Resonant decay of a Langmuir wave in the presence of dust grains in a cylindrical plasma

The resonant parametric decay of a Langmuir wave into a backward propagating Langmuir wave and an ion acoustic (IA) wave is studied in a cylindrical dusty plasma. The analysis shows that the frequency of the IA mode decreases with the parameter δ_c (where δ_c is the ratio of the ion density to the electron density) for negatively charged dust grains. The growth rate of the resonance decay instability (RDI) and the threshold required for its onset also decrease with δ_c and are strongly dependent on the electron to ion temperature ratio for both positively and negatively charged dust grains. The results obtained also illustrate the dependence of the threshold of the resonance decay instability (μ_th) on the plasma cylinder radius.     

Fluorescence decay characteristics of the green emission from CaF2:Ho3+

Variations in the decay times of the characteristic green emissions at 522.7, 551.3, 549.6, 547.6, 542.2, 540.2, 535.9 and 533.5 nm from CaF₂:Ho³⁺ with concentration are studied at RT and LNT. A pulsed N₂ laser beam of power density 1.5 MW cm^-2 is used for the excitation. Temperature dependent concentration quenching of the decay times are observed for all the emission bands. But an increase in the decay time due to the reabsorption process is also observed for a few of the above bands.     

Relative change of the electron density at the Cu nucleus in CuSCN and Cu(SCN)2

The change of the decay constant of⁶⁴Cu in CuSCN and Cu(SCN)₂ compared to metallic Cu has been measured. Values of the relative change of the electron density at the Cu nucleus are deduced taking condensed matter effects on overlap and exchange corrections into account.     

Time‐decay of magnetisation in high-Tc superconductors

An extension of Bean's model, incorporating the observed exponential decay, with increasing field, of critical current density in the high-T_c superconductors, is developed. It is used for explaining the various features of the isothermal magnetisation curves. Using this model, a calculation is presented for the time-decay of magnetisation after the sample is exposed to a magnetic field, and also after this applied field is switched off. The field dependence of the decay rate in these two conditions is calculated, and then compared with available results on the 90 K and the 40 K class of oxide superconductors. The results, in the limit of field-independent current density, also agree qualitatively with experimental results for the heavy fermion superconductor CeCu₂Si₂.     

The participant Coster–Kronig preceded Auger transition in the resonant L2,3-M2,3V Auger electron spectrum of Ti metal

The L_2,3-M_2,3V resonant Auger electron spectroscopy (RAES) spectrum of Ti metal measured by Le Fêvre et al. [P. Le Fêvre, J. Danger, H. Magnan, D. Chandesris, J. Jupille, S. Bourgeois, M.-A. Arrio, R. Gotter, A. Verdini, A. Morgante, Phys. Rev. B69 (2004) 155421] is analyzed in the light of relaxation and decay of the resonantly excited L_2,3-hole states. The relaxation time of the resonantly excited L_2,3-hole state to the fully relaxed (screened) one is much shorter than the L_2,3-hole Auger decay time, whereas the participant Coster–Kronig (CK) decay time of the resonantly excited L₂-hole state to the fully relaxed L₃-hole state at the L₂ resonance is as short as the relaxation time of the resonantly excited L₂-hole state to the fully relaxed one. The excited electron is predominantly either rapidly decoupled from the L_2,3-hole decay or annihilated by the participant CK decay. Thus, near the L_2,3 edges the L_2,3-M_2,3V RAES spectral peak appears at constant kinetic energy. The L_2,3-M_2,3V RAES spectrum shows a normal L_2,3-M_2,3V Auger decay profile not modulated by the density of empty d states probed by the resonant excitation. Not only the relaxation time but also the participant CK decay time depends on photon energy because they depend on the density of empty d states probed by the resonant excitation. As a result, the L_2,3 X-ray absorption spectroscopy spectral line broadening depends on photon energy.     

14N Pulsed nuclear quadrupole resonance. 4. Two-pulse sequences for the determination of T1 and T2 relaxation times

A general theory, based on density matrix calculations, has been developed for the special case of a two-pulse sequence applied to spin 1 (¹⁴N) nuclear quadrupole resonance (NQR) of a powder sample. It is shown that the homolog of the NMR inversion-recovery experiment leads easily to the spin-lattice relaxation time T ₁ (associated with the diagonal elements of the density matrix) provided that an appropriate phase cycling is used. Conversely, in spite of two-step phase cycling schemes adapted to spin-spin relaxation measurements, the homolog of the NMR Hahn spin-echo sequence may pose some problems if the results are displayed in the magnitude mode. First, at short decay times, the echo may be corrupted by unwanted signals. Secondly, in that case, the amplitude of the resulting signal can evolve unexpectedly and differently as a function of the phase of the second pulse. Thirdly, at long decay times, the echo maximum occurs earlier than expected. All these problems in principle disappear with a complete four-step phase cycling scheme and the echo decay curve yields reliably the spin-spin relaxation time T ₂ (associated with off-diagonal elements). This theory allowed the exploitation of many test experiments performed at different frequencies on hexamethylenetetramine (HMT) and sodium nitrite.     

On the possibility of stimulated emission in GaP : N

The luminescence decay times of NN_i-pair emission are found to decrease when the photoexcitation density is increased from several W/cm² to 10⁶–10⁷ W/cm². For lightly N-doped GaP, the reduction is typically from 3000 to 30 nsec (A-line, low temperature); for heavily N-doped GaP it is from about 1000 nsec to a few nanoseconds. From this decay time reduction the onset of a new recombination mechanism is concluded. No mechanism is likely to explain previously reported high gain values for GaP : N.     

Decay dynamics of Eu/Tb in solution-processed Y2O3

As new synthesis techniques are developed to create phosphors with smaller particles the physical processes that govern luminescence behaviors, like efficiency and decay rates, are influenced by the large surface-to-volume ratios of the nano-structured materials. Decay of lanthanide excited states, following pulsed laser excitation, is used to probe the affect of size-induced decay channels on the luminescent properties. We report results on the decay dynamics of europium and terbium excited states in solution-produced yttrium oxide. Radiative decay from the Eu³⁺⁵D₀ state is found to be well described by a single exponential function over a range of concentrations (0.01-2.0%). The Tb³⁺⁵D₄ decay exhibits a distribution of decay rates over the concentration range 0.01-5.0%. The distribution of decay rates is discussed using a probability density function based on the Kohlrausch decay function.     

Determination of nonneutral plasma microstate from synchrotron radiation measurements

The spectrum of the synchrotron radiation emitted from the University of Maryland mirror confined nonneutral plasma is measured during the decay of the pulsed magnetic field and used to obtain the energy distribution ?(γ) and electron density profile n_e⁰(r). One result is that the electron density profile evolves from a relatively thin E-layer to a broad profile with density build-up on axis.     

Probable Decay Modes at Limits of Nuclear Stability of the Superheavy Nuclei

The modes of decay for the even–even isotopes of superheavy nuclei of Z = 118 and 120 with neutron number 160 ≤ N ≤ 204 are investigated in the framework of the axially deformed relativistic mean field model. The asymmetry parameter η and the relative neutron–proton asymmetry of the surface to the center (R _η ) are estimated from the ground state density distributions of the nucleus. We analyze the resulting asymmetry parameter η and the relative neutron–proton asymmetry R _η of the density play a crucial role in the mode(s) of decay and its half-life. Moreover, the excess neutron richness on the surface, facets a superheavy nucleus for β^? decays.     

Stability of two-dimensional nanostructures

We investigate atomic and molecular nanostructures on metal surfaces by variable low-temperature scanning tunnelling microscopy. In combination with molecular dynamics calculations we achieve a detailed understanding of the stability of these structures.?Atomic nanostructures in homoepitaxial metallic systems are thermodynamically only metastable. Two-dimensional islands on Ag(110) decay above a threshold temperature of T _l=175 K. Caused by the anisotropy of the surface, distinct decay behaviours exist above and below a critical temperature of T _c=220 K. Calculations based on effective medium potentials of the underlying rate limiting atomic processes allow us to identify the one-dimensional decay below T _c as well as the two-dimensional decay above T _c.?In contrast to atoms, the intermolecular electrostatic interaction of polar molecules leads to thermodynamically stable structures. On the reconstructed Au(111) surface, the pseudo-chiral 1-nitronaphthalin forms two-dimensional supermolecular clusters consisting predominantly of ten molecules. Comparison of images with submolecular resolution to local density calculations elucidates the thermodynamical stability as well as the internal structure of the decamers. Received: 25 March 1999 / Accepted: 17 August 1999 / Published online: 6 October 1999     

掺铒Si/Al2O3多层结构中结晶形态对1.54 μm发光的影响

利用脉冲激光沉积的方法制备掺铒 Si/Al₂O₃多层结构薄膜,获得了由纳米结构的Si作为感光剂增强的Er³⁺在1.54 μm高效发光.利用拉曼散射、高分辨透射电镜和光致发光测量研究了在不同退火温度下（600—1000 ℃）纳米结构Si层的结晶形态变化,及对Er³⁺在1.54 μm的发光的影响特征.研究发现最佳发光是在退火温度600—700 ℃.在这个条件下纳米Si的尺寸和密度,Si和Er的作用距离以及Er^{3+ 关键词： 铒 纳米硅 能量转移 氧化铝}     

The Fisher-Widom line for systems with low melting temperature

We present Monte Carlo results for the pair distribution function of three simple fluid models, with pairwise interactions, which have low triple point temperatures and mimic some aspects of Na and Hg liquid metals. The results are then used to get the direct correlation function, by numerical solution of the Ornstein-Zernike equation, and to characterize the decay modes of any density distribution towards the bulk fluid. The Fisher-Widom line is obtained from the crossing of the two lowest inverse decay lengths, associated to monotonic and to oscillatory decay modes. For the pair potential models with a soft repulsive core, the Fisher-Widom line appears well below the critical temperature and has positive slope of the temperature with respect to the density, contrary to previous results for the Lennard-Jones and square-well potentials which had located that line quite close to T _c and with negative slope.     

Calculations of the excitation functions and isomer ratios of the K(α, n)Sc reaction in the equilibrium and pre-equilibrium statistical models

Angular-momentum effects are incorporated in the pre-equilibrium statistical model using Williams' spin-dependent level density as a function of the particle and hole numbers. Excitation functions and isomer ratios for the ⁴¹K(α, n)⁴⁴Sc reaction are analyzed both in the equilibrium and pre-equilibrium models in the energy range 8 to 40 MeV. Conventional pairing-energy corrections are introduced and their influence on the pre-equilibrium decay is investigated. The results are compared with published experimental data. The best fit was obtained for almost-pure pre-equilibrium decay with the initial excited-particle-plus-hole number n_o = 5 when pairing corrections were taken into account. No success was achieved in fitting the decrease of the isomer ratios at high energies, which is probably caused by the contribution from direct processes.     

Biexciton decay and multimagnon sideband in emission spectra of CsMnF3

Emission spectra of intrinsic and impurity-trapped excitons and their side- bands have been measured in CsMnF₃ by a polarized time-resolved spectroscopy. From a biexcitonic decay of an emission an exciton- exciton interaction was found to be very small in contrast with those of MnF₂ and KMnF₃. Multimagnon sidebands have been assigned with the aid of calculation of magnon density of states.     

Self-quenching effects of excitons in CaWO4 under high density XUV free electron laser excitation

Using free electron laser excitation in the XUV range, CaWO₄ samples were exposed to ultrashort intense photon pulses (photon energy, 89.84 eV; average pulse energy, 10 μJ; pulse length, 25 fs), and their luminescence was studied with time-resolved spectroscopy. In the decay curves measured in the temperature range 8–300 K, a nonexponential emission decay with shortening of the lifetimes over the first few microseconds was observed, depending on the excitation density. Using a model for dipole-dipole interaction of excitons under nonuniform excitation densities, the structure of the decay curves can be reproduced in good agreement with the experimental data, and parameters for the initial exciton interaction can be calculated. The text was submitted by the authors in English.     

Fragmentation channels of neutral and charged sodium clusters

The Density Functional Formalism (with the local density approximation for exchange and correlation) is applied to jellium-like spherical particles to obtain heats of fragmentation of Na_n; Na⁺_n and Na²⁺_n (n ⩽ 90) following different decay paths. Masses of the fragments are analyzed resulting that the energetically more favorable channels are controlled by the tendency to fragments with a magic number of electrons, mainly 2 or 8. We have also obtained that, provided we consider parent clusters with the same number of electrons and decay modes involving a neutral fragment, the fragmentations of neutral and ionized clusters are very similar.     

Linear and Nonlinear Wavetransformation and Absorption in the Inhomogeneous Plasma Capacitor

The parametric decay process in inhomogeneous layers existing near the plasma boundaries or in front of antennas and probes in a plasma has been investigated. The linear enhancement of the pump field near ω = ω_p, the threshold fieldstrength, the wavenumber selection rules and the influence of spontaneous low frequency fluctuations are discussed in detail using a one-dimensional model of the inhomogeneous plasma capacitor. According to this model the instabilities appear in the layers with maximum linear transformation and (linear) absorption. In addition, a strong nonlinear part of absorption in the presence of the instability has been observed. The level of the spontaneous low frequency fluctuations influences strongly the spectrum of the parametrically excited ion waves. The experiments show a redistribution of the transferred ion acoustic wave energy over the whole wave continuum up to ω_pi, if a sufficient strong spontaneous fluctuation level exists in the plasma. It is impossible, however, to excite ion acoustic turbulence by the decay of the high frequency pump field under the present conditions. The conditions for the linear field enhancement are disturbed by the action of the ponderomotive forces changing the density profile near the critical point before reaching the strong pump amplitude being necessary for the excitation of a cascade of decay processes.     

Decay of photoinduced oscillations of the optical reflection coefficient of bismuth

A model describing the decay of photoinduced oscillations of the optical reflection coefficient R of bismuth is constructed, taking the crystal lattice anharmonicity into account. The decay time of oscillations of R is calculated as a function of the energy density of a laser pulse. The results of calculations explain the experimental data on the anomalously strong decay of oscillations of the optical reflection coefficient of bismuth (the decay time decreases by more than an order of magnitude with an increase in the laser pulse energy density from 0 to 4 mJ/cm²).     

Effective Radiative Lifetimes

Measurements of trapped decay rates for the xenon 146.9 nm resonance transition in the positive column of low current xenon discharges are reported. These rates, also called “effective lifetime of resonance photons”, were measured in contrast to other authors in an active discharge by means of a laser produced disturbance in the density of the 6s[3/2]₁-xenon resonance atoms at the discharge axis. The 146.9 nm radiation resulting from this temporary overpopulation was measured and then analysed for the determination of the trapped decay rates. The dependence of the measured quantities of tube diameter is discussed. A comparision with decay rates obtained by other experimental methodes and simulations of resonance radiation trapping shows good agreement.