Recombination Of Hydrogen Atoms Research Articles

Three-body recombination of spin-polarized atomic hydrogen in very strong magnetic fields

The study of spin-polarized atomic hydrogen in the three-dimensional gas phase has been extended into a new regime of magnetic field strength. Analysis of decay data for B=10--20 T reveal for the first time the characteristic peak in the three-body dipolar-decay rate constant as predicted by Kagan, Vartanyants, and Shlyapnikov (Zh. Eksp. Teor. Fiz. 81, 1113 (1981) [Sov. Phys.---JETP 54, 590 (1981)]). Beyond 17 T we observe a rapid falloff of the three-body-decay rate constant. We also determine a value for the three-body-exchange rate constant which is in accord with the calculation of Kagan et al. In contrast to the picture which was emerging from previous studies at more moderate fields, the data presented here suggest that existing theories are capable of describing the decay of the system in the very-high-field limit. The new picture which is emerging suggests that Bose-Einstein condensation may be achievable at somewhat higher fields which are currently available at high-magnetic-field facilities.

Effect of the recombination of hydrogen atoms on the implantation of hydrogen ions

Effect of the recombination of hydrogen atoms on the implantation of hydrogen ions

Hydrogen transport and particle-induced hydrogen release in carbonization layers

The transport of hydrogen through amorphous hydrogenated carbon films (a-C : H) has been studied by investigating the molecular and plasma-driven permeation of hydrogen through an iron membrane which had been coated in-situ with a-C: H and a-C: D films of up to 100 nm thickness. A pronounced initial increase of the plasma-driven permeation flux compared with that of a clean iron membrane is observed during the early stage of the film deposition. This is attributed to a progressive decrease of the recombination rate σk r on the exposed surface of the membrane. At larger film thickness, d, the plasma-driven permeation rate decreases and is inversely proportional to d. The contribution of hydrogen isotopes incorporated during film deposition and that of the plasma species to the permeation is separated in measurements with a-C: H layers exposed to deuterium plasmas. The permeation from molecular driving pressures through the membrane is unaffected by the carbon film up to the largest thicknesses investigated (100 nm). The data are analyzed in the framework of a simple model assuming recombination of hydrogen atoms to molecules within their implantation range and rapid transport of H 2 molecules through the film. Tentative values for the transport coefficient have been derived.

Theory of surface recombination of spin-polarized hydrogen

A theory is presented, based on the Faddeev equations, for direct two-body recombination of hydrogen atoms on a liquid helium surface. The equations developed are applicable to hydrogen or deuterium atoms in any spin state, but are applied in particular to dipolar recombination ofb state hydrogen atoms. The equations yield terms corresponding to one- and two-step processes. These terms are calculated for low temperatures (T-0.1 to 1.1 K) and high field strengths (B=4 to 14 T). The one-step term increases slowly withB, while the two-step term is rapidly decreasing. While the overall rate is quite small (∼5×10−18 cm2/s) compared to recombination by two-body spin-relaxation, the results have important consequences in understanding the experimentally measured three-atom dipolar surface recombination rates. In three-atom recombination, where the role of spin-relaxation and the two-atom one-step processes are repressed, the role of the underlying two-atom, two-step process is enhanced. The field dependence of the process relevant to the three-atom system is calculated and found to be in fairly good agreement with the experimental three-atom data. The role of possible liquid excitations in enhancing the contribution of the two-step processes is also discussed.

Spin-polarized atomic hydrogen in very strong magnetic fields.

We calculate the effective rate constant for three-body dipolar recombination of spin-polarized atomic hydrogen in conditions of high density and very high magnetic field strengths (beyond the <10-T range of previous calculations). We find that the decay of the gas sample is not sufficiently suppressed at magnetic fields available in the forseeable future.

Role of three-body correlations in recombination of spin-polarized atomic hydrogen.

We present results of a calculation of the volume rate constant Lgeff of dipole recombination in H↓↓, in which the bbb incoming state is determined exactly by means of the Faddeev formalism. Inclusion of all three-particle correlations in the initial state does not resolve the discrepancies between Kagan’s approach and experiment. As a first step towards an exact determination of the outgoing atom-molecule state, we present a calculation in which all three-particle collision aspects are taken into account, except for rearrangement. This leads to values for Lgeff, which are a factor of 5 smaller than experiment, while the B dependence of the rate constant still shows a slowly increasing behavior. On the basis of this state-of-the-art calculation, we thus localize the cause for the existing discrepancies in rearrangement processes.

Resonance recombination of spin-polarized atomic hydrogen at low temperatures

In sufficiently large magnetic fields, the weakly bound levels of H2 acquire finite lifetimes owing to hyperfine predissociation and become manifest as scattering resonances in spin-polarized atomic hydrogen (H↓). In this work we investigate in detail the contribution to the reaction kinetics in H↓ due to the resonance, denoted [Formula: see text], arising from the predissociation of the (v,J) = (14,4) level of H2. To characterize the formation and disintegration of [Formula: see text], we calculate the predissociation as a function of both the applied magnetic field and the dissociation energy, D, of the level. Recombination occurs when [Formula: see text] is stabilized in collisions that cause transitions to lower molecular levels. For low H↓, densities, collisions with atoms of the saturated 4He vapour dominate; we calculate the cross section in the distorted wave Born approximation and also use a coupled-channels approach. At higher densities, stabilization occurs in collisions with H↓ as well; the effect of proton exchange between the atom and the molecule is derived. Previous experiments, analyzed neglecting proton exchange, are reanalyzed including this; the dissociation energy D is not as well determined by the data as was previously thought. Prospects for future experiments are discussed.

Nascent vibrational/rotational distribution produced by hydrogen atom recombination

The nascent vibrational/rotational distribution produced by the reaction H+H+H→H2(v, J)+H is calculated using resonance complex theory. These calculations are carried out using the accurate Siegbahn–Liu–Truhlar–Horowitz surface. Results are presented for the dependence of the total recombination rate coefficient on temperature and specific rate coefficients into individual vibrational/rotational states.

Resonant Recombination of Spin-Polarized Atomic Hydrogen at Low Temperatures

We describe calculations of the predissociation of the (ν,J) = (14,4) level of molecular hydrogen and its contribution to the recombination of spin-polarized atomic hydrogen.

The Role of Final State Correlations in Recombination of Atomic Hydrogen

We calculate the rate-constant for recombination in the bulk of a spin-polarized atomic hydrogen gas. We use an exact initial state and include the most essential collision aspects of the final state, except for rearrangement.

Theoretical aspects of heterogeneous catalysis

Quantum chemical aspects of chemical bonding to metal surfaces are discussed. It is demonstrated that a Frontier Orbital Theory of chemisorption can be developed in which the group orbital local density of states at the Fermi level replaces the HOMO and LUMO interactions familiar in organic or organo-metallic chemistry. This appears to be a useful tool to describe elementary reactions at metal surfaces. Dissociation of the H2 molecule and hydrogen atom recombination is analysed in detail. Symmetry considerations can be applied. It is shown that the interaction with antisymmetric group orbitais lowers the activation energies. Such orbitais are also available at s-valence electron metal surfaces as long as the molecule interacts in bridging coordination sites.One finds that the interaction with metal d-valence electrons stabilizes coordination in the atop position. The relative contribution to bonding of metal s and d-valence electrons differs significantly for the transition metals. It is not only a function of metal-electron occupation, number, but also of the row in the periodic system in which the metal is placed. This information is used to explain the different hydrogenolysis behaviour of Ni and Pt.

Observation of inverse predissociation of spin-polarized atomic hydrogen at low temperatures.

Measurements of the two-body recombination of spin-polarized atomic hydrogen in a magnetic field of 40 kG have been extended to temperatures above 0.5/sup 0/K. The rate constant for the formation of parahydrogen shows an unexpected increase with temperature, which is explained by inverse predissociation into the v = 14,J = 4 level of H/sub 2/. Data indicate the level is bound by 0.7 +- 0.1/sup 0/K.

Hydrogen desorption from tantalum with segregated oxide surface layers

The combination of thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES) was used to examine the correlation between elementary processes on an oxidized tantalum surface and the acceleration of the dehydriding rates at increasing temperatures. We present the TDS of TaD x foils (foil thickness ~ 25μm) for various initial concentrations (0.038 ⩽ x ⩽ 0.108; α-phase), various heating rates and for high and ultra high vacuum conditions. Activation energies for desorption were derived from the TDS spectra on the assumption that the rate determining process is the recombination of hydrogen atoms to hydrogen molecules at the surface. We find two different activation energies for the low and for the high temperature region. The AES measurements show that the corresponding change in the desorption process is correlated with the dissolution of the segregated oxide surface layer.

Surface recombination in spin-polarised atomic hydrogen

The recombination of spin-polarised hydrogen atoms is found to arise from their interaction with the third-sound ripplons on the surface of the superfluid helium. The rate of recombination is found to have new terms in addition to those experimentally measured.

Adsorption and recombination of hydrogen atoms on a model graphite surface

The adsorption and recombination of atomic hydrogen on a model graphite grain have been examined in a series of calculations in which a modified, iterative, extended Hueckel program was used. The hydrogen atom is found to be chemisorbed at a site with a zero-point binding energy of 0.7 eV and at an equilibrium distance of 2.25 A above the site. Despite a barrier of about 0.4 eV between adjacent sites, calculations suggest that at temperatures as low as 10 K, an H atom will tunnel through to adjacent sites in less than one nanosecond. However, a potential barrier to the recombination of two hydrogen atoms has been found which displays high sensitivity to the mutual arrangement of the two hydrogen atoms with respect to the graphite surface. Results show that at very low temperatures, recombinations can occur only by tunneling. Consistent with experiment, the region in which H2 begins to form exhibits a repulsive potential with respect to possible chemisorption of the incipient H2 entity.

Decomposition of water by cerium oxide of δ-phase

Reduced cerium dioxide (CeO 2−x) can reduce water, producing hydrogen at ⩾298 K. Kinetic studies were focused on the stoichiometric reaction of δ-phase cerium oxide (CeO 1.818) with water vapor. Different activation energies of 18.1 and 33.4 kJ mol −1 were observed for the reactions at the temperature ranges above and below ca. 453 K, respectively. Rate equations observed in the two temperature ranges were also different. These results strongly suggest that the rate-determining steps are different between the two temperature ranges. Rapid oxygen exchange observed between H 2 18O and lattice oxygen in cerium oxide of δ- phase at ⩾ 298 K indicated that neither the adsorption of water molecules not the diffusion of oxygen ions in the bulk of the oxide can be the rate-determining step. H 2D 2 exchange occurred rapidly at 373 K compared to the rate of water decomposition, suggesting that the recombination of hydrogen atoms on the surface is not rate- determining either. A tentative reaction mechanism was proposed to explain the results of the kinetic studies. The rate-determining step at high temperatures (>453 K) is the reduction of OH − by the six-coordinated Ce 3+ which is present in the nonstoichiometric cerium oxide, while that at low temperatures (<453 K) is the subsequent reduction of H + by the seven-coordinated Ce 3+.

Desorption kinetics of LaNi 5, LaNi 4.7Al 0.3, (CFM)Ni 5 and MNi 4.5Al 0.5 hydrides

A kinetic study was performed on the hydrides of LaNi 5, LaNi 4.7Al 0.3, (CFM)Ni 5 (CFM  cerium-free misch metal) and MNi 4.5Al 0.5 (M  misch metal). In all cases isobaric and nearly isothermal conditions were maintained by intimately mixing a small quantity of hydride-forming material with a large amount of aluminum powder. Some of the desorption kinetics were conducted at temperatures in the 25–45 °C range while others were done at temperatures in the range 70–90 °C. First-order kinetic behavior was observed for all materials studied in the lower range with activation energies in the range 35–40 kJ mol −1. This indicates a probable diffusion-controlled process. In the higher temperature range second-order kinetics occurred with significantly higher activation energies. The second-order process is believed to be recombination of hydrogen atoms at the surface to form molecular hydrogen.

Recombination of hydrogen atoms in solution. Model calculations on spin effects

A computer model is presented for reactions in a spur containing two radicals plus two solvated electrons or two hydrogen atoms. Spin-correlation effects are calculated explicitly. Electron exchange in encounters can transfer singlet character, i.e. ability to react, from one pairing to another. When hydrogen atoms are involved the hyperfine coupling leads to distinctive magnetic-field effects on the product yields and to changes in the ortho-/para-hydrogen ratio.

INFRARED SPECTROSCOPY OF STAR-FORMING REGIONS - SOME RECENT RESULTS OF INTEREST.

ABSTRACT A brief summary of the major events in star formation is given. Three infrared spectroscopic phenomena currently being studied in star-forming regions are discussed: atomic hydrogen recombination lines in protostars, molecular hydrogen emission lines in shocked clouds, and solid-state absorption features due to molecules on grain mantles.

On the possibility of attaining the Bose-condensation region in spin-polarized atomic hydrogen at ultra-low temperatures

It is shown that the adsorption of atomic hydrogen on the He 4 surface is predominantly accompanied by excitation of the surface wave. At ultra-low temperatures a strong decrease in the adsorption rate with temperature is found to result in a surface density of adsorbed atoms essentially lower than its equilibrium value. Therefore the channels of recombination of atomic hydrogen on the surface prove to be suppressed and it becomes possible to make compatible the Bose-condensation and the long lifetime of the spin-polarized atomic hydrogen gas.