On the electric activity of superfluid helium at the excitation of first and second sound waves

We show that the electric activity of superfluid helium (HeII) observed in the experiments [3] during the excitation of standing second sound waves in an acoustic resonator can be described in terms of the phenomenological mechanism of the inertial polarization of atoms in a dielectric, in particular, in HeII, when the polarization field induced in the medium is proportional to the mechanical acceleration, by analogy with the Stewart-Tolman effect. The variable relative velocity w = v _n − v _s of the normal and superfluid HeII components that emerges in the second sound wave determines the mean group velocity of rotons, V _g ≈ w, with the density of the normal component related to their equilibrium number density in the temperature range 1.3 K ≤ T ≤ 2 K. Therefore, the acceleration of the 4He atoms involved in the formation of a roton excitation is proportional to the time derivative of the relative velocity.w. In this case, the linear local relations between the variable values of the electric induction, electric field strength, and polarization vector should be taken into account. As a result, the variable displacement current induced in the bulk of HeII and the corresponding potential difference do not depend on the anomalously low polarizability of liquid helium. This allows the ratio of the amplitudes of the temperature and potential oscillations in the second sound wave, which is almost independent of T in the above temperature range, consistent with experimental data to be obtained. At the same time, the absence of an electric response during the excitation of first sound waves in the linear regime is related to an insufficient power of the sound oscillations. Based on the experimental data on the excitation of first and second sounds, we have obtained estimates for the phenomenological coefficient of proportionality between the polarization vector and acceleration and for the drag coefficient of helium atoms by rotons in the second sound wave. We also show that the presence of a steady heat flow in HeII with nonzero longitudinal velocity and temperature gradients due to finite viscosity and thermal conductivity of the normal component leads to a change in the phase velocities of the first and second sound waves and to an exponential growth of their amplitudes with time, which should cause the amplitudes of the electric signals at the first and second sound frequencies to grow. This instability is analogous to the growth of the amplitude of long gravity waves on a shallow-water surface that propagate in the direction of decreasing basin depth.     

Relaxation times for establishing steady state populations in optically thin helium plasmas

Population densities of HeI and HeII excited states are calculated from a collisional radiative model for non-LTE optically thin helium plasmas. Effect of direct ionisation-excitation of HeI to HeII states on the population density of HeII states is shown. Relaxation times for HeI states calculated from the CR model is reported forT _e from 3 to 18 eV andn _e from 10⁹ to 10¹⁶ cm⁻³. Part of a research project supported by the Department of Atomic Energy.     

Asymmetry of spontaneous helium radiation in retarded excitation in an electric discharge

The dynamics of formation of optical radiation in spectral lines in a longitudinal nanosecond helium discharge under conditions of wave breakdown is investigated. A discharge tube with a diameter of 0.4 cm and a length of 50 cm was placed in a metallic shield 1 cm in diameter. The gas pressure was varied within 1–60 torr for an amplitude of the voltage pulses of up to 50 kV. Asymmetry of spontaneous radiation recorded from different ends of the discharge tube is detected. This is shown to be a consequence of nonuniform distribution of the functions of the primary brightness of the flux and the absorption coefficient along the tube length. Dagestan State University, 43a, M. Gadzhiev Str., Makhachkala, 367025, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 415–420, May–June, 1999.     

Effect of a photonic band gap in the optical range on solid-state SiO2 cluster lattices — opals

It is shown that synthetic opals — cubic face-centered lattices of SiO₂ clusters — are systems which exhibit a number of properties of photonic crystals in the visible-light range. By filling the voids (pores) in such lattices with different materials it is possible to vary the optical contrast of the medium and to obtain crystals of both the lattice of spheres type and its three-dimensional replica. It is shown that under conditions of identical optical contrast and in the presence of an additional optical inhomogeneity of the spheres, the transparency of the lattice of spheres is lower than that of its replica based on homogeneous media. A refractive index modulation of 1.266 was achieved in the lattice of spheres. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 496–501 (10 April 1996)     

ZnO spheres and nanorods formation: their dependence on agitation in solution synthesis

ZnO nanostructures including nanorods, dense, and partially hollow spheres were synthesized via a solution synthesis method with temperature ranging from 65 to 95 °C. Scanning electron microscopy (SEM) revealed that the diameter of the spheres is in the range of 200–500 nm. Transmission electron microscopy (TEM) showed that some of the spheres are hollow or partially hollow. Powder X-ray Diffraction (XRD) and TEM-Selected area electron diffraction (SAED) analysis showed that the spheres consist of polycrystalline nanoparticles. It was found for the first time that the agitation during the synthesis plays a critical role on morphology of the ZnO nanostructures formed in solution. The oriented attachment of nanocrystals without agitation during the synthesis could guide the nanocrystals to form an ordered nanorod structure. However, the disordered aggregation of the nanocrystals under shear force resulted in a spherical morphology. It was also found that the composition of spheres is different from that of nanorods: the spheres consist of both ZnO and Zn(OH)₂, but nanorods consist of single-crystal ZnO only. Zn(OH)₂ presented in the spheres could decompose to ZnO by calcination, resulting in the formation of hollow spheres.     

Nanoparticle formation during laser ablation of metals at different pressures of surrounding noble gases

We demonstrate that the nanoparticle formation during laser ablation of metals by short (of a few tens of ps) laser pulses strongly depends on the concentration of surrounding gas. While, at vacuum conditions, nanoparticle formation shows very “sharp” atomic force microscope images of aggregated clusters, following with clear appearance of plasmon resonance on the absorption spectra of deposited films, an addition of gas particles starts to decrease the probability of cluster formation. This process shows a threshold for both helium (33 torr) and xenon (12 torr) above which no surface plasmon resonance and correspondingly no observable nanoparticles on the deposited surfaces were detected. The destruction of nanoparticle formation was attributed to the negative influence of surrounding gas particles on ablated particles aggregation.     

Metallization of DNA on silicon surface

New simple way for silver deoxyribonucleic acid (DNA)-based nanowires preparation on silicon surface was developed. The electrochemical reduction of silver ions fixed on DNA molecule provides the forming of tightly matched zonate silver clusters. Highly homogeneous metallic clusters have a size about 30 nm. So the thickness of nanowires does not exceed 30–50 nm. The surface of n-type silicon monocrystal is the most convenient substrate for this procedure. The comparative analysis of DNA metallization on of n-type silicon with a similar way for nanowires fabrication on p-type silicon, freshly cleaved mica, and glass surface shows the advantage of n-type silicon, which is not only the substrate for DNA fixation but also the source of electrons for silver reduction. Images of bound DNA molecules and fabricated nanowires have been obtained using an atomic force microscope and a scanning ion helium microscope. DNA interaction with silver ions in a solution was examined by the methods of ultraviolet spectroscopy and circular dichroism.     

Temporal evolution of plasma from a highly ionized helium capillarydischarge

The generation of a highly ionized helium capillary plasma and the study of its temporal evolution are discussed. A 30-cm-long and 1-mm-diameter helium plasma was created with well-terminated kiloamp current pulses of 90-ns full width at half-maximum (FWHM). Emission spectroscopy was used to study the recombination of totally stripped ions into hydrogenic helium ions and to measure the evolution of the plasma density from the Stark broadening of HeII transitions. A 1.2-kA discharge current pulse was observed to create a plasma density of 8×10¹⁶ cm^-3 in 1 torr of helium. The maximum intensity of HeII transitions occurs in the afterglow of the discharge pulse, following the collisional recombination of totally stripped ions with plasma electrons when the plasma cools. The study is of interest in relation to the possibility of obtaining amplification in the 164.0-nm line of HeII in a capillary discharge     

Equation of state of metallic helium

The effective ion-ion interaction, free energy, pressure, and electric resistance of metallic liquid helium have been calculated in wide density and temperature ranges using perturbation theory in the electron-ion interaction potential. In the case of conduction electrons, the exchange interaction has been taken into account in the random-phase approximation and correlations have been taken into account in the local-field approximation. The solid-sphere model has been used for the nuclear subsystem. The diameter of these spheres is the only parameter of this theory. The diameter and density of the system at which the transition of helium from the singly ionized to doubly ionized state occurs have been estimated by analyzing the pair effective interaction between helium atoms. The case of doubly ionized helium atoms has been considered. Terms up to the third order of perturbation theory have been taken into account in the numerical calculations. The contribution of the third-order term is significant in all cases. The electric resistance and its temperature dependence for metallic helium are characteristic of simple divalent metals in the liquid state. The thermodynamic parameters—temperature and pressure densities-are within the ranges characteristic of the central regions of giant planets. This makes it possible to assume the existence of helium in the metallic state within the solar system.     

Negative Ca− and Ba− ions of large radius on the surface and in the volume of liquid helium

Negative Ca⁻ and Ba⁻ ions of large radii on the surface of and in bulk liquid helium have been studied. Our results indicate that these ions are adsorbed on the helium surface. Ions on free liquid helium surfaces have not been studied previously because it was thought impossible to confine them on the surface. Ca⁻ and Ba⁻ ions have very low binding energies, therefore, like electrons, they form a bubble of large radius in bulk helium, whose energy is higher than on the surface. The behavior of ions on the surface exhibits a number of previously unknown features owing to their large masses and strong localization in the horizontal plane. Even in the absence of confining electric field, a hole is formed under an ion due to the polarization attraction between the liquid helium and the charged ion. This hole formation reduces the ion mobility by several orders of magnitude and increases its effective mass severalfold. The critical density of electrons and ions is approximately the same on the surfaces of thin and thick helium films. Zh. éksp. Teor. Fiz. 115, 593–604 (February 1999)     

Phosphorus cluster production by laser ablation

Neutral and charged phosphorus clusters of a wide size range have been produced by pulsed laser ablation (PLA) in vacuum at 532, 337, and 193 nm ablating wavelengths and investigated by time-of-flight mass spectrometry. The neutral P_n clusters are even-numbered with local abundance maxima at n=10 and 14, while the cationic and anionic clusters are preferentially odd-numbered with P₇⁺, P₂₁⁺, and P₁₇^- being the most abundant ions. The dominance of the magic clusters is more pronounced at 337-nm ablation that is explained by efficient direct ejection of their building blocks under these conditions. Nanocrystalline phosphorus films have been produced by PLA in ambient helium gas. PACS 52.38.MF; 61.46.+w; 79.20.Ds; 81.07.B; 81.16.Mk     

On the mechanism of electromagnetic microwave absorption in superfluid helium

In experiments on electromagnetic (EM) wave absorption in the microwave range in superfluid (SF) helium [1?C3], a narrow EM field absorption line with a width on the order of (20?C200) kHz was observed against the background of a wide absorption band with a width of 30?C40 GHz at frequencies f ₀ ?? 110?C180 GHz corresponding to the roton gap energy ?? _r (T) in the temperature range 1.4?C2.2 K. Using the so-called flexoelectric mechanism of polarization of helium atoms (⁴He) in the presence of density gradients in SF helium (HeII), we show that nonresonance microwave absorption in the frequency range 170?C200 GHz can be due to the existence of time-varying local density gradients produced by roton excitations in the bulk HeII. The absorption bandwidth is determined by the roton-roton scattering time in an equilibrium Boltzmann gas of rotons, which is t _r-r ?? 3.4 × 10^?11 s at T = 1.4 K and decreases upon heating. We propose that the anomalously narrow microwave resonance absorption line in HeII at the roton frequency f ₀(T) = ??r(T)/2??? appears due to the following two factors: (i) the discrete structure of the spectrum of the surface EM resonator modes in the form of a periodic sequence of narrow peaks and (ii) the presence of a stationary dipole layer in HeII near the resonator surface, which forms due to polarization of ⁴He atoms under the action of the density gradient associated with the vanishing of the density of the SF component at the solid wall. For this reason, the relaxation of nonequilibrium rotons generated in such a surface dipole layer is strongly suppressed, and the shape and width of the microwave resonance absorption line are determined by the roton density of states, which has a sharp peak at the edge of the roton gap in the case of weak dissipation. The effective dipole moments of rotons in the dipole layer can be directed either along or across the normal to the resonator surface, which explains the experimentally observed symmetric doublet splitting of the resonance absorption line in an external dc electric field perpendicular to the resonator surface. We show that negative absorption (induced emission) of EM field quanta observed after triggering a Kapitza ??heat gun?? occurs when the occupation numbers for roton states due to ??pumping?? of rotons exceed the occupation numbers of EM field photons in the resonator.     

Dynamics of the plume formation and parameters of the ejected clusters in short-pulse laser ablation

The dynamics of the early stages of the ablation plume formation and the mechanisms of cluster ejection are investigated in large-scale molecular dynamics simulations. The cluster composition of the ablation plume has a strong dependence on the irradiation conditions and is defined by the interplay of a number of processes during the ablation plume evolution. At sufficiently high laser fluences, the phase explosion of the overheated material leads to the formation of a foamy transient structure of interconnected liquid regions that subsequently decomposes into a mixture of liquid droplets, gas-phase molecules, and small clusters. The ejection of the largest droplets is attributed to the hydrodynamic motion in the vicinity of the melted surface, especially active in the regime of stress confinement. Spatially resolved analysis of the dynamics of the plume formation reveals the effect of segregation of the clusters of different sizes in the expanding plume. A relatively low density of small/medium clusters is observed in the region adjacent to the surface, where large clusters are being formed. Medium-size clusters dominate in the middle of the plume and only small clusters and monomers are observed near the front of the expanding plume. Despite being ejected from deeper under the surface, the larger clusters in the plume have substantially higher internal temperatures as compared to the smaller clusters. The cluster-size distributions can be relatively well described by a power law Y(N)∼N^-τ with exponents different for small, up to ∼15 molecules, and large clusters. The decay is much slower in the high-mass region of the distribution. Received: 13 October 2001 / Accepted: 18 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-434/982-5660, E-mail: lz2n@virginia.edu     

Single-pulse drilling study on Au,Al and Ti alloy by using a picosecond laser

Picosecond laser single pulse ablation of Au, Al and Ti alloy (Ti6Al4V) was experimentally investigated with a laser pulse width of 10 ps at a wavelength of 1064 nm for potential industrial micromachining applications. The diameters, depths and morphologies of the drilled craters were studied. Two novel phenomena were found: as hole diameters decreased with fluence, a change of slope of the trend line indicated a change in ablation mechanism for Al and Ti alloy, metallic materials with short electron-phonon coupling times (<10 ps), while Au showed no such transition: an isolated island structure was also observed on Au due to significant melt expulsion. A one-dimensional two-temperature model has been used to discriminate different ablation phenomena. It is shown that metallic materials with different electron–phonon coupling constant have different ablation characteristics in the ps regime. This study could be very helpful for metallic material micromachining with high repetition rate ps lasers pulses which indicates that high throughput may be achieved as well as good machining quality.     

Features of the development of pulsed microwave discharges in various gases in a quasioptical beam

Photographs of pulsed microwave discharges initiated by a metallic sphere placed at the focus of a quasioptical electromagnetic beam with linear polarization of the field in air, sulfur hexafluoride, hydrogen, and helium under a pressure of several hundred Torr are presented. The common and distinctive features of the discharges in these gases are noted. Zh. Tekh. Fiz. 68, 33–36 (April 1998)     

Laser ablation and micropatterning of thin TiN coatings

Laser ablation of thin TiN films deposited on steel substrates has been studied under wide-range variation of irradiation conditions (pulsewidth, wavelength, energy density and spot size). It has been demonstrated that both picosecond (150–300 ps) and nanosecond (5–9 ns) laser pulses were suitable for controllable ablation and microstructuring of a 1-μm-thick TiN film unlike longer 150-ns pulses. The ablation rate was found to be practically independent of the wavelength (270–1078 nm) and pulsewidth (150 ps–9 ns), but it increased substantially when the size of a laser spot was reduced from 15–60 μm to 3 μm. The laser ablation technique was applied to produce microstructures in the thin TiN films consisting of microcraters with a typical size of 3–5 μm in diameter and depth less than 1 μm. Tests of lubricated sliding of the laser-structured TiN films against a steel ball showed that the durability of lubricated sliding increased by 25% as compared to that of the original TiN film. Received: 28 July 1999 / Accepted: 17 April 2000 / Published online: 20 September 2000     

Acid functionalized,highly dispersed carbonaceous spheres: an effective solid acid for hydrolysis of polysaccharides

Highly dispersed carbonaceous spheres with sulfonic acid groups were successfully prepared from glucose by hydrothermal method. Transmission electron microscopy (TEM) showed the as-synthesized carbonaceous materials were uniform, spherical in shape with an average diameter of about 450 nm. Fourier transform infrared (FT-IR) proved that –SO₃H, –COOH, OH groups were grafted on the surface of the carbonaceous spheres during the sulfonation. Interestingly, the functionalized carbonaceous spheres exhibited high dispersibility in the polar solvent due to the hydrophilic groups on the surface. The mechanism of the formation for the carbonaceous spheres was also discussed based on the analysis of structure and composition. At last, the functionalized carbonaceous spheres were employed as solid acid to hydrolyze starch and cellulose. By comparison, the as-synthesized catalyst showed considerable high yield of glucose.     

Condensed water in superfluid He-II

It was found that when ⁴He gas containing water vapor as an impurity condenses on the surface of superfluid He-II cooled to ∼ 1.4 K, semitransparent clouds (icebergs) form in the volume of a glass cell filled with He-II below the He-II surface. The form of the icebergs extracted from the superfluid liquid remains virtually unchanged on heating up to ∼ 1.8 K. In the temperature range 1.8–2.2 K the thermometers register sharp temperature jumps, which are accompanied by jumps in the gas pressure in the cell and a repeated decrease, by more than two orders of magnitude, in the total volume of the condensate, i.e., the water content in the volume of an iceberg does not exceed 10²⁰H₂O molecules per 1 cm³. It can be inferred that porous icebergs, permeated with superfluid liquid and containing cores consisting of small clusters surrounded by a layer of solidified helium, form in the volume of He-II as the gas mixture condenses. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 744–748 (10 December 1999)     

HeII的粘度理论分析与计算

在一般的分析与计算中,超流氦的粘度往往成为一个被忽视甚至被省略的一个物理量。但是在某些低温系统中,需要完成超流氦长距离输送,粘度成为确定其流动阻力损失的一个重要物理量。目前各具特色的理论和计算关系式很多,但是迄今为止,还没有任何一种理论体系或计算关系式可以适用于所有的物质或所有的温度和压力范围,因此如何应用和借鉴现有的粘度理论或关系式以及实验结果,实现对HeII这种特殊流体粘度的分析和计算是一项十分重要的工作。     

Method for measuring chemical diffusion coefficients in solids

Experiments were performed with temperature programmed desorption of hydrogen and deuterium adsorbates on small platinum spheres. Beyond the expected desorption peak of these adsorbates at around 300 K sample temperature an additional desorption peak at higher temperatures was observed. This additional peak is explained by the diffusion of hydrogen or deuterium atoms from the inside of the spheres to their surfaces with final desorption from these surfaces. The visibility of this second high temperature desorption peak is supported by a small diameter of the platinum spheres. Platinum spheres with diameters around 64 μm were used. The sample temperature at which the second peak was observed depends on the parameters: diameter of the platinum spheres, heating rate of the sample and chemical diffusion coefficient of hydrogen or deuterium in platinum. A theory, which assumes that the chemical diffusion coefficient can be described with an Arrhenius ansatz, was developed to simulate the occurrence of the second peak. The combination of these kinds of experiments with the theory gives a method to measure chemical diffusion coefficients. This method can be called temperature programmed diffusion. At 510 K sample temperature the diffusion coefficient 1.61×10⁻¹² m²/s of hydrogen in platinum and the diffusion coefficient 1.40×10⁻¹² m²/s of deuterium in platinum was measured.