Diffusion processes and atomic collisions of Na-rare gas systems studied by laser induced population grating method

Diffusion processes and atomic collisions of Na-He and Na-Ar systems have been studied by measurement of the time development of population gratings (Laser-Induced Population Grating; LIPG) induced in atomic levels by two intersecting resonant laser pulses and monitored through the Bragg diffraction of a probe laser beam. The diffusion process in the systems was observed through the decay of the LIPGs in the sodium ground-state $\text{(3}{}^2\text{S}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}$ sublevels. The pressure dependence of the diffusion constants were measured. The values showed rough agreement with the previously reported values which had been measured indirectly by the optical-pumping method. The gaussian process model and a simple collision kernel model were considered for the diffusion process. It was also found that with increasing buffer-gas pressure, the population gratings in the excited state $\text{3}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ were transferred to $\text{3}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, which was interpreted as an effect of fine-structure state-changing collisions.     

On the quantum kinetic equation for molecular gases: II. Noncentral interaction

A rigorous derivation of a quantum kinetic equation for dilute homogeneous diatomic gases with noncentral interactions is presented. This equation generalizes the Waldmann-Snyder equation, describing the relaxation of both diagonal and nondiagonal terms of the quantum distribution function $\text{f}{}_{\mathrm{\delta \delta \prime }}\text{(t,}\text{P}\text{)}$. The collision integral contains a term which describes the long-lived collisions and collisions with a large exchange of internal energy. The macroscopic conservation laws for mass density, total momentum density, total angular momentum density and energy density are derived. In the limit of monoatomic gases all known results are obtained.     

Rotational relaxation of spherical-top molecules by atoms

A theoretical study of rotational relaxation of spherical-top molecules in an inert gas is presented. Under the assumption that the molecules and atoms collide as rough spheres, two previous theoretical methods obtained apparently conflicting results. These two methods are shown to be in agreement, the discrepancy being related to the definition of a fundamental relaxation time. When applied to the recent ultrasonic-absorption experiments, the relevant relaxation time is equal to that first reported by Widom. Throughout, the modified rough-sphere collision theory developed by Offerhaus is used. If ?_α is a reduced moment of inertia, defined by $\text{?}{}_{\mathrm{\alpha }}\text{= I/μσ}{}^2{}_{\mathrm{\alpha \beta }}$, where I is the moment of inertia of the rotating molecule, μ is the reduced mass of the atom-molecule pair, and σ_αβ is the sum of the atomic and molecular radii, then N_rot, the number of collisions suffered by each molecule in a time equal to the relaxation time, is $\text{N}{}_{\mathrm{rot}}\text{=}\text{3}\text{4}\text{(1 + ?}{}_{\mathrm{\alpha }}\text{)}{}^2\text{/?}{}_{\mathrm{\alpha }}\text{(1 ? cos φ)}$. Here φ is a parameter related to the rotation of a particular relative velocity vector, and ranges from zero (smooth spheres) to л (rough spheres). A correction to this result related to the attractive potential between atoms and molecules is obtained by multiplying N_rot by the factor exp (?ε/k_BT), in which ε is the depth of the attractive potential well, k_B is Boltsmann's constant and T is the temperature. Values for N_rot are tabulated for a variety of spherical-top molecule-inert gas systems.     

The law of corresponding states for metals

Using the similarity of the effective potentials seen by ions in metals a reduced phonon equation of state is derived. It is shown that the melting point T_m(0) and the atomic volume Ω₀ at T = 0 K and at p = 0 are suitable macroscopic parameters for scaling ? and σ characterizing the interatomic potentials of metals having similar structures. The temperature and pressure dependence of thermodynamical quantities reduced with the above parameters are discussed and the results are compared with the experiment. It is shown that the pressure dependence of the reduced thermodynamic quantities can be described by the pressure dependence of the scaling parameters T_m(p) and Ω₀(p).The general form of the reduced equation of state (containing the electronic contributions as well) obtained gives that the reduced pressure is a universal function of the following reduced variables: the volume, temperature, de Broglie wavelength, Gibbs free energy of electrons $\text{3}\text{5}\text{zE}{}_{\mathrm{fo}}\text{?}$ (E_fo is the Fermi energy at T = 0 K) and depe of the valence z as well. It is shown that $\text{E}{}_{\mathrm{fo}}\text{?}$ is a function of $\text{Ω}{}_{\mathrm{o}}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}$ and $\text{(}\text{E}{}_{\mathrm{fo}}\text{/?}\text{)Ω}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ is approximately constant within the same sub-group of the periodic table.     

Asymptotic power laws for hadron calorimeter experiments

This is a study of asymptotic QCD predictions for collisions in which a finite fraction of the available energy is carried by hadrons into an angular region $\text{Ω}$ whose complement is $\text{Ω}{}_{\mathrm{<mtext>a</mtext>}}$ or $\text{Ω}{}_{\mathrm{<mtext>b</mtext>}}$ or $\text{Ω}{}_{\mathrm{<mtext>a</mtext>}}\text{UΩ}{}_{\mathrm{<mtext>b</mtext>}}\text{, Ω}{}_{\mathrm{<mtext>a</mtext>}}$ being fixed cones about the directions of motion of the initial particles. Apart from slow (logarithmic) corrections the asymptotic power laws derived from the covariant parton model appear to be justified provided certain conditions on $\text{Ω}{}_{\mathrm{<mtext>a</mtext>}}$ and/or $\text{Ω}{}_{\mathrm{<mtext>b</mtext>}}$ and the energy deposited in them are met.     

Heating of electrons in superconductor in the resistive state due to electromagnetic radiation

The effect of heating electrons with respect to phonons in a thin superconducting film driven into the resistive state by the current and the external magnetic field has been observed and investigated. This effect caused by the electromagnetic radiation is manifested in the increased resistance of the film and is not selective over the frequency range from 10¹⁰ to 10¹⁵ Hz. That the effect is frequency independent under the conditions of strong electron scattering caused by static defects is explained by the decisive role of electron -electron collisions in forming the distribution function. The characteristic time of resistance change, obtained experimentally, corresponds to the relaxation time of the order parameter $\text{τ}{}_{\mathrm{\Delta }}$ near the superconducting transition and to the relaxation time of the nonelastic electron-phonon interaction $\text{τ}{}_{\mathrm{<mtext>eph</mtext>}}$ at lower temperatures and in lower magnetic fields.     

Exact two-point correlation functions of turbulence without pressure in three dimensions

We investigate exact results of isotropic turbulence in three dimensions when the pressure gradient is negligible. We derive exact two-point correlation functions of the density in three dimensions and show that the density-density correlator behaves as , where $\text{α}{}_3\text{=2+}\text{1}\text{6}\text{√33}$. It is shown that, in three dimensions, the energy spectrum E(k) in the inertial range scales with exponent $\text{2−}\text{1}\text{12}\text{√33⋍1.5212}$. We also discuss the time scale for which our exact results are valid for strong 3D turbulence in the presence of pressure. We confirm our predictions by using the recent results of numerical calculations and experiment.     

Structural evidence of 2kF commensurability effects in TTF-TCNQ under high pressure

We have shown that the Fermi wave vector describing the low temperature longitudinal distorsion of TTF-TCNQ moves under hydrostatic pressure from an incommensurable value ($\text{2k}{}_{\mathrm{F}}\text{= .295 b}{}^1$) at ambient pressure to a commensurable one ($\text{2k}{}_{\mathrm{F}}\text{= b}{}^1\text{/3}$) around 14.5 Kbar. This value is maintained until 17.5 Kbar at least. Between these two pressures the commensurable low temperature superstructure has a periodicity a × 3b × c.     

A measurement of the energy dependence of the inclusive yield of high-Pt events triggered by a large solid angle calorimeter

Large aperture calorimeter measurements of high transverse momentum events from 100, 200 and 340 GeV pp collisions are reported. Yields are higher by one to two orders of magnitude than the yield expected from uncorrelated statistical fluctuations in multiparticle events. The exponential slopes of the yields versus x_t exhibited a systematic change with $\text{s}\text{(x}{}_{\mathrm{<mtext>t</mtext>}}\text{= 2P}{}_{\mathrm{<mtext>t</mtext>}}\text{/}\text{s}\text{)}$. The events are shown to be more diffuse than pure jets as produced in e⁺e^? collisions. However, these events are consistent with a QCD-inspired 4-jet model.     

Light scattering study of dispersion and attenuation of hypersound in adamantane

The Brillouin scattering techniques have been used to measure the velocity dispersion of hypersonic acoustic waves in the “high temperature” disordered cubic phase of adamantane. Shear waves, characteristic of the C₄₄ elastic constant, show no significant dispersion. Longitudinal waves propagating in the (001) plane show strong velocity dispersion. The measures have been performed at the same temperature T = 295.7 K. Using a classical single relaxation time model for the dispersion as a function of frequency at temperature T, the L-mode data have been correctly fitted.The importance of the dispersion $\text{(C}{}_{\mathrm{\infty }}\text{? C}{}_{\mathrm{o}}\text{)}\text{C}{}_0$ for the elastic constants is 20% for C₁₁, 51% for C₁₂ #1% for C₄₄ and ?2.8% for $\text{(C}{}_{11}\text{? C}{}_{12}\text{)}\text{2}$. The fitted relaxation time is τ ? 9 × 10^?11 sec.     

Nuclear magnetic resonance and relaxation in liquid zinc

Measurements of the Knight shift K and the nuclear spin-lattice relaxation rate $\text{(}\text{1}\text{T}{}_{\mathrm{l}}\text{)}$ of Zn⁶⁷ are reported for liquid Zn. Measurements of K extend from 409°C (supercooled) to 700°C; measurements of $\text{(}\text{1}\text{T}{}_{\mathrm{l}}\text{)}$ cover the range 425 – 620°C. The Knight shift shows a weak positive temperature dependence attributed to thermal expansion. The reciprocal enhancement factor of the Korringa relation K(α) is found to increase with temperature from .77 ± .04 to .82 ± .04 in the range 425 – 620°C. The quadrupolar contribution to the relaxation rate was concluded to be very small with an estimated upper limit given by $\text{(}\text{1}\text{T}{}_{\mathrm{l}}\text{)}{}_{\mathrm{Q}}\text{≤ 15 sec.}{}^{\mathrm{-l}}$     

Pressure-induced low-temperature transitions in TMMC

Measurements of the pressure dependence of the low-frequency dielectric constant ? and high pressure, low temperature X-ray diffraction photographs on (CH₃)₄ (TMMC) have shown that the low temperature $\text{P2}{}_1\text{aa}$ phase of TMMC transforms to the low temperature $\text{P2}{}_1\text{m}$-like phase of (CH₃)₄ NCdCl₃ (TMCC) at high pressure. The pressure-temperature phase diagram has been deliniated from the triple point at 2.0 kbar and 137° K to lower temperatures, and, from changes in the dielectric constant anomaly, it appears that another triple point may occur at ～ 1.0 kbar and 95° K. The pressure and temperature dependences of ? together with the observed doubling of the unit cell suggest that the $\text{P2}{}_1\text{a}$ low temperature phase of TMMC may be antiferroelectric.     

Dynamic viscosity of macromolecular liquids for a superposition of static and oscillating velocity gradients

Two coupled, inhomogeneous relaxation equations for the friction pressure tensor and the alignment tensor are derived within the framework of irreversible thermodynamics. These equations are solved for a specific geometry, viz. flow between flat plates, and for a velocity gradient of the form $\text{Γ}{}_0\text{+ Γ}{}_1\text{cos Ωt}$ with small Γ₁. From the resulting relation between the (time-dependent) friction pressure tensor and the velocity gradient, the dynamic viscosity and the normal pressure can be inferred. The frequency dependence of the relevant viscosity coefficients is discussed. If Г₀, the magnitude of the static part of the velocity gradient is large enough, a type of resonance behavior is found with the resonance frequency $\text{Ω}{}_{\mathrm{res}}\text{≈(1 + ξ)}{}^{-1}\text{Γ}{}_0$ where ξ is the ratio between the relation times of the friction pressure tensor and of the alignment tensor.     

Spin dynamics in the quasi-1 dimensional spin-glass FeMgBo4

The results of an inelastic neutron scattering investigation performed on the insulating title compound are presented. The spin dynamic is well described by a relaxation model leading to a quasi-elastic Lorentzian neutron-scattering line. The relaxation rate $\text{1}\text{τ}{}_{\mathrm{R}}$ is a linear temperature function for T>20 K, which is ascribed to the isotropic and 1-D character of the spin correlations. At T = 20 ? 5 K a drastic drop in $\text{1}\text{τ}{}_{\mathrm{R}}$ is observed reflecting a “crossover” to a highly frustrated 3D-spin glass state.     

Transverse energy per charged particle and freeze-out criteria in heavy-ion collisions

In relativistic nucleus–nucleus collisions the transverse energy per charged particle, ET/Nch$E_T/N_{\mathrm{ch}}$, increases rapidly with beam energy and remains approximately constant at about 800 MeV for beam energies from SPS to RHIC. It is shown that the hadron resonance gas model describes the energy dependence, as well as the lack of centrality dependence, qualitatively. The values of ET/Nch$E_T/N_{\mathrm{ch}}$ are related to the chemical freeze-out criterium E/N≈1 GeV$E/N\approx 1\text{GeV}$ valid for primordial hadrons.     

Diabatic emission of neutrons: A probe for the energy-dissipation mechanism in nucleus-nucleus collisions

The precompound emission of neutrons in central nucleus-nucleus collisions is investigated within the framework of dissipative diabatic dynamics. For ⁹²Mo + ⁹²Mo at bombarding energies between 7.5 and 20 MeV/u the differential neutron multiplicities $\text{d}\text{M}{}_{\mathrm{<mtext>n</mtext>}}\text{d}\text{E}{}_{\mathrm{<mtext>n</mtext>}}$ are estimated from the decay of highly excited diabatic single-particle states. The energy spectra have an almost exponential high-energy tail with effective temperatures up to 10 MeV for 20 MeV/u bombarding energy.     

Energy dependence of the reaction mechanism of 11B induced single nucleon transfers on 26Mg

Angular distributions for elastic scattering and for single nucléon stripping reactions induced by a 31 MeV ¹¹B beam on ²⁶Mg have been measured. The DWBA calculations provide a good account of the shape and magnitude of the proton transfer data and of the neutron transition to the $\text{j-}\text{favoured}\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ level in ²⁷Mg, but fail to reproduce the features of the $\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ transitions to ²⁷Mg. The j-dependent effects and l-matching conditions are investigated as a function of beam energy and their role in determining the reaction mechanism is examined.