Average Relaxation Rate Research Articles

Bursting liability indices of coal

Natural coal's ability to store and release rapidly elastic strain energy seems to be a fundamental condition for the occurrence of rock-bursts in deep underground coal mines. In Upper Silesia where a number of existing mines work below a critical depth, several indices have been proposed and their numerical values found by experiment in order to classify the potential liability of coal seams to create rock-burst hazards. Three of these indices are most popular, namely the Strain Energy Storage Index ( W ET ), the Bursting Efficiency Ratio (η), and the Rheologic Ratio (ϑ). The first is defined as the proportion of strain energy retained to that dissipated during a single loading-unloading cycle of uniaxial compression. The second is the ratio of the kinetic energy of chips thrown out on failure to the maximum elastic strain energy. The Rheologic Ratio is calculated as a dynamic resistance stress rate (failure velocity) divided by an average stress relaxation rate. W ET values may be obtained through laboratory tests or by direct (double-hole method), or indirect (rebound method) in-situ evaluations, while both η and ϑ values are obtainable in laboratory solely. Each of the indices quoted leads to classifications and/or criteria for the bursting liability of coal. When complete predictions of the rock-burst hazard for a specific location in a mine are to be made, data on in-situ stress concentrations must also be collected. Since direct stress measurements in coal are hard to recommend as a routine procedure in mines, a number of indirect methods are currently being practiced, drilling yield and seismic wave velocity methods giving the most reliable results.

Dynamical Properties of a Large Electron-Hole Drop in Germanium. II. Temperature Dependence of the Carrier Relaxation Rate and of the Pair Density in the Large Drop

The Alfven wave dimensional reonance for a strain-confined large electron-hole drop in germanium is observed in microwave absorption experiments at various temperatures. From the linewidth measurement, the average carrier relaxation rate in the drop is calculated. It has temperature dependence of the form 1/τ av = a T 2 + b with a =(1.2±0.1)×10 9 K -2 s -1 and b =(1.4±0.1)×10 10 s -1 . It is strongly suggested that the temperature dependence of the relaxation is mainly due to carrier-carrier scattering. In addition, the isothermal compressibility for the large drop is estimated to be K T =(2.3±0.2)×10 -2 cm 2 dyne -1 .

Phenomenological theory of laser damage in insulators

A phenomenological theory of laser damage is presented. The physical mechanism for laser damage in insulators is presumed to be similar to the one for breakdown in semiconductors. A few starter electrons in the conduction band are excited by the external field to the point where an electron avalanche occurs. Heat is generated by electron relaxation by phonon emission, and damage occurs when the temperature of the irradiated volume reaches the melting point. The phenomenological theory contains two parameters: an average cross section for excited-state absorption, which for NaCl is 1.8 x 10/sup -17/ cm/sup 2/ and an average relaxation rate by phonon emission, which is about 10/sup 14/ sec/sup -1/ for a 200-cm/sup -1/ phonon in NaCl. These values overcome problems with previous models, which require very large electron relaxation rates (> 10/sup 15/ sec/sup -1/). The parameters of the theory are determined by fitting the results of calculations to experiment. With this theory, subthreshold properties such as the hot-electron distribution, rate of electron avalanche, and rate of heat generation can also be calculated. Ways of verifying the theory are discussed in detail.

Nuclear relaxation and anisotropic molecular reorientation of o-dichlorobenzene

Proton recovery curves are reported for nonselective 180°-τ-90°-FT spin-lattice relaxation of a dilute solution of o-dichlorobenzene in carbon disulfide. An exact density matrix analysis, combined with measured 13C relaxation rates, is used to obtain the principal reorientational correlation times. Differences between these values and those found previously for 1,2,3-trichlorobenzene in the same solvent can be rationalized in terms of molecular shape. Exact density matrix calculations of proton relaxation are compared with an approximate treatment in which dipolar cross correlation and nonlinear mixing effects of strong pulses are ignored. It is shown that simple formulae arising from the approximate treatment successfully account for the average relaxation rate of a given multiplet, but fail to account for differential relaxation of lines within a multiplet.

Thermally denatured proteins. Nuclear magnetic resonance, binding isotherm and chemical modification studies of thermally denatured bovine serum albumin

The properties of thermally denatured bovine serum albumin have been investigated by n.m.r., binding isotherm and chemical modification studies. The marked reduction observed in the protein spin-spin relaxation times on thermal denaturation indicates that the protein chains become rigid. There was no change in the bulk water properties on thermal denaturation but a slight reduction in the amount of water bound to the protein was observed. The increase in the average water proton spin-spin relaxation rate on thermal denaturation results from a decrease in the bound water spin-spin relaxation time due to a reduction in the mobility of protein chains to which water molecules are bound. At temperatures higher than about 285 K, there is a contribution to the observed water proton relaxation times from exchange between labile protein protons and water protons, which results in a minimum in the spin-spin relaxation time at 328 K. The native structure of bovine serum albumin is not extensively disrupted on thermal denaturation. The results are consistent with a two step mechanism for thermal denaturation, involving aggregation of initially unfolded protein molecules. The initial unfolding involves mainly changes in the tertiary structure rather than in the secondary structure and aggregation results in the formation of a rigid network. The molecules are held together by co-operative forces involving many amino acid residues and, although intermolecular disulphide bonds do form on thermal denaturation of bovine serum albumin, chemical modification studies show that aggregation does not depend upon formation of chemical cross-links, as is often supposed.

Die kinetischen Konstanten des Triplett-Zustandes von Fluoren-Molekülen / The Cinetic Constants of the Triplet Stale of Fluorene Molecules

Abstract The metastable triplet state T1 of fluorene molecules in a fluorene crystal, disturbed by the presence of dibenzothiophene (“X-traps”) was investigated at 1.6 and 4.2 °K. From ESR measurements a model for the fluoren X-traps is derived. The X-trap molecule is misoriented by an angle of 2,5° in the fluorene crystal. The rate constants for population (s), radiative decay (kD) and total decay (k) of the three magnetic sublevels of T1 were determined by analysis of phosphorescence intensity and decay time in high magnetic field and by optically detected ESR. We find (with z = axis perpendicular to the molecule, x = long axis): The average spin lattice relaxation rate constant is w = (0.8 ± 0.2) sec- 1 .

Chemiluminescence of CH in the O +C2H2 Reaction : Rotational Relaxation and Quenching

Rotational distributions of CH (A 2Δ, v=0) were measured at pressures from 0.1 to 8.5 torr in the O+C2H2 reaction in the presence of large excess of N2, Ar, or He. The reaction N+NO→N2+O was used to produce O without O2. Under all conditions the distributions are found to be a superposition of two Boltzmann distributions, one at 1200°—1400°K characteristic of the process leading to the formation of excited CH (A 2Δ), and one which stays close to the temperature of the reactor. Assuming that the observed behavior of the rotational distributions is caused by collisions of excited CH with heat-bath molecules and interpreting the fraction of molecules in this low-temperature distribution as a measure of the extent of relaxation of the initial rotational distribution of excited CH, average relaxation rate constants are derived. With the collision partners mentioned above, 10 to 30 collisions are required for relaxation. A 14-level phenomenological model including only transitions between neighboring rotational levels (K→K±1) fits the experimental distributions well at all pressures studied if the downward rate constants kK−1,K are approximately proportional to exp(−2BK/kT), where B=14.6 cm−1 is the rotational constant for CH(A 2Δ, v=0) and T=360°K. Other models are discussed. Rotational and vibrational distributions for CD(A 2Δ) are also described. Quenching of the CH(A→X) emission by added oxygen was studied quantitatively and the results are consistent with a mechanism in which not CH(A 2Δ) but a precursor to its formation is removed by O2. In the absence of O2 the lifetime of the precursor is determined primarily by the atomic-oxygen concentration. When an electrical discharge through O2 is used as the source of oxygen atoms for reaction with acetylene, the rotational distribution of CH(A 2Δ) and its pressure dependence is quite different from that described above. When acetylene reacts with the products of a discharge through CO2 with excess CO2 as heat bath, the rotational distribution of CH(A 2Δ) has a Boltzmann form at all pressures, with only the rotational temperature depending on pressure and approaching the gas kinetic temperature at high pressure.