Solid CH Research Articles

Direct ESR evidence for S H2 type reaction of methyl radical with methylsilane and methylgermane in a low temperature solid: A deuterium labeling study

The first ESR spectroscopic evidence of a bimolecular homolytic substitution (S H2) reaction in a 77 K solid phase was presented, where CD 3 radicals were substituted for the CH 3 group of methylsilane and methylgermane (CH 3XD 3, X = Si, Ge). The CD 3 radicals were generated by in situ UV-photolysis of CD 3I in the solid CH 3XD 3 in an ESR cavity at 77 K. Using the selectively deuterium labeled compounds, it was demonstrated that the S H2 reaction proceeded over a competitive H atom abstraction reaction with a large H/D isotope effect. Ab initio and density functional theory calculations complement the experimental results.

Cold neutron production in solid and liquid [formula omitted] moderators. II: on the reentrant-hole configuration

It has been shown in our previous paper (Part I) that a solid CH 4 moderator produces higher density of cold neutrons inside and greater cold-neutron leakage flux from the surface, in comparison with those for liquid H 2 and CH 4 . To take such advantages of the moderator, a porous configuration with many thin holes for emerging cold-neutron beam is being studied. In the present preliminary report, neutronic characteristics of the hole are described together with the results of Monte Carlo calculation.

Cross-section models of liquid para H 2 and solid CH 4 for design of an advanced cold-neutron source

(a) Coherent cold-neutron scattering in liquid para-H 2 and D 2 is described theoretically in terms of a mean field approximation. The cross-section results, both double-differential and total, show good agreement with experimental ones at temperatures between melting and boiling points. (b) A cross-section library (group constant set) of solid and liquid CH 4 is generated: 80 groups between 0.1 μeV and 10 eV at 20.4, 50, 90.7 and 111.7 K . It is shown through neutron transport analysis that cold neutron production in a solid CH 4 moderator is significant especially at lower temperature as an efficient convertor of fast and thermal neutrons into cold ones.

Physical and chemical characterization of gas hydrates and associated methane plumes in the Congo–Angola Basin

As a part of the ZAIANGO IFREMER/TOTAL-FINA-ELF program, gas hydrates were collected from a gravity sediment core on the Congo–Angola margin during the ZAI-ROV cruise (December 2000). Gas hydrates, associated with a deep giant (800 m in diameter) active depression called “pockmark,” occurred from the sediment surface down to 12 m depth, at 3160 m water depth. Elevated concentrations of particles, manganese, iron, and methane detected in the water column close to the sediment surface reveal intense fluid circulation transporting methane-rich turbid fluids into the overlying bottom waters. Free gas is liberated via diffusion and/or advection and from destabilizing gas hydrates. The gas hydrates occur as small fragments and massive crystal aggregates, mostly disseminated irregularly in the highly disturbed sediment and escaping in the overlying deep seawater, creating methane-rich plumes. The dissociation of solid CH 4 hydrate particles rising in a turbulent flow may explain the high heterogeneity of methane concentration and CH 4 peaks measured in vertical profiles. Raman spectroscopy revealed that the gas hydrates recovered from the Congo–Angola are mainly 100% methane gas hydrate of structure I cubic with a lattice constant of a=12 A°, but H 2S and CO 2 are co-clathrated with CH 4 in cages. Analyses of hydrate water show depletions of Cl, SO 4, Na, Mg, Ca, and Sr, and enrichments of Si, Cs, and Ba, compared to the ambient deep seawater. Gas analysis shows that methane is the major component (99.1%), but CO 2 (0.83%) and heavier gases such as C 2H 6 (0.043%) and H 2S (0.02%) are also present as traces. Helium concentrations were in the range of 0.04–1.3 ppm, with slightly radiogenic 3He/ 4He ratios between 6.6×10 −7 and 7.6×10 −7. The hydrate methane has δ 13C of −69.3‰ (PDB) and δ D of −199‰ (V-SMOW), and the hydrate CO 2 has δ 13C of −17.5‰ (PDB). These values indicate a primarily microbial origin for the CH 4, which is generated through bacterial CO 2 reduction, as previously observed on many continental margins where solid gas hydrates were sampled.

Cold neutron production in solid and liquid CH 4 moderators

A numerical study of cold neutron production in cryogenic moderators composed of solid or liquid CH 4 at temperatures ranging from 20.4 to 111.7 K is made. The study is performed by a multigroup neutron transport analysis with a set of energy-averaged cross-sections (group constants) generated by the authors. Particular attention is paid to the total and angular properties of a space–energy cold-neutron flux, especially in terms of an equilibrium energy spectrum at each temperature, a storage property of high-density cold neutrons and an anisotropic emission of cold neutrons from the moderator surface.

Cross-section model for cold neutron scattering in solid and liquid methane. II: group-constant generation

A set of energy-averaged cross-sections (group constants) for neutron scattering in solid and liquid CH 4 is generated: 80 groups between 0.1 μeV and 10 eV at 20.4, 50, 90.7 and 111.7 K . Use is made of the double-differential scattering cross-section model developed by the authors, which includes molecular translations and rotations as well as intramolecular vibrations. In view of the computed results on scattering and slowing-down properties, it is expected that the group constants may serve as a powerful tool for design of an advanced cold neutron source.

Development of cross-section models for liquid H 2 and solid CH 4

In order to clarify neutronic performance of a pulsed cold-neutron source, scattering properties of some typical hydrogenous moderators are studied. Namely, cross-section models for liquid ortho- and para-H2 at temperatures between 14 and 20.4 K and solid and liquid CH4 at temperatures down to 20 K are developed. A set of group constants for liquid H2 is also generated: 80 groups between 0.1 μeV and 10 eV in a P3 approximation. The major results are as follows. (a) Cold-neutron production due to a para-to-ortho transition and up-scattering of lower-energy neutrons by ortho-H2. (b) Neutron slowing-down by rotational excitation of a CH4 molecule, especially at 20 K. And (c) an optimum condition of a liquid-H2 source model for cold-neutron production.

Cross-section model for cold neutron scattering in solid and liquid methane

Incoherent neutron scattering cross-sections for solid CH 4 in the temperature range of 20.4– 90.7 K and liquid CH 4 at temperatures between 90.7 and 111.7 K are evaluated. A space–time correlation approach is used to describe a double-differential scattering cross-section which is basically expressed by a generalized frequency distribution. The cross-section model includes molecular translations and rotations as well as intramolecular vibrations. The former are concerned with very short-time free-gas like translation, short-lived vibration and long-time diffusion (only in liquid state). The latter consists of short-time free rotation and long-time isotropic rotational diffusion. Numerical calculations on double-differential and total cross-sections are carried out for incident neutron energies covered 0.1 μeV to 10 eV . Good agreement with experimental results at many different temperatures is found.

Experimental measurements of deep directional columnar heating by laser-generated relativistic electrons at near-solid density.

In our experiments, we irradiated solid CH targets with a 400 J, 5 ps, 3 x 10(19) W/cm(2) laser, and we used x-ray imaging and spectroscopic diagnostics to monitor the keV x-ray emission from thin Al or Au tracer layers buried within the targets. The experiments were designed to quantify the spatial distribution of the thermal electron temperature and density as a function of buried layer depth; these data provide insights into the behavior of relativistic electron currents which flow within the solid target and are directly and indirectly responsible for the heating. We measured approximately 200-350 eV temperatures and near-solid densities at depths ranging from 5 to 100 microm beneath the target surface. Time-resolved x-ray spectra from Al tracers indicate that the tracers emit thermal x rays and cool slowly compared to the time scale of the laser pulse. Most intriguingly, we consistently observe annular x-ray images in all buried tracer-layer experiments, and these data show that the temperature distribution is columnar, with enhanced heating along the edges of the column. The ring diameters are much greater than the laser focal spot diameter and do not vary significantly with the depth of the tracer layer for depths greater than 30 microm. The local temperatures are 200-350 eV for all tracer depths. We discuss recent simulations of the evolution of electron currents deep within solid targets irradiated by ultra-high-intensity lasers, and we discuss how modeling and analytical results suggest that the annular patterns we observe may be related to locally strong growth of the Weibel instability. We also suggest avenues for future research in order to further illuminate the complex physics of relativistic electron transport and energy deposition inside ultra-high-intensity laser-irradiated solid targets.

Laser-plasma interactions in long-scale-length plasmas under direct-drive National Ignition Facility conditions

Laser-plasma interaction experiments have been carried out on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] under plasma conditions representative of the peak of a 1.5 MJ direct-drive laser pulse proposed for the National Ignition Facility (NIF). Plasmas have been formed by exploding 18–20 μm thick CH foils and by irradiating solid CH targets from one side, using up to 20 kJ of laser energy with phase plates installed on all beams. These plasmas and the NIF plasmas are predicted to have electron temperatures of 4 keV and density scale lengths close to 0.75 mm at the peak of the laser pulse. The electron temperature and density of the exploding-foil plasmas have been diagnosed using time-resolved x-ray spectroscopy and stimulated Raman scattering, respectively, and are consistent with predictions of the two-dimensional Eulerian hydrodynamics code SAGE [R. S. Craxton and R. L. McCrory, J. Appl. Phys. 56, 108 (1984)]. When the solid-target or exploding-foil plasmas were irradiated with an f/6 interaction beam at 1.5×1015 W/cm2, well above the NIF f/8 cluster intensity of ∼2×1014 W/cm2, stimulated Brillouin backscattering (SBS) was found to be completely inhibited. A conservative upper limit of direct-backscattered SRS was found to be ∼5% from the solid targets. SRS and SBS are thus unlikely to have a significant impact on target performance at the peak of the NIF direct-drive laser pulse.

13C chemical shielding anisotropy in the solid phases of CH 313CN

The principal elements of the axially symmetric nitrile 13C chemical shift tensor, and the CN bond distance of solid CH 3CN have been determined from both static and sample spinning NMR experiments at 78 and 140 K. The spectra show no significant differences between the α and β forms of the solid though a large temperature coefficient is observed for both the isotropic chemical shift, σ , and the 14N nuclear electric quadrupole interactions as seen through the carbon-nitrogen dibole interaction.

The photochemical behaviour of matrix-isolated W(CO) 6 in the presence of alkynes

The outcome of UV photolysis of W(CO) 6 molecules isolated in a solid CH 4 or Ar matrix in the presence of an alkyne RCCR′ (R,R′H,H; Me,H; or Me, Me) at ca. 20 K has been monitored by recording the IR and UV-vis absorption spectra of the deposit. The results indicate that irradiation with light having wavelengths near 313 nm results in the formation of the ( η 2-alkyne)tungsten pentacarbonyl complex, [( η 2-RCCR′)W(CO) 5], in addition to [X … W(CO) 5] (X  CH 4 or Ar), and free CO. In the case of a terminal alkyne, e.g. HCCH or MeCCH, however, photolysis under these conditions gives, in addition to the η 2-alkyne complex and in significantly greater yield, what appears to be the vinylidene derivative [R(H)CCW(CO) 5] (RH or Me). Prolonged irradiation with UV light ( λ = ca. 313 or 369 nm) causes the various pentacarbonyl complexes to lose CO with the formation of tetracarbonyl species; prominent among the products issuing from the W(CO) 5/C 2H 2 system are [( η 2-HCCH)( η 1-H 2CC)W(CO) 4] (in C 2H 2-rich matrices) and [( η 2-HCCH)W(CO) 4] in which the alkyne is acting formally as a 4e ligand. The results are significant in relation to the likely intermediate stages in the photocatalytic polymerization of terminal alkynes induced by tungsten hexacarbonyl.

Spectra and structure of silicon-containing compounds. XVII. Vibrational and microwave spectra, molecular structure, barrier to internal rotation and ab initio calculations of methyltrichlorosilane

The vibrational spectra of gaseous, liquid and solid CH 3SiCl 3 and CD 3SiCl 3 have been recorded from 3500 to 40 cm −1. The fundamental vibrations have been assigned, and the degree of vibrational coupling among the normal modes has been determined. A normal coordinate analysis has been carried out by using the force constants obtained from ab initio calculations with the 3-21G* basis set. The microwave spectrum of CD 3SiCl 3 has been recorded from 26.5 to 40.0 GHz and the A rotational constants of seven isotopomers in a weighted least-squares procedure are: r(SiC)=1.841±0.005 Å, r(SiC)=2.030±0.001Å ∢ClSiC=110.5±0.1°, ∢ClSiCl=108.4±0.2°, ∢HCSi=109.7±0.1° and ∢HCH=109.3±0.1°. The barrier to internal rotation has been estimated from the relative intensity to the torsional satellites in the microwave spectrum to be 2.3 and 2.6 kcal/mol, for the − d 0 and − d 3 molecules, respectively, and from direct observation of the two-quantum transitions in the Raman effect, to be ∼2.5 kcal/mol for both isotopomers. The structural parameters, barrier to internal rotation, and fundamental vibrational frequencies which have been determined experimentally are compared to those obtained from ab initio calculations employing the RHF/3-21Gu* and/or RHF/6-31G* basis sets. Additionally, structural parameters have been calculated with the 6-31G* basis set with electron correlation at the MP2 level. All of the results are discussed and compared to the corresponding quantities obtained for some similar molecules.

Surface photochemical determination of the mean free path of subvacuum electrons in adsorbate films

Dissociative electron attachment (DEA) of adsorbates was used to detect photoexcited substrate electrons with energies below the vacuum level. When adsorbate layers of variable thickness were deposited on a Pt(111) surface and capped with a topmost, DEA active, detector layer of CH 3Br the mean free path of substrate derived electrons could be measured. The averaged electron scattering cross section through solid Xe at 25 K was σ s = 4.7 +0.7 −0 A ̊ 2 for electrons of energy −0.6 ⩽ E v ⩽ −0.27 eV with respect to the vacuum level. The cross section in solid CH 3Br at 25 K was σ s ⩾ 4.7 A ̊ 2 averaged over an approximately 0.4 eV FWHM range of electron energies centred at E v = −0.7 eV.

A study of the tunneling frequencies of CH 3D in various crystalline environments

The tunneling frequencies of CH 3D molecules in phase II of solid methane, as well as CH 3D molecules adsorbed on graphite and MgO at sub-monolayer coverages, are computed considering both the crystalline and molecular fields. The procedure for computing the rotational energy levels of a hindered symmetric top in fields of C 2 and C 1 symmetry is outlined. The first step in the study of the tunneling frequencies of CH 3D, is to determine the parameters in the potential function of CH 4 by using atom-atom potentials. Using these parameters together with those determined from the analysis of the tunneling frequencies of CH 3D using atom-atom potentials, it is possible to fully determine the potential function of CH 3D. The computed tunneling frequencies of CH 3D in phase II of solid methane and CH 3D adsorbed on graphite are in excellent agreement with the observed results. The tunneling frequencies of CH 3D adsorbed on MgO have been computed, but no experimental results are available.

Molecular Motions and Phase Transitions in Solid CH3NH3PbX3 (X = C1, Br, I) as Studied by NMR and NQR

Abstract The temperature dependence of 35C1, 81Br, and 127I NQR frequencies and 1Hspin-lattice relaxation times (T1) for CH3NH3PbX3 (X = Cl, Br, I) was measured through the successive phase transitions in these solids. The isotropic reorientation of the CH3NH3 ions takes place in the higher-temperature phases (tetragonal [I4/mcm] and cubic) of the three salts (Ea= 11 kJ mol -1). T1's in the lowest-temperature phases (orthorhombic) indicate that the cations undergo correlated C3-reorientation in the chloride (Ea = 5.45 kJ mol -1)and in the iodide (Ea = 5.80 kJ mol -1, whereas correlated (Ea = 2.40 kJ mol-1) and uncorrected (Ea = 7.50 kJ mol-1) C3- reorientations are excited in the bromide. It is also revealed that the rotational tunneling of the cations governs T1 at lowtemperature region in the orthorhombic phases of these salts

Far-infrared spectrum of solid methane: phases III and IV

The far-infrared spectrum of phases III and IV of solid CH 4 have been investigated. Five bands were observed in phase III. Discontinuous changes in the linewidths and the emergence of a new band are associated with the transition to phase IV. On the basis of the measured Grüneisen parameters, the band around 49 cm −1 in phase III was identified as a librational mode and all the others as translational modes. The present results are consistent with a P4 2/mbc structure for phase III.

Comment on “Crystal field effects on the infrared spectra of phases I and II of crystalline CH 4”

The possibility that no nearly-free rotors exist in the low-temperature phase II of solid CH 4, as proposed in a paper that recently appeared in Spectrochimica Acta Part A [1], is discussed.

The low temperature phase transitions in solid CH 4-CD 4 mixtures : II. The absence of the II–III phase transition at x between 0.05 and 0.15

The low temperature phase transitions in a (1 − x)CH 4 − xCD 4 mixture for x = 0.125 have been investigated by means of proton spin-lattice relaxation time measurements up to 700 bar. The II–III transition as it occurs in the pure substances was not found in the mixture at this CD 4 concentration. Instead, another transition to a low temperature phase was found. Once this phase is produced at a pressure above 500 bar it can be maintained down to zero pressure. The experimental results indicate that this low temperature phase is metastable. Spin conversion in this mixture was observed as an increase of the proton relaxation time below 8 K.

The low temperature phase transitions in solid CH 4-CD 4 mixtures : I. The shift of the II–III phase transition at low CD 4 concentrations

The low temperature phase transitions in solid (1 − x)CH 4 − xCD 4 mixtures for x below 0.07 have been investigated up to 2 kbar by means of proton spin-lattice relaxation time measurements as a function of pressure and temperature. In the mixtures investigated, the low temperature phase transitions occur only at elevated pressures. In an x = 0.031 and an x = 0.048 mixture at a pressure below about 2 and 1.2 kbar, respectively, we observe low temperature transitions with the same characteristics as the II–III transition in pure CH 4. These transitions give rise to curved phase transition lines similar to those in pure CH 4, but shifted to lower pressure and higher temperature. At higher pressure, the characteristics of the transitions change and they occur at temperatures much lower than the II–III transition temperatures in pure CH 4. In x = 0.063 and x = 0.069 mixtures, we do not find a II–III transition as it occurs in pure CH 4. Instead, we observe only a transition above 1 kbar of the same kind as in the x = 0.031 and x = 0.048 mixtures at high pressure.