Distribution Of Hydrogen Atoms Research Articles

Spatial distributions of H, CN, and C 2 in a diamond-growing oxyacetylene flame

Two-dimensional laser-induced fluorescence (2D-LIF) measurements are applied to the chemical vapor deposition (CVD) of diamond by an oxyacetylene flame to visualize the distributions of atomic hydrogen, C 2 , and CN in the gas phase during diamond growth. Experiments are carried out in laminar flames and reveal that atomic hydrogen is ubiquitous at and beyond the flame front. Its presence extends to well outside the diamond deposition region, whereas the C 2 distribution is limited to the flame front and the acetylene feather. CN is found to be present mostly at the outer edge of the flame, where ambient air interacts with flame gases. The diamond layers obtained are characterized by optical as well as scanning electron microscopy (SEM) and cathodoluminescence topography (CL). Clear relations are observed between the local variations in growth rate of the diamond layer and the distribution of H, C 2 , and CN in the boundary layer just above the substrate. Further relations between CN and the morphology and the nitrogen incorporation as identified by CL of the deposited diamond layer are found as well. These relations agree with theoretical models describing the importance of the mentioned species in (flame) deposition processes of diamond. Three separate regions can be discerned in the flame and the diamond layer, where the gas phase and diamond growth are predominantly governed by the flame source gases, the ambient atmosphere, and the interaction of both, respectively.

Plateau in above-threshold-ionization spectra and chaotic behavior in rescattering processes

An improved quasistatic model is used to describe the ionization process of atoms in intense linearly polarized fields. Numerical calculations of above-threshold-ionization (ATI) energy spectra and photoelectron angular distributions (PAD) of hydrogen atoms clearly reveal a plateau and side lobe structures, which are in good agreement with recent ATI experiments. Our results show that the existence of this plateau and the side lobes is a consequence of the classical kinematics of the electrons in combined atom and laser fields. Furthermore, we find that the onset of these unusual phenomena is related to the onset of chaotic behavior in the system.

Revised crystal structure of molybdenum hydroxymonophosphate, MoO2 · H2O · P(O3OH)

Abstract The structure of the Kierkegaard hydroxymonophosphate has been revised. The chain-like structure of this compound is confirmed. The valence-strength based on valence length calculations, allows a new distribution of hydrogen atoms to be proposed. The structure consists of P(O3OH) tetrahedra and MoO5(H2O) octahedra, leading to the formulation MoO2 · H2O · P(O3OH) instead of Mo(OH)3PO4. The cohesion of the structure is effectuated by strong hydrogen bonds between the different chains.

Optical emission spectroscopy of the plasma during microwave CVD of diamond thin films with nitrogen addition and relation to the thin film morphology

It is common knowledge that the presence of gas impurities such as nitrogen during microwave plasma assisted chemical vapour deposition of diamond can drastically affect the film morphology and even favour the growth of oriented textured films. It was shown previously that diamond films could be grown with various preferred orientations depending on the fraction of nitrogen present in the process gas mixture. In this work, we show how the methane and nitrogen fractions in the feed gas can alter the morphology of the diamond film deposited at high substrate temperature (1600 K). The domain of high substrate temperatures (>1300 K) has not yet been widely investigated and offers the opportunity of growing diamond films with other textures and morphologies with respect to their counterparts grown at lower temperatures. Optical emission spectroscopy is applied to study the spatial distribution of atomic hydrogen, CN and C 2 emitting radicals from the substrate surface to the bulk plasma. Scanning electron microscopy and X-ray diffraction are used, respectively, to identify the different diamond film morphologies and to establish the preferred growth orientations ranging from {111} to {100} textured films.

Picosecond molecular motions in bacteriorhodopsin from neutron scattering

The characteristics of internal molecular motions of bacteriorhodopsin in the purple membrane have been studied by quasielastic incoherent neutron scattering. Because of the quasihomogeneous distribution of hydrogen atoms in biological molecules, this technique enables one to study a wide variety of intramolecular motions, especially those occurring in the picosecond to nanosecond time scale. We performed measurements at different energy resolutions with samples at various hydration levels within a temperature range of 10-300 K. The analysis of the data revealed a dynamical transition at temperatures Td between 180 K and 220 K for all motions resolved at time scales ranging from 0.1 to a few hundred picoseconds. Whereas below Td the motions are purely vibrational, they are predominantly diffusive above Td, characterized by an enormously broad distribution of correlation times. The variation of the hydration level, on the other hand, mainly affects motions slower than a few picoseconds.

Effect of the global neutral hydrogen distribution on the spatial structure and thermal balance in the upper ionosphere

Abstract A global numerical self-consistent and time-dependent model of the thermosphere, ionosphere and protonosphere (GSM TIP) was used to study the influence of atomic neutral hydrogen distribution in the exosphere on the upper ionosphere parameters for summer solstice conditions during high solar activity (F10.7 = 200). Theoretical model calculations were compared with measured ion and electron densities obtained by in situ observations along early morning and afternoon orbits of the satellite pair Active and Magion-2 on 31 July (1990). The satellites passed over Europe around 03:25 and 15:40 UT at an altitude of about 2000–2500 km during orbits 3805 and 3812. The numerical results were compared with O+, H+ Ne densities in the dawn sector and with the Ne concentration in the afternoon sector. We found that the atomic neutral hydrogen density is an essential model input parameter for upper ionosphere and plasmasphere studies. The inaccurate model input of the exospheric neutral hydrogen distribution can cause dramatic variations of the H+ density up to a factor of 2–4 and a 40% variation of the Ne concentration under the conditions of our study. On the other hand, the variability of the thermal latitudinal structure (Ti and Te) in the upper ionosphere in the altitude range from 2000 to 2500 km reaches only about 10–15%.

Distribution of hydrogen atoms in YPd 3H x studied by neutron diffraction and inelastic neutron scattering

The YPd 3H(D) x system was studied by means of neutron powder diffraction as well as incoherent inelastic neutron scattering at various temperatures for x=0, 0.19 and 0.30. The L1 2 structure of the YPd 3 was conserved on hydrogenation. The position and site occupation of H(D) atoms were determined from the neutron diffraction data. The H(D) atoms occupy preferentially the characteristic octahedral interstices composed by 6 Pd atoms. This result is discussed in view of the recently proposed criteria predicting the relative hydrogen occupancy of various interstitial sites in intermetallic hydride.

Photodissociation and size analysis of (HBr)n clusters

AbstractHydrogen bromide clusters that are generated in an adiabatic expansion with Ar are size‐selected by scattering from a He or a Ne beam and a subsequent separation by a velocity selector. The complete fragmentation pattern for electron impact ionization of clusters n = 3,5,7,10, and 15 is obtained for electron energies of 70 eV. Up to cluster sizes of n = 15 strong fragmentation is observed. Based on these results the cluster sizes are measured as a function of different source conditions. For the same experimental conditions the (HBr)n clusters are photoionized at 243.1 nm. A time‐of‐flight mass spectrometer with low extraction fields is used to obtain the translational energy distributions of hydrogen atoms. Several distinct features are observed which sensitively depend on the cluster size. At small cluster sizes a tail appears towards lower energies in the fast hydrogen kinetic energy distribution which is caused by few collisions with the cluster molecules. Peaks at higher energies are attributed to internally excited HBr molecules which gain their energy in collisions with photofragmented H atoms. Small clusters exhibit pure rotational excitation with averaged J = 7, while the larger ones show vibrational excitations to v = 1 with J = 8 and v = 2. Finally, we observe a narrow H component peaking near zero velocity and exhibiting an isotropic angular distribution. This effect appears only at larger cluster sizes and is attributed to a complete caging of the photofragments.

Radiative emission analysis of an expanding hydrogen arc plasma—II. Plume region diagnostics through radial emission

An experimental investigation of the plasma plume of a 1 kW class radiatively-cooled hydrogen arcjet thruster is presented. The line-of-sight emission of the Balmer-alpha line is measured at several radial positions using a high resolution monochromator and photon counting detection. The line-of-sight spectra are deconvolved radially into Voigt profiles, from which radial distributions of translational temperature and electron number density are determined. Absolute intensity calibration is also performed providing radial distributions of atomic hydrogen m = 3 excited state number density. Electron temperature profiles are calculated using a non-equilibrium collisional-radiative model. The hydrogen translation temperature displays good agreement with previous measurements. The electron number density profiles are consistent with Langmuir probe measurements made downstream of the exit plane. The results of the model indicate that the plasma at the nozzle exit plane is strongly recombining.

Species characterization for a direct-current-biased hot filament growth of diamond using spatial resolved optical emission spectroscopy

Spatial resolved optical emission spectroscopy was applied for a direct-current biased hot filament diamond growth system with a spatial resolution of better than 1 mm. Emission lines from atomic hydrogen, CH, CH+, H2, and Ar were observed in the visible region. It is found that emission intensity of all the observed lines decreases rapidly as the probe distance from the filament center raises, and increases near the substrate surface. The relative concentration of atomic hydrogen has been estimated by using the emission line of Ar at 750.4 nm as an actinometer. The concentration of atomic hydrogen decreases from filament to the substrate and drops sharply at the substrate surface. The emission spectra and the distribution of hydrogen atom along the substrate surface have also been measured, which are closely correlated with the homogeneity of diamond film growth. And the low concentration of atomic hydrogen at the edge of the substrate results in a worsening of diamond films.

The distribution of hot hydrogen atoms produced by electron and proton precipitation in the Jovian aurora

The energy distribution functions of nonthermal thermospheric hydrogen atoms are calculated for electron and proton precipitation in the Jovian aurora. A numerical model taking into account the production, elastic and inelastic relaxation and transport processes for hot H atoms is developed. This model is based on a Monte Carlo solution of the nonlinear Boltzmann equation for hot H atoms produced by electron and proton impact on H and H2 and exothermic chemical reactions. The distribution functions show a much higher energy tail for proton than electron precipitation. It is shown that the steady state flux of hot atoms (E ≥ 2 eV) is essentially isotropic. The peak and column hot H densities are about 3 × 105 cm−3 and 1 × 1014 cm−3 for a 100 erg cm−2 s−1 precipitation combining hard (22 keV) and soft (0.2 keV) electrons mixed with a 10 erg cm−2 s−1 flux of soft (0.3 keV) protons. These column densities, coupled with the wide range of hot H atom energies, may play an important role in the formation of the Lyman α line profile. Multiple scattering in the wings of the Ly α line by the fast H atoms is shown to partly account for the broad Ly α profile observed in the Jovian aurora with the Hubble space telescope.

Negative surface ionization of hydrogen atoms and molecules

The total yield of H− ions, Y(Ein), produced in backscattering of low-energy H+ and H+2 ions from polycrystalline gold, tungsten, and molybdenum converter surfaces was measured at normal incidence in the energy range Ein=2–30 eV per nucleus. The yield per nucleus is independent of the ion mass. This indicates that the molecular ions are dissociated before colliding with the converter surface. A universal expression for Y(Ein) was developed by combining the electron tunneling theory with atomic scattering theory. This expression agrees well with measurements. The yield is completely characterized by two parameters, Eth/RE and RNη0, which can be determined experimentally: Y=0 for Ein=Eth/RE, and Y approaches the maximum yield Rη0 as Ein increases. These parameters were determined from measured H− yields in ion beam backscattering experiments, as well as for backscattering of thermal distributions of hydrogen atoms. For beam experiments, the maximum yield of 0.3 per nucleus was obtained for Mo/Cs converters with 1.5 eV work function. A higher maximum yield of 0.42 was obtained from experiments on backscattering thermal distributions of H atoms. This is attributed to high extraction fields. The universal yield formula made it possible to compare the results of the two different types of experiments.

Modelling of the hydrogen distribution at a crack tip

A model has been developed to predict the distribution of hydrogen atoms at a crack tip with the unique feature of incorporation of generalised boundary conditions which account realistically for the electrochemistry-diffusion interface. The use of boundary conditions involving expressions for the flux allows identification of the conditions for diffusion and surface reaction control of hydrogen transport. For ferritic steels, which have a relatively large diffusivity compared with face centred cubic alloys, the model predicts that hydrogen transport is controlled by the kinetics of cathodic generation of hydrogen atoms. The crack-tip concentration is approximately an order of magnitude less than that predicted assuming the conventional constant concentration boundary conditions derived from diffusion controlled transport. The results imply that crack growth rates will be limited by the kinetics of surface reactions and that predictive models of crack growth rates and thresholds for cracking based on diffusion control should be reassessed. The use of generalised boundary conditions also enables the calculation of the time evolution of the hydrogen distribution in initially precharged metals. It is shown that the crack tip and crack walls act as sinks for hydrogen atoms in these circumstances, depleting the hydrogen atoms in the crack-tip region. The loss of hydrogen through the crack tip in precharged samples may affect the location of hydrogen cracking and is an additional factor to be considerred in comparing internal and external hydrogen embrittlement. It also raises questions concerning the meaning of tests on cadmium plated samples because of the loss of hydrogen through the crack tip once the coating has been fractured due to dynamic straining or crack advance.

The Distribution of Neutral Atomic Hydrogen in the Interacting Ringed Barred Spiral NGC 5850

NGC 5850 is a large, nearby SB(r)b spiral that is an apparent member of the NGC 5846 group of galaxies (de Vaucouleurs 1975; Turner & Gott 1976). The mean corrected redshift of this group is ≈1900 km s−1 (Haynes & Giovanelli 1991). We have obtained broadband imaging and VLA HI interferometry of this galaxy as part of a general study of its properties. The most noteworthy characteristic of the galaxy is a severe distortion of the outer regions. This distortion is most likely due to an interaction with another member of the group, possibly NGC 5846 or its companion NGC 5846A, which are only 10' to the northwest. In this paper we summarize some of the results of the optical and HI analysis, and highlight the effects of the interaction.

A Comparison of Neutron Diffraction and Molecular Dynamics Structures: Hydroxyl Group and Water Molecule Orientations in Trypsin

A comparison is presented of experimentally observed hydroxyl and water hydrogen atoms in trypsin determined from neutron density maps with the results of a 140 ps molecular dynamics simulation. Experimental determination of hydrogen and deuterium atom positions in molecules as large as proteins is a unique capability of neutron diffraction. The comparison addresses the degree to which a standard force-field approach can adequately describe the local electrostatic and van der Waals forces that determine the orientations of these hydrogen atoms. The molecular dynamics simulation, based on the all-atom AMBER force-field, allowed free rotation of all hydroxyl groups and movement of water molecules making up a bath surrounding the protein.The neutron densities, derived from 2.1 Å 2H 2O-H 2O difference Fourier maps, provide a database of 27 well-ordered hydroxyl hydrogen atoms. Virtually all of the simulated hydroxyl orientations are within a standard deviation of the experimentally observed positions, including several examples in which both the simulation and the neutron density indicate that a hydroxyl group is shifted from a “standard” rotamer. For the most highly ordered water molecules, the hydrogen distributions calculated from the trajectory were in good agreement with neutron density; simulated water molecules that display multiple hydrogen-bonding networks had correspondingly broadened neutron density profiles. This comparison was facilitated by development of a method to construct a pseudo 2 Å density map based on the hydrogen atom distributions from the simulation. This method is particularly useful for statically disordered water molecules, in which the average location assigned from a trajectory may represent a site of relatively low occupancy. The degree of disorder of internal water molecules is shown to result primarily from the electrostatic environment surrounding that water molecules as opposed to the cavity size available to the molecule.

Diagnostics for the spatial distribution of hydrogen atoms around the divertor region

For measurements of spatial distributions of hydrogen atoms around the divertor region, laser-induced fluorescence (LIF) based on various two-phonon excitation schemes from the ground level is discussed, and merits and demerits under different experimental conditions are compared. Excitation to the n=3 level using an ArF excimer laser (wavelength = 193 nm) and its Raman shifts in deuterium gas (205 and 219 nm) and observation at Balmer alpha transition (656 nm) was found to be the best for the above purpose. A design study of the fluorescence observation and estimation of expected signal to noise (SN) ratios were performed for measurements of atomic hydrogen densities on plasmas around the JFT-2M tokamak divertor, showing that such a measurement is possible.

Cascade processes and the kinetic-energy distribution of pionic hydrogen atoms.

The previously unmeasured neutron time-of-flight distributions for the reaction ${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$p\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{0}$n in gaseous targets at pressures of 17 and 40 bar have been measured. The kinetic energy of the ${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$p atoms at the instant of the nuclear reaction has been evaluated from the Doppler broadening of the neutron time-of-flight spectra. Evidence was found for ${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$p atoms with kinetic energies of 75 eV. The present experimental data were interpreted within a cascade model that takes the evolution of the kinetic-energy distribution during the cascade into account. The parameters of the model were determined from experiments measuring neutron time of flight in liquid hydrogen and x-ray yields in gas. Coulomb deexcitation is responsible for the significant fraction of the high-energy component, whose intensities are compatible with the calculations of Bracci and Fiorentini [Nuovo Cimento 43A, 9 (1978)]. Stark mixing is found to be significantly stronger than in the commonly used straight-line approximation; the initial mean kinetic energy of 1--2 eV is consistent with the results of muonic hydrogen. The model therefore describes the cascade of pionic hydrogen over a range of pressures of three orders of magnitude. The implications for high-resolution x-ray measurements of the 1S-level nuclear width are discussed.

95/02152 Non-Darcy forced convection boundary layer flow

Illinois No. 6 coal and its reductive methylation and butylation products have been studied by magnetic resonance techniques. Conventional CP-MAS13Cn.m.r. spectroscopy indicates that 62% of the carbon atoms in the coal are aromatic and that about 6% of the carbon atoms are carbonyl. Esters are more abundant than carboxylic acids. The resonances of methoxy groups and other novel etheric carbon atoms are apparent in the high field region. Dipolar dephasing experiments suggest that methyl carbon atoms constitute no more than 16% of the carbon, methylene and methine carbon atoms about 14% and quaternary aliphatic carbon atoms about 2%. The dipolar dephasing experiments in conjunction with previous work also permit estimates of the hydrogen atom distribution. The THF-insoluble products obtained in the reductive alkylation reactions with13C-enriched alkylating agents contain paramagnetic and ferromagnetic substances that adversely influence the solid state n.m.r. spectra. However, good 13C n.m.r. spectra were obtained after these substances were extracted with aqueous hydrochloric acid. The O:C alkylation ratios are 1.2 and 1.0 for methylation and butylation, respectively. Dipolar dephasing experiments establish that the decay constants of functional groups in the whole coal and of C- and O-methyl-13C and C- and O-butyl-1 -13C nuclei in the labelled coal molecules are very similar to those of reference compounds. These findings suggest that the decay constants measured for the 13C nuclei in coals and coal-derived solids provide reliable information about their degree of substitution.

Neutron scattering from amorphous metals

Detailed information about the structure of amorphous metallic alloys, the so-called metallic glasses, is obtained from neutron diffraction in combination with the isotopic substitution technique. In many cases neutron diffraction and X-ray diffraction are complementary techniques. The atomic structure of these materials is characterized by distinctive topological and chemical ordering effects. The establishment of theoretical models on the basis of accurate structural data provides a deeper insight into the 3-dimensional arrangement of the atoms in amorphous metals. Neutron diffraction is particularly suited for the investigation of the distribution of hydrogen atoms in amorphous alloys. Small angle scattering presents a means for the characterization of the inhomogeneous structure of metallic glasses.

Electron-impact dissociation of HCl: Translational energy and angular distributions of excited hydrogen atoms

Electron-impact dissociation of HCl for the formation of excited hydrogen atoms (n=4) has been investigated by measuring Doppler profiles of the Balmer lines and their angular dependence at a high optical resolution using a Fabry–Perot interferometer. The translational energy distribution (TED) and the angular difference Doppler profile were obtained. There are five major dissociation processes for the formation of H* (n=4). The threshold energy and the TED peak of the five components are (1) 19 and 2.5; (2) 25 and 7.2; (3) 29 and 1.7; (4) 36 and 5.1; and (5) ≳40 and 8–12 eV, respectively. Formation of components 1 and 4 is anisotropic and parallel with respect to the electron beam. Component 1 should be produced by predissociation through the Rydberg states with the Σ symmetry converging to either the 4Σ or 2Σ− state and then those converging to the A 2Σ+ state. The asymmetry parameter (β) of component 1 was determined to be about 0.62, and the intermediate excited state for the formation of component 1 has a lifetime equal to the rotational period. Component 2 would be produced through the Rydberg states converging to the 2Π state. Component 3 would be produced through high-lying doubly excited Rydberg states converging to either the (4)2Π or (4)2Σ+ state. Component 4 should be produced through doubly excited repulsive states with the Σ symmetry.