of molecular hydrogen clouds through radiative dissociation

We suggest that the dissociation of molecular hydrogen by ultraviolet radiation may provide a substantial portion of the momentum required to accelerate molecular clouds to their high, observed velocities. The process, however, is important only when the dissociation front velocity, u, is small compared to the speed of sound, c, in the molecular cloud ahead of the front, or if u

Electron Capture by Fast H+, He+, and He++ Ions in Collisions With Atomic and Molecular Hydrogen

Total cross sections for electron capture by H+ (250-600 keV), 4He+ and 4He++ (0.3-2 MeV) incident on atomic and molecular hydrogen have been measured. A directly heated tungsten oven provided an atomic-hydrogen target, and the degree of dissociation of molecular hydrogen and the target-thickness calibration were performed utilizing earlier results reported by Shah and Gilbody.

Physics of particle exhaust in pump limiters

Monte Carlo calculations are presented of neutral particle flows in hollow collection probes and large area limiters used in recent experiments. The collected neutral hydrogen flux scales linearly with plasma density for n i < 2 × 10 11 cm −3 where the plasma is essentially transparent to the neutrals. For 2 × 10 11 < n i < 2 ×10 12 cm −3 the neutral flux varies mildly nonlinearly as n 1.26 i . The transition between regimes is driven by the mean free path for molecular hydrogen dissociation being of the order of the probe dimensions. Calculations of the neutral energy spectra emerging from the pump ducts of various lengths indicates that within the accuracy of avialable low energy surface data, energy transfer to the duct walls is very inefficient. Furthermore, the results indicate that “energy freezing” occurs at epithermal energies as a consequence of particle and energy reflection coefficients approaching unity at low energies.

Formation ofH−ions by backscattering thermal hydrogen atoms from a cesium surface

We have investigated the formation of negative hydrogen ions by backscattering thermal hydrogen atoms incident on a thick cesium surface. The incident hydrogen atoms are produced by thermal dissociation of molecular hydrogen at temperatures of about 2500 K. The negative ions are produced with an efficiency of ${10}^{\ensuremath{-}4}$ ions per incident atom. Our data show that the backscattering theory of Hiskes-Gardner-Karo, originally developed for incident hydrogen atoms with energies above 100 eV, can be extrapolated down to the 1-eV range. One way of comparing our experimental results with the above theory is to determine the average survival probability for the negative hydrogen ions as they leave the cesium surface. We obtain a survival probability of 0.005 for hydrogen from a 2500-K oven, in close agreement with the value of 0.004 calculated by extrapolating the experimental results of Hiskes and Schneider down to 1 eV. In addition to the negative hydrogen ions we see electrons produced with efficiencies about one hundred times greater than for the negative ions. The electrons appear to be produced by an Auger process accompanying a chemical reaction of hydrogen with cesium.

Collision-induced dissociation of H2 and CO molecules

A discussion is presented of the collision-induced dissociation of molecular hydrogen and carbonn monoxide in shock-heated regions of the interstellar medium. The modifications to the dissociation rate caused by radiative stabilization are considered, and estimates are presented of the effective rates of collision-induced dissociation as functions of density and temperature in gases in atomic and molecular hydrogen. Below a certain density, n/sub asterisk/, radiative stabilization greatly reduces the rate of collision-induced dissociation with substantial consequences for the thermal and chemical evolution of the postshock gas.

Evolution of giant gaseous protoplanets embedded in the primitive solar nebula

Evolutionary calculations are presented, in a spherically symmetric approximation, for a protoplanet of 1 Jovian mass with homogeneous solar composition during the early phase of quasi-static contraction prior to the dissociation of molecular hydrogen. In contrast to earlier calculations which assume that protoplanets are isolated, this study invokes a time-dependent surface boundary condition that simulates physical conditions in an evolving primitive solar nebula. In a first set of calculations the protoplanet is surrounded by a “thermal bath” whose temperature varies with time and whose pressure is small and constant in time. Under a wide range of parameters the result is evaporation and complete dispersal of the object. Conditions required for the protoplanet to survive are discussed. In a second set of calculations both the temperature and pressure at the surface vary with time according to models of the solar nebula. In this case the protoplanet is not dispersed, but the evolution is accelerated or retarded relative to that of an isolated protoplanet, depending upon whether the entropy in the nebula is lower than or higher than, respectively, the entropy in the outer layers of the protoplanet. Processes by which terrestrial planets can form in the cores of giant gaseous protoplanets are discussed.

Pulsed laser stimulated field desorption of hydrogen from molybdenum

The recently developed pulsed laser atom probe has been used to investigate the adsorption and dissociation of molecular hydrogen in high electric fields. The results indicate that at temperatures of 60 K and fields of 2.0 V/Å and higher, hydrogen is field adsorbed on molybdenum surfaces in molecular form. The activation energy of desorption is found to depend on the square of the applied electric field, in agreement with an expression previously derived for noble gas atoms. The desorption mechanism is shown to be a two-step process consisting of thermal release followed by field ionization and/or dissociation above the surface at xc (the critical distance of field ionization). Values of xc are obtained for H+, H2+, and He+, and it is shown that the probability of molecular dissociation at a given field strength can be predicted from simple energetic considerations. An expression relating the number of field adsorbed molecules on a surface with N adsorption sites to various experimental parameters is derived from an empirical rate equation, and data from the present study are shown to be in good agreement with the theoretical predictions.

Mg-based hydrogen storage materials: Surface segregation in Mg2Cu and related catalytic effects

Fresh surfaces of Mg2Cu polycrystals cleaved in UHV have been investigated by AES, XPS and X-ray induced AES at room temperature. They show chemical decomposition of the surface leading to pronounced increase of surface Mg content. In contrast to the Mg the Cu remains essentially metallic, even on air exposed samples. It is suggested that this segregation prevents the formation of a compact oxide or hydroxide layer, thereby enabling dissociation of molecular hydrogen at the metallic Cu precipitations and/or the metallic Mg2Cu subsurface. The results support comparable conclusions drawn earlier for LaNi5, FeTi and Mg2Ni.

Calculations of the evolution of the giant planets

Evolutionary calculations are presented for spherically symmetric protoplanetary configurations with a homogeneous solar composition and with masses of 10 −3, 1.5 × 10 −3, 2.85 × 10 −4, and 4.2 × 10 −4 M ⊙. Recent improvements in equation-of-state and opacity calculations are incorporated. Sequences start as subcondensations in the solar nebula with densities of ∼10 −10 to 10 −11 g cm −3, evolve through a hydrostatic phase lasting 10 5 to 10 7 years, undergo dynamic collapse due to dissociation of molecular hydrogen, and regain hydrostatic equilibrium with densities ∼1 g cm −3. The nature of the objects at the onset of the final phase of cooling and contraction is discussed and compared with previous calculations.