Structural transitions in ice samples at low temperatures and pressures

The structure of ice samples formed in the decay of a water impurity gel at temperatures above 4 K and atmospheric pressure has been examined. The X-ray diffraction analysis indicates that three phases coexist in the initial sample at temperatures of 85–110 K. These phases are amorphous ice occupying up to 30% of the sample volume, cubic-phase ice I _c metastable at low pressures (∼60%), and normal hexagonal ice I _h (≤6%). The characteristic sizes of crystals of the cubic and hexagonal phases are about 6 and 30 nm, respectively. The amorphous phase at annealing above 110 K is gradually transformed to the crystalline phase both cubic and hexagonal. This transition is accompanied by two processes, including a fast increase in the sizes of cubicphase nanocrystals and the partial transition of the cubic phase I _c to the hexagonal one I _h. Hexagonal ice I _h prevails in the bulk of the sample above 200 K.     

Zur nichtlinearen Theorie der Strahl-Plasma-Wechselwirkung IV. Numerische Ergebnisse: Elektronenschwingungen

The spatial variations of the electric field E and the electron distribution function f_a for the case of beam plasma amplification are computed on the basis of a nonlinear approximation developed in the earlier parts of the present paper [1–3]. The essential results are: With increasing distance x from the point, where the beam enters the plasma, the amplitude Ã₁ of the first harmonic increases, reaches a maximum value at some point x_m and than decreases monotonically without reaching a further maximum. In the region around x_m we find a large increase of the higher harmonics, i.e. increasing nonlinearity, and also an increasing phase velocity, which soon approches the beam velocity. The non-oscillating part of the distribution function of the beam electrons deformes, but not so much as the quasilinear theory predicts, so that amplitude settling must be interpreted as a nonlinear but not as a quasilinear effect. The results show a good agreement with some typical experiments on beam plasma interaction.     

Zur nichtlinearen Theorie der Strahl-Plasma-Wechselwirkung. II. Das asymptotische Verhalten des Systems

The asymptotic behavior of an amplifying beam-plasma-system for great distances x from the point where the beam enters the plasma is investigated on the basis of an expansion in powers of the electric field amplitude A₁. It is shown, that a necessary condition for approaching a steady state with a constant wave amplitude is that the first 2N ?1 derivations of the distribution function f₀(u) at the point u_φ vanish if the approximation is to N-th order in A₁. Further it is demonstrated, that an approximation including the third power of A₁ involves the important physical effects as mode coupling and wave amplitude saturation, and that fifth order correction terms are unimportant, if the beam parameters are chosen so that the difference between beam and phase velocities is not to small. To illustrate this point some numerical examples are considered.     

Self-pulsation and optical chaos in self-pumped photorefractive BaTiO3

Sinusoidal intensity oscillations of the phase conjugated wave reflected from a self-pumped photorefractive BaTiO₃ crystal have been observed. The oscillation amplitude is shown to be increased by providing optical feedback of the light scattered during grating formation. The oscillation frequency ? can be tuned from 0.024 Hz to 2 Hz by changing the light intensity I₀ of the object beam from 0.7 W/cm² to 100 W/cm². A power law ? ∝ I^β₀ ∝ σ with β = 0.89 was observed over this range of intensities (σ = photoconductivity). Transitions to a chaotically oscillating or a stable reflecting state have been observed at higher power levels or with increased optical feedback. An additional pump beam near the entrance face of the signal beam can decrease the build-up time for the self-pumping process by a factor of five or more.     

Fast LEED-intensity measurements with a computer controlled television system

A standard television system in combination with a processing computer is used for measurements of LEED beam intensities of a spot pattern. The video signal of the TV camera contains the intensity distribution of each spot. This can be integrated for one selected spot by the computer which also takes care of automatic background subtraction. The processing program controls the energy of the incident electron beam and follows the moving spot position. One integration of a profile and thus the measurement of one point of anI(E) curve takes 0.5 s or less. Diffracted beam currents down to 10⁻¹¹ A can be detected with the present experimental set-up. The output is immediately displayed and recorded in final reduced formI/I ₀.     

Phase control of iron silicides using femtosecond laser irradiation

Phase control of Fe–Si amorphous thin film in micro area is demonstrated using femtosecond laser irradiation. A femtosecond laser beam with a high repetition rate over 200 kHz and tightly focused through an objective lens promotes both crystallization and phase transformation from an amorphous phase into crystalline β-FeSi₂, α-FeSi₂, or ε-FeSi phases. Formation of each crystalline phase is possible by changing the pulse energy or the scanning speed of the incident laser beam.     

Oscillation of nonvolatile holographic recording in LiNbO3:Fe:Cu crystals

The oscillation of diffraction efficiency is observed in the nonvolatile holographic recording of lithium niobate crystals doped with iron and copper. The physics of oscillation in doubly doped lithium niobate crystals is studied by using Runge–Kutta methods, and the oscillation can be attributed to the redistribution of electrons in the deeper and shallower traps of the crystals in the initial phase of holographic recording. The effects of Fe concentration and intensity ratio of red beams to UV beam (I_R/I_UV) on the oscillation are investigated theoretically. The results show that with lower Fe concentration, the amplitude of oscillation is larger and with lower intensity ratio I_R/I_UV, the duration of the oscillation is longer.     

Laser-induced spark for simultaneous ignition and fuel-to-air ratio measurements

This work investigates the use of laser-induced gas breakdown for simultaneously igniting and measuring fuel-to-air ratio of CH₄–air and H₂–air combustible mixtures. The fuel-to-air ratio is determined using the measured spectral peak ratio I_o,Hα/I_o,OI. Sparks are produced using a single-mode, Q-switched Nd–YAG laser. The laser produces a beam of 6 mm in diameter at the wavelength of 1064 nm and pulse duration of 5.5 ns. The beam optics is designed to have mainly a beam splitter and a focusing lens. The beam splitter is coated to reflect the laser beam and transmit emission lines with wavelengths from 600 to 900 nm which are then collected by a fiberoptic cable and detected by an imaging spectrometer–detector assembly. The results showed a linear dependence of the spectral peak ratio on the equivalence ratio that can be generally expressed by φ=a(I_o,Hα/I_o,OI)+b, where a and b are the parameters that depend on the gas pressure. Using the least-square curve fitting technique to fit the experimental data, a calibration curve for calculating the equivalence ratio as a function of the ratio of (I_o,Hα/I_o,OI) was generated.     

Path integral approach for superintegrable potentials on spaces of nonconstant curvature: I. Darboux spaces <Emphasis Type="Italic">D</Emphasis><Subscript>I</Subscript> and <Emphasis Type="Italic">D</Emphasis><Subscript>II</Subscript>

In this paper, the Feynman path integral technique is applied for superintegrable potentials on two-dimensional spaces of nonconstant curvature: these spaces are Darboux spaces D _I and D _II. On D _I, there are three, and on D _II four such potentials. We are able to evaluate the path integral in most of the separating coordinate systems, leading to expressions for the Green functions, the discrete and continuous wave-functions, and the discrete energy-spectra. In some cases, however, the discrete spectrum cannot be stated explicitly, because it is either determined by a transcendental equation involving parabolic cylinder functions (Darboux space I), or by a higher order polynomial equation. The solutions on D _I in particular show that superintegrable systems are not necessarily degenerate. We can also show how the limiting cases of flat space (constant curvature zero) and the two-dimensional hyperboloid (constant negative curvature) emerge. The text was submitted by the authors in English.     

Effect of thermal nonlinearity in high-absorption media on the parameters of the photoacoustic signal detected by the gas microphone method: The fundamental and second harmonics

A perturbation theory is put forward that describes the effect of thermal nonlinearity due to the temperature dependence of the thermophysical parameters of high-absorption systems with a low thermal conductivity on the parameters of the photoacoustic signal detected by the gas microphone technique. It is found that the dependence of the photoacoustic signal amplitude on incident beam intensity I ₀ stems from the dependence of the illuminated surface temperature on I ₀. This dependence is a complicated function instead of being a simple quadratic function as was expected. In the limiting cases (μ_sβ ? 1 and μ_sβ ? 1), this contribution to the photoacoustic signal amplitude is described by simple expressions, which are convenient for determining the thermal coefficients of the thermophysical parameters of the medium. It is found that the thermal nonlinearity significantly affects the photoacoustic signal phase in the frequency region meeting the condition μ_sβ ～ 1. In the above limiting cases, its effect is insignificant. A theory of generation of the photoacoustic signal second harmonic is proposed. The second harmonic is related to the temperature dependence of the thermophysical parameters of the buffer gas and sample. It is shown that the amplitude of the signal is a quadratic function of the incident beam intensity and varies with its frequency as ω^?3/2 for μ_sβ ? 1 and ω^?5/2 for μ_sβ ? 1.     

87Rb NMR study of the temperature and electric field dependence of the soliton density near the lock-in transition of incommensurate Rb2ZnCl4

The frequency distribution of the⁸⁷Rb NMR 1/2–1/2 central transition is measured in a detailed manner as a function of the temperature and the crystal orientation in the normal (N), incommensurate (IC), and commensurate (C) phases of high quality single crystals of Rb₂ZnCl₄ (RZC). Special emphasis is laid upon the region around the lock-in transition atT _c . While in the N and C phases discrete lines are observed, in the IC phase a continuous distribution of frequencies is detected. In a temperature interval not exceeding 15 K aboveT _c some centers of intensity termed C lines can be observed in the frequency distribution. They reflect the fact that in the IC phase close toT _c several nuclei feel a local surrounding very similar to that of the low temperature C phase. For the first time in the region aroundT _c the influence could be studied which an electric field favouring the ferroelectric C phase has on the NMR spectra. The experimental results are related to the temperature and electric field dependence of the soliton density near the lock-in transition. A general definition of the soliton density is developed whose validity is not limited to the very narrow soliton limit. The meaning of this definition is also discussed in terms of well known phenomenological theories of the three phases of RZC. The experimental NMR data can be well described by this general definition.Dedicated to Prof. Dr. H.E. Müser on the occasion of his 60th birthday     

Low-temperature specific heat of icosahedral and amorphous Pd−U−Si alloys

Icosahedral (I) Pd_0.588U_0.206Si_0.206 can be obtained from melt-spun amorphous (A) ribbons by annealing. The specific heatC (measured betweenT=0.1 K and 20 K) shows very similar behavior for both phases. The main features ofC are as follows. (i) The vibrational heat capacityC _ph dominatesC at highT.C _ph is almost identical in both phases, in agreement with recent inelastic neutron scattering data. (ii) Shallow maxima in (C–C _ph)/T vs.T are found at 5.4 and 4.3 K forI andA phases, respectively, associated with magnetic order. These maxima are suppressed by 20% in an applied magnetic field of 6 T. (iii) A large quasi-linear contribution is observed with a low-T coefficient =165 mJ/mole U K² for theI phase and =120 mJ/mole U K² for theA phase. In the low-T region,C is hardly affected by a field of 6 T. This hints at the formation of a narrow 5f band with a comparable density of states for bothI andA phases.     

Conversion of high-order Laguerre-Gaussian beams into Bessel beams of increased, reduced or zeroth order by use of a helical axicon

We propose a new method for transformation of a Laguerre-Gaussian beam of azimuthal index l and radial index n = 0 (LG_l,0) into a vortex, diverging or nondiverging Bessel beam, which can have increased or decreased phase singularity order, or into a zeroth order Bessel beam, by means of a helical axicon. The Bessel beam divergence or nondivergence depends upon the waist position of the input Laguerre-Gaussian beam, regarding the plane where the helical axicon is situated.The expressions for the amplitude and the intensity distribution of the diffracted wave field, in the process of Fresnel diffraction, are deduced using the stationary phase method. The theoretical analysis for the vortex radius and the maximum propagation distance of the Bessel beams obtained is presented.     

Holographic interferometry using a reference wave with a sinusoidally modulated amplitude

A method of holographic interferometry which uses an amplitude-modulated reference wave is proposed for investigating vibration phases. Sinusoidal amplitude modulation of a reference wave yields hologram fringes which are characterized by a function J²₁(α)cos²Δ, where α and Δ are related to vibration amplitudes and phases on object points, respectively. The resultant fringes indicate that vibration amplitude information and phase information are stored separately. The phase information of the vibration is easily obtained as brightness variations of the fringes, independently of the amplitude information.     

Spatial transport phase transitions in systems with high polariton densities

The behaviour of high intense laser beams in matter is examined with respect to a possible pattern formation. The latter is thought to be a collective phenomenon. We establish a microscopic transport theory for anharmonically interacting polaritons, which is valid even for regions far from thermal equilibrium. It is shown that then there must necessarily be phase transitions in the space distribution of polaritons as a function of the laser power. Numerical calculations are performed for an ionic crystal. They yield two critical intensitiesI _c1 andI _c2, where qualitative alterations in the spatial distribution are to be expected. We ascribe the transport phase transitions atI _c1 andI _c2 to the formation of large and small scale filaments respectively. The phenomenon appears to be very similar to the Bénard problem.     

Study of the influence of the injection section on the stability of a high-current relativistic electron beam propagating through the gaseous medium of a plasmochemical reactor

Results are presented from experimental study of the influence of the injection section on the stability of a high-current relativistic electron beam (REB) propagating through the gaseous medium of a plasmochemical reactor (PR). An REB with the electron energy E _e =1 MeV, beam current I _b =10–22 kA, and pulse duration t=60 ns was generated by the Tonus accelerator and, then, injected into a 0.1-m-diameter PR filled with air. The PR consisted of two sections with lengths L ₁= 0.3–1.0 m and L ₂=1.1–2.5 m; the total length of the system was no longer than 3.5 m. The first section was filled with air at a pressure of P ₁=0.8–1.5 torr, and the pressure in the second section was varied within the range P ₂=0.1–760 torr. The current I _b of an REB passed through both sections of the PR was measured with the help of a sectioned vacuum Faraday cup. The transportation efficiency of the beam was determined as the ratio I _b /I _inj, where I _inj is the beam current measured at the point of injection into the PR. It is shown that, for the optimal dimensions of the first PR section, it has a stabilizing action on the REB with a current density of up to 3 kA/cm², which makes it possible to increase the effective length of the second (working) PR section, which is filled with a gas at various pressures, to L ₂=(25–35)L _bet, where L _bet is the beam betatron length.     

Reactive scattering of oxygen atoms: O+I2, ICl,Br2

Angular and velocity distribution measurements of IO reactive scattering from crossed beams of O atoms and halogen molecules I₂, ICl are reported. Angular distribution measurements are reported for BrO from O + Br₂. The O atom beam was generated at ～350 K from a microwave discharge source and the halogen molecule beam from a supersonic nozzle source at ～380 K. The product time-of-flight distribution was recorded at each laboratory scattering angle by a mini-computer. The scattering data are found to be in excellent agreement with the RRKM-AM model of reactive scattering via a long-lived collision complex. The observation of IO product from O + ICl identifies the complex with a bound O-I-Cl triplet state, previously observed for O-Cl-Cl in matrix isolation studies, as proposed by Herschbach. The maximum centrifugal barrier B _m′ for dissociation of the long-lived complex can be accurately determined, particularly for O + I₂. The B _m′ values indicate that both the entrance and exit valleys of the potential energy surface are governed by centrifugal barriers in the region of long-range van der Waals potentials. The comparatively small reaction cross section (e.g. Q ～ 2 Ų for O + Br₂ from discharge flow measurements) is attributed primarily to an orientation requirement for reaction. The RRKM-AM model indicates a ‘tight linear’ transition state for dissociation of the O-I-I complex, corresponding to significant long-range IO orienting forces in the exit valley of the potential energy surface.     

谱合成光束特性的模拟分析

建立了光束谱合成的传输模型.应用光线追迹方法，将闪耀光栅引起的相位变化用槽间光程差和槽内光程差来表示，建立了光束倾斜入射到光栅时相位变化的计算模型.利用衍射积分方法，给出了阵列光源各子光束经谱合成系统后输出光场的解析表达式.依据光束非相干叠加的原理，计算得到合成光束的光强分布.在此基础上，利用强度二阶矩方法，分别计算了阵列光束和合成光束的M²因子，并定量分析了谱合成系统参数对合成光束特性的影响.研究结果表明：通过谱合成系统合成光束的M²关键词： 光束谱合成 阵列光源 闪耀光栅 光束质量     

Multiple-Quantum J-Resolved NMR Spectroscopy (MQ-JRES): Measurement of Multiple-Quantum Relaxation Rates and Relative Signs of Spin Coupling Constants

The technique of multiple-quantum J-resolved NMR spectroscopy (MQ-JRES) is introduced and applied to the spin system SI₃–M (such as in the example given here, the ¹³CH₃–¹²CH in alanine). The SI₃ spin system was excited to its highest quantum state (8S_yI_xI_yI_y), which consists of four coherences: quadruple quantum of (3I + S), double quantum of (3I − S), double quantum of (I + S), and zero quantum of (I − S). In the MQ spectrum generated from the projection onto the F₁ dimension, the resonances of the different multiple-quantum coherences are resolved by their coupling constants to the remote spin (M). The absorptive lineshapes in both F₁ and F₂ dimensions enable accurate measurements of transverse relaxation rates, and both amplitude and relative signs of the long-range coupling constants are to be derived from either frequency or time domain data. The selective detection of MQ-JRES spectra of the individual MQ coherences using either phase cycling or pulsed field gradients is presented.     

Ab Initio Simulation of Ta2O5: A High Symmetry Ground State Phase with Application to Interface Calculation

It is reported that the recently predicted triclinic γ‐phase ground state Ta₂O₅ by Yang and Kawazoe can be assigned a much more symmetric I4₁/amd space group, and is isomorphic to P‐Nb₂O₅. Interestingly, the well‐known high temperature α‐phase Ta₂O₅ also has the I4₁/amd symmetry, but is unstable at zero temperature according to our phonon dispersion calculation. A thorough energy comparison of the β_AL, δ, λ, Β, L_SR, β_R, Pm, Cmmm, γ, and α phases of Ta₂O₅ is carried out using density functional theory under the generalized gradient approximation (GGA). The GGA‐1/2 method is applied in calculating the electronic structure of various phases, where the tetragonal γ‐phase demonstrates a 4.24 eV indirect band gap, close to experimental value. The high symmetry tetragonal phase together with computationally efficient GGA‐1/2 method greatly facilitates the ab initio simulation of Ta₂O₅‐based devices. As an example, the Ohmic contact nature between metal Ta and Ta₂O₅ by calculating an interface model of b.c.c. Ta and tetragonal γ‐Ta₂O₅, using GGA‐1/2 has been explicitly shown.