Conduction mechanism in biopolymers

By studying the frequency dependence of the conductivity for different models, it is argued that hopping conduction rather than band conduction is occuring in some biopolymers that have been experimentally studied.     

Diffusion mechanism of hydrogen in amorphous silicon: ab initio molecular dynamics simulation

The mechanism of H migration in amorphous Si has remained an unresolved problem. The main issue is the small activation energy (1.5 eV) relative to the known strength of Si-H bonds (2-3.5 eV). We report first-principles finite-temperature simulations which demonstrate vividly that H is not released spontaneously, as proposed by most models, but awaits the arrival of a floating bond (FB). The "migrating species" is an FB-H complex, with H jumping from Si to Si and the FB literally floating around it. Migration stops when the FB veers away.     

Transport mechanism at a metal-gallium arsenide contact

A study was made of the temperature dependence of the current-voltage characteristics of surface-barrier metal-n-type gallium arsenide junctions with various doping levels. The transport mechanism is shown to be governed by the temperature and by the electric field in the space-charge layer; thermionic or thermionic-field emission occurs.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 82–87, January, 1971.     

Superfluid molecular hydrogen

In order to produce a supercooled liquid phase of molecular hydrogen that may possibly change at a sufficiently low temperature to a superfluid state, it is suggested to reduce the temperature of its equilibrium coexistence with the solid phase by means of developing different pressures in these phases through the use of linear mechanical pressure on the solid phase or of external electric field. The thermodynamic functions of hydrogen are calculated in both the stable and metastable regions; its phase diagram and the region of possible transition to a superfluid state are also found. The values of excess pressure on the solid phase and of external electric field intensity are estimated, which are necessary for the stabilization of this state.     

Dissociation energies of molecular hydrogen and the hydrogen molecular ion

We have obtained improved values for the dissociation energies of molecular hydrogen and its ion by using a high-resolution pulse-amplified laser to probe the second dissociation limit. The onset of the vibrational continuum is observed by state-selective detection of the atomic products of dissociation, and several auxiliary measurements link the results to the ground state. The dissociation energies are accurate to 0.010-0.026 cm(-1), improving previous measurements by a factor of 3-7. Agreement with ab initio calculations is good for H2, D2, and their ions, but not for HD and HD+.     

Feasibility of a low-voltage discharge in pure molecular hydrogen

The feasibility of initiating a low-voltage discharge in pure (free of readily ionizable impurity) molecular hydrogen is studied theoretically. A discharge with cathode fall φ₁ = 10 V, interelectrode gap L = 2 cm, and total hydrogen concentration \(N_{H_2 }^{(0)} = 2 \times 10^{15}\) cm^?3 is considered by way of example. The plasma parameters, including concentration \(N_{H^ - }\) of negative hydrogen ions H^?, are calculated. The concentration of H^? ions is maximal in the near-anode plasma and may reach \(\left( {N_{H^ - } } \right)_{\max } = 0.5 \times 10^{12}\) cm^?3. Concentration \(N_{H^ - }\) can be increased severalfold by introducing a small amount of cesium into the discharge, \(N_{Cs}^{(0)} \leqslant 10^{13}\) cm^?3. Cesium ionizes completely and concentrates in narrow near-electrode layers, which are depleted with the plasma in the purely hydrogen discharge. The discharge modifications studied in this work may be of interest as low-voltage volume plasma sources of H^? ions under conditions when a high concentration of cesium in the source plasma is undesirable.     

Electron-electron separation in molecular hydrogen

Calculations of the mean distance and mean inverse distance between the electrons in the hydrogen molecule show that a Heitler-London wave function exhibits more correlation and a molecular orbital function less correlation than a covalent-ionic wave function. Introduction of an atomic screening constant decreases the mean distance between the electrons.     

Is superdense fluid hydrogen a molecular metal?

Recent experiments on the compression of liquid hydrogen in reverberating shock waves, which indicate the transition into a metallic state at about nine times the liquid H₂ density [S. T. Weir, A. C. Mitchell, and W. J. Nellis, Phys. Rev. Lett. 76, 1860 (1996)], have been interpreted by a microscopic percolation in a virtual molecular structure with a continuous spectrum of the electron excitations. The scaling dependence of the electron mobility on the energy above the percolation threshold has been used to qualitatively describe the electrical conductivity of fluid molecular hydrogen in the vicinity of the insulator-metal transition point. Zh. éksp. Teor. Fiz. 113, 1094–1100 (March 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

OPE molecular junction as a hydrogen gas sensor

Oligo(phenylene ethynylene) (OPE) molecular junction has been suggested as a H₂ molecule sensor based on calculations using the first principles of density–functional theory and non-equilibrium Green's function. The electronic transport properties of the OPE molecule between two Au electrodes with or without adsorbed H₂ molecules are investigated. Results show that the adsorbed H₂ molecule significantly changes the characteristics of the current–voltage curve of the OPE molecular junction. The pure OPE molecular junction exhibits a significant negative differential resistance, but this kind of phenomenon will disappear or weaken after hydrogen molecules are adsorbed. The conductance of the junction also obviously decreases in the bias range of [−0.4, 0.4] V after adsorbing H₂ molecules. These effects can be used to design a H₂ molecule sensor.     

Evaporative cooling of hydrogen molecular ions in a Penning trap

Evaporative cooling of singly charged ions in a Penning trap is studied. The ions are created by in-situ electron bombardment of hydrogen molecules and trapped in a cylindrical Penning trap. Cooling of the ions is observed by their axial motion after trapping of a few hundred milliseconds. The ions temperature decreases by a factor of more than 6 in 800 ms, while the bunch density of the coldest ions increases by up to a factor of 10. By studying the time constants of the dependence of ion loss and axial temperature on the magnetic field strength, we exclude the effects of ions loss through the cyclotron motion on the axial ion temperature.     

Coherent Raman scattering in molecular hydrogen in a dc electric field

The polarization properties of light in coherent Raman scattering in molecular hydrogen in a dc electric field are investigated. A nonlinear polarization-optical method of determining the absolute magnitude and orientation of the dc electric field is proposed and realized. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 6, 371–375 (25 September 1999)     

Spontaneous radiative dissociation in molecular hydrogen

The spontaneous radiative dissociations of the discrete vibrational levels of the B¹Σ⁺_u electronic states of H₂, HD and D₂ of the C¹Π_u electronic state of H₂ into the vibrational continuum of the ground X¹Σ⁺_g state are calculated as a function of the emission wavelength. The fluorescent spectra of HD in the Lyman system and of H₂ in the Werner system resulting from an excitation source uniform in wavelength are predicted. The vibrational radiative lifetimes are tabulated as are the fractions of radiative decays that lead to dissociation. The effects of centrifugal distortion are discussed briefly. An appendix describes a sum rule used to check the numerical accuracy of the calculations.     

The mechanism of oxidation of FeS nanoparticles by molecular oxygen and hydrogen peroxide in dilute aqueous solutions

The kinetic characteristics of the oxidation of iron monosulfide with molecular oxygen and hydrogen peroxide in a neutral aqueous medium were studied. The rate of oxidation of FeS with oxygen was proportional to the concentration of O₂ and quadratically depended on the initial concentration of FeS, W ₀ = k _ef [FeS]²[O₂], where k _ef = (2.5 ± 0.5) × 10⁵ M^?2 s^?1 (pH 8, 22°C). During the oxidation of FeS, the iron ion retained the valence state +2. Oxidation products contained about 50% elemental sulfur. The interaction of FeS with H₂O₂ was characterized by the half-time of reaction less than 15 s. The products of oxidation of initially nontoxic FeS contained toxic compounds, supposedly the sulfoxylate ion HS ₂ ^? .     

Nanoscale molecular superfluidity of hydrogen

We present a microscopic quantum theoretical analysis of the nanoscale superfluid properties of solvating clusters of para-H2 around the linear OCS molecule. Path-integral calculations with N=17 para-H2 molecules, constituting a full solvation shell, show the appearance of a significant superfluid response to rotation around the molecular axis at T=0.15 K. This low-temperature superfluid response is highly anisotropic and drops sharply as the temperature increases to T approximately 0.3 K. These calculations provide definitive theoretical evidence that an anisotropic superfluid state exists for molecular hydrogen in this microscopic solvation layer.     

Compton profile of molecular hydrogen

The Compton profile of molecular hydrogen has been determined at an incident photon energy of 20 ke V based on the third generation synchrotron radiation, and the statistical accuracy of 0.2% is achieved at p z = 0. Different theoretical methods, i.e., the density functional method, and the Hartree–Fock method, were used to calculate the Compton profiles of hydrogen with different basis sets, and the theoretical calculations are in agreement with the experimental observation in the whole p z region. Compared with the HF calculation, the DFT-B3 LYP ones are in better agreement with the present experiment, which indicates the electron correlation effect is very important to describe the wavefunction in the ground state of hydrogen.     

Chirped-pulse four-wave Raman mixing in molecular hydrogen

Four-wave Raman mixing (FWRM) in molecular hydrogen was studied using chirped pump and Stokes pulses emitting at 802 and 1,203 nm, respectively. The group delay dispersion (GDD) of the anti-Stokes pulse was examined employing a frequency-resolved optical gating system at different GDDs of the pump and Stokes pulses (0 or ±1,000 fs²). As a result, the energy and the sign of GDD for the anti-Stokes pulse remained unchanged, when the pump and Stokes pulses had the GDD with the same sign. When the sign was not the same, the energy decreased and only the portion useful for resonant FWRM was converted into a Raman emission. This technique has a potential for use in compensation of dispersion by passing the negatively chirped high-order Raman sidebands through the optics with positive chirps in the spectral region from the deep-ultraviolet to the near-infrared, to generate multiple transform-limited Raman pulses and then to produce an ultrashort optical pulse by a Fourier synthesis of these Raman emissions.     

Absence of metallization in solid molecular hydrogen

Being the simplest element with just one electron and proton the electronic structure of a single Hydrogen atom is known exactly. However, this does not hold for the complex interplay between them in a solid and in particular not at high pressure that is known to alter the crystal as well as the electronic structure and eventually causes solid hydrogen to become metallic. In spite of intense research efforts the experimental realization of metallic hydrogen, as well as the theoretical determination of the crystal structure has remained elusive. Here we present a computational study showing that the distorted hexagonal P6₃/m structure is the most likely candidate for Phase III of solid hydrogen. We find that the pairing structure is very persistent and insulating over the whole pressure range, which suggests that metallization due to dissociation may precede eventual bandgap closure. Due to the fact that this not only resolve one of major disagreement between theory and experiment, but also excludes the conjectured existence of phonon-driven superconductivity in solid molecular hydrogen, our results involve a complete revision of the zero-temperature phase diagram of Phase III.