Broad Frequency Distribution Research Articles

Spectral and temporal dynamics of far infrared generated monochromatic nonequilibrium phonons in LaF 3:Pr 3+

The spectral and temporal dynamics of monochromatic, nonequilibrium phonons in LaF 3:Pr 3+ are investigated at low temperatures (1.8 K). High occupation number, narrow band phonon populations are generated via far infrared (FIR) pumping of defect induced one phonon absorption. Anti-Stokes absorption spectroscopy is utilized in an “optical boxcar” scheme to directly observe both the spectral and temporal evolution of these phonons. The monochromatically generated phonons are seen to decay anharmonically to a broad low frequency distribution. Brckfeeding via phonon confluence of these decay products produces a distinct broad distribution around the generation frequency.

Electrointercalation of Cu into 2H-TaS2: in situ monitoring of reaction and Cu mobility

We investigated the181Ta nuclear quadrupole interaction (NQI) in crystals of Cux TaS2 by time differential perturbed angular correlation (TDPAC) both in situ during galvanostatic intercalation at 300 K and subsequently between 11 K and 813 K. During electrointercalation the spectra show broad frequency distributions due to slowly diffusing Cu+ in the van der Waals gap.

Time differential perturbed angular correlation studies of hafnium hydrides

The dependence of the electric field gradients (EFG) in Hafnium-Hydride systems as a function of the composition H/Hf in the cubic δ and tetragonal e phases were investigated using the time-dependent perturbed angular correlation (TDPAC) technique, with181Ta as the nuclear probe. Two EFGs were found in the e phase, indicating the existence of two major symmetries surrounding the Ta probe. The results indicate that the trend of the EFGs, in the e phase, are due to the changes in the lattice parametersao,co as hydrogen is added to the Hf-H system. In the δ phase, only one major symmetry was found. Both phases are characterized by broad frequency distributions and large anisotropies.

Surface oxidation of copper observed by perturbed angular correlation and Rutherford backscattering

The oxidation process of copper was investigated by the Rutherford backscattering spectrometry (RBS) and perturbed angular correlation (PAC) techniques. The variation of the oxygen concentration below the surface was analysed by RBS. The PAC method was applied to obtain a microscopic picture of the oxidation process. Some 10 12 radioactive 111In + ions were implanted at 400 keV into copper foils, which were oxidized at a fixed oxygen pressure either isothermally or isochronally at increasing temperature ( T ox = 300–1200 K). The PAC spectra were taken at room temperature. The hyperfine interaction parameters were compared with those found after 111In implantation into Cu 2O and CuO pressed powder samples, respectively oxide layers on copper foils. The broad frequency distribution observed during the oxidation process is suggested to be due to a complex formed by indium, oxygen and copper vacancies in the Cu 2O lattice.

The electric field gradient of111Cd in an α-Al2O3 single crystal

Radioactive111In+ ions were implanted into an α-Al2O3 single crystal. The hyperfine parameters of111Cd at substitutional Al lattice sites were identified by measuring the perturbed angular correlation for different sample orientations. The electric field gradientV zz =1.04(17)·1022V/m2 was obtained from the quadrupole coupling constant. This result is compared with the efg values of27Al in α-alumina and111Cd in α-Fe2O3, which also has the corundum structure. Two additional fractions with broad frequency distributions were observed, one of which is attributed to111Cd atoms in a strongly distorted Al2O3-lattice.

PAC-study on 111In implanted into YBa 2Cu 3O 7 and constituent materials

111In was implanted at 400 keV into Y 2O 3 and BaCO 3, which are often used for the ceramic powder preparation of YBa 2Cu 3O 7 bulk samples. The electric field gradients (efg) obtained with perturbed angular correlation (PAC) measurements from these samples and recent results for 111Cd in CuO and Cu 2O are compared with the results of PAC measurements performed after implantation of 111In into YBaCuO. The bulk samples were prepared by ceramic powder technology after a solid state reaction or a citrate synthesis of YBaCuO. After annealing in oxygen at T ⩾ 600 °C the broad frequency distribution caused by the radiation damage changed to a complex frequency spectrum with its main amplitudes centered around ω 1 ≈ 50 MHz and ω 1 ≈ 250 MHz respectively, depending on the production method. For one of the observed efgs an identification with the Cu1 site is proposed.

Perturbed angular-correlation experiments on 111In in oxidized fcc metals and their oxides.

The perturbed angular correlation (PAC) method has been employed to study the hyperfine interaction of the 245-keV state in $^{111}\mathrm{Cd}$, following $^{111}\mathrm{In}$ implantation into the fcc metals Ag, Al, Cu, Ni, Pd, Au, and Pt and subsequent oxidation. Broad quadrupole frequency distributions around a peak frequency 〈${\ensuremath{\omega}}_{1}$〉=160 MHz were found in all systems and attributed to oxygen trapping at $^{111}\mathrm{In}$. The fraction of oxidized $^{111}\mathrm{In}$ increases with the oxidation temperature in such a way that about 50% of the implanted In ions have trapped oxygen at 60% of the melting temperature of the host. An interpretation of these frequency distributions has been attempted on the basis of PAC measurements for $^{111}\mathrm{In}$ implanted into the metal oxides AgO, ${\mathrm{Ag}}_{2}$O, and ${\mathrm{In}}_{2}$${\mathrm{O}}_{3}$. In these systems, well-defined electric field gradients have been found which can be related to the respective crystallographic structures. In addition, the magnetic hyperfine interaction of $^{111}\mathrm{In}$ in antiferromagnetic NiO proves substitutional implantation.

Photon correlation spectra of poly(phenylmethyl siloxane) under high pressures

AbstractWe report polarized photon correlation measurements of bulk poly(phenylmethyl siloxane) (PPMS) with a molecular weight of about 2500 at different pressures from 1 to 1750 bar and temperatures between 269 and 308 K. In the Frequency range accessible by present‐day digital correlators, the time correlation function can measure a slow q2‐dependent diffusional process due to local concentration fluctuations and a fast α‐mode structural relaxation process. The mean characteristic times of the two processes approach each other at low temperatures and high pressures, in contrast to the behavior of the α‐ and β‐relaxation times. We should be cautious in relating the mean characteristic times for the structural relaxation process obtained by different experimental techniques, such as Brillouin scattering and photon correlation spectroscopy, because of the differnces in the probe emphasis and the extremely broad frequency distribution of such relaxation processes.

Mechanism of tetanolysin-induced membrane damage: studies with black lipid membranes.

Tetanolysin produced similar rates of leakage of K+ and hemoglobin from erythrocytes. When studied by using cholesterol-containing black lipid membranes, this hemolysin induced conductance steps with a broad frequency distribution. These findings are inconsistent with the formation of structural channels and suggest that tetanolysin acts by causing lipid perturbations.

OBSERVATION OF STRONGLY FREQUENCY DEPENDENT LIFETIMES OF ACOUSTIC PHONONS IN CaF2

A strongly frequency dependent lifetime of acoustic phonons in CaF2 at low crystal temperature is observed. The phonon lifetime decreases proportionally to v-= indicating spontaneous phonon decay. Furthermore, we report measurements of the spectral distribution of phonons generated by nonradiative transitions. Various attempts have been made to detect spontaneous decay of high-frequency acoustic phonons without, however, conclusive results. 1 In this paper we report experimental evidence of spontaneous decay of acoustic phonons. Our experiment was performed on CaF2 at low crystal temperature. The principle of our experiment is shown in Fig.1. Uniaxial stress is applied to a CaF2 crystal doped with 0.003 mole% EU~' ions. The 2+ stress leads to a splitting of the lowest excited state level of Eu into a doublet with variable energy separation hv where h is Planck's constant. This system allows tunable phonon detection up to a frequency v of 3.2 THz.' For phonon generation the crystal is optically pumped with radiation of a nitrogen laser. The radiation is absorbed in broad bands of the E U ~ + ions. Nonradiative transitions lead to population of the stress split energy levels and, simultaneously, to generation of phonons with a broad frequency distribution. An additional phonon source is due to one-phonon spin-lattice relaxation in the doublet levels. Phonons are detected by the phonon induced fluorescence radiation S 2 (Fig.1). Since the spin-lattice relaxation time is shorter than 1 ns the relative population of the doublet levels is in equilibrium with phonons of the frequency v. Therefore, we can directly measure phonon Fig.1. Principle of occupation numbers by observing S1 and S2 phonon detection. fluorescence. In addition, from the decay of Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981635

Effectiveness of size frequency distributions in regulating intraspecific spatial overlap of stream insects

Spatial niche breadth and overlap of size groups of four species of epilithic stream insects were studied to determine if broad size frequency distributions serve to reduce intraspecific competition for space. Mean niche overlap decreased with greater breadth of size frequency distributions for only one species (Baetis intercalaris) and did not vary in relation to population density. Similarly, the ratio of the probabilities of intergroup and intragroup encounter did not vary as a function of the breadth of size frequency distributions. Therefore, it was concluded that changes in size frequency distribution have a minimal effect on intraspecific competition for space among epilithic species. It was suggested that observed size frequency distributions may be more a function of extant environmental conditions (temperature and flow) than of previous or extant intraspecific competition.

MESSENGER RNA TRANSCRIPTION IN REGENERATING KIDNEY

As part of a larger project studying potential reactivation of genes active in fetal organogenesis which are not expressed in adult tissue, we have examined the control of transcription in kidney regeneration, a process involving hypertrophy and hyperplasia of several cell types. Renal polyadenylated mRNA binding to oligo(dT)-cellulose was prepared from mice 2 and 4 days following unilateral nephrectomy and from controls and used as template for the synthesis of 3H complementary DNA (cDNA). The complexity of the mRNA population was evaluated by hybridization to its homologous cDNA. Hybridization occurred over 5 log units of ROt (mols nucleotide/liter × seconds) and reached 90% saturation indicating a broad frequency distribution of mRNA populations. The 4 day post-nephrectomy curve showed a twofold increase in the complexity of the most abundant class of mRNA molecules, but no significant difference in the complexity of mRNA species present in lesser abundance. Heterologous hybridizations failed to show a significant change in saturation values. We conclude that few if any new species of mRNA are required for kidney regeneration, but that some species must be produced in increased numbers. In vitro translation of these mRNAs substantiated these conclusions, as no new proteins unique to compensatory growth were identified, although several proteins were present in significantly different amounts compared to control.

Pulsed-beam study ofCd111nuclear quadrupole interaction with recoil-induced lattice defects in cubic palladium

The quadrupole interaction of excited $^{111}\mathrm{Cd}$ nuclei with electric field gradients due to lattice defects in fcc palladium has been studied by the technique of time-differential observation of perturbed angular distribution of $\ensuremath{\gamma}$ rays. The lattice defects were induced by the recoiling ions following the nuclear reaction $^{108}\mathrm{Pd}(\ensuremath{\alpha},n)^{111}\mathrm{Cd}$. A sharp frequency of 87(2) MHz is assigned to trapped interstitials. A broad temperature-dependent frequency distribution arising from various other lattice defects was found. A migration energy of 0.10(2) eV for free interstitials is deduced.

Radiation temperature and radiation power of thermal phonon radiators using diamond as transmission medium

Thermal phonon radiators allow short pulses of incoherent thermal phonon beams to generate in a simple manner and the center frequency of the broad frequency distribution of the emitted phonons to vary up to the acoustic cutoff frequencies. This paper reports experimental investigations of the connection between radiation temperature and radiation power using films of several metals (copper, nickel, gold, and lead) as phonon radiators on diamond. The measured dependencies of the radiation temperature on the radiation power show that the radiation process is by far better described by assuming acoustic mismatch between radiator and transmission medium than using the model of perfect match. Some metals on diamond give a phonon radiation power of several kilowatts per square millimeter radiator area. With a diamond substrate held at liquid-helium temperature, a radiation temperature of a few hundred degrees Kelvin was achieved. If the phonon radiator deposited on diamond is immersed in liquid helium, the transport of heat to the liquid helium becomes important at radiation temperature below 30 °K and is dominant below 20 °K. This effect can be explained by assuming a thin vapor layer between the hot radiator and the liquid helium, and a gas kinetic energy transfer.

The Acoustical Concomitants of Cavitation and Boiling, Produced by a Hot Wire. II

The properties of an electrically heated wire in various liquids have been investigated in an effort to explain qualitatively the properties of the hot-wire sound generator of the preceding paper. The sound from the hot wire is primarily a cavitation noise—its properties can all be ascribed to the continuous formation and collapse of vapor cavities. Many of the phenomena of cavitation, including corrosion, have a counterpart in boiling. It is shown that under certain conditions the hot wire has two stable forms of operation for the same input power. In one the wire is relatively cold, and the sound emitted has a broad frequency distribution. In the other the wire is red hot, and for some liquids the sound emitted is a single pure tone a whistle of 1000–2000 cycles/sec. Swishing the wire through the liquid increases the intensity of this whistle.

The Acoustical Concomitants of Cavitation and Boiling, Produced by a Hot Wire. I

This paper presents data on the properties of a very simple, low power underwater noise generator—a wire heated by a heavy electric current. The dependence of the sound generated upon frequency, input power, wire size, material, and wire history has been determined. The sound has a very broad frequency distribution. For small wires a large part of the sound is supersonic. About one part in a million of the input electrical energy appears as sound. With increasing electric power the sound power passes through a broad maximum. This paper can also be considered as a study of cavitation noise, the cavities being produced by heat rather than by reduced pressure. Thus a new approach to the problem of cavitation noise is provided. As a study in heat flow, this report provides quantitative data on the acoustical concomitants of boiling.