Dependence of the longitudinal relaxation time of the polarization of cesium atoms in the ground state on the temperature of an antirelaxation coating

The dependence of the time of longitudinal relaxation of the ground-state cesium vapor on the temperature of the antirelaxation coating of the cell walls is studied experimentally. It is found that the fast component of relaxation is independent of the coating temperature, while the slow component depends on it. The temperature dependence of the slow relaxation component is used to estimate the energy of activation of desorption of cesium atoms from the antirelaxation coating, E _desorp = 0.13 eV.     

Electron-stimulated cesium atom desorption from the Cs/CsAu/Au/W system

This paper reports on a continuation of the investigation of electron-stimulated Cs-atom desorption from a tungsten surface on which cesium and gold films had been adsorbed at T = 300 K. Earlier studies revealed that Cs atoms start to desorb only after more than one monolayer of gold and more than one monolayer of cesium had been deposited on the tungsten surface. In this case, a coating consisting of a gold adlayer on tungsten, a CsAu compound possessing semiconducting properties, and a cesium monolayer capping CsAu (Cs/CsAu/Au/W) is formed on the tungsten surface at 300 K. The yield of atoms from this system exhibits a resonant dependence on the incident electron energy E _e , with an appearance threshold of 57 eV and a maximum at 64 eV. In this case, Cs atoms desorb in two channels, with one of them involving Cs desorption out of the cesium monolayer, and the other, from the CsAu monolayer. The Cs yield at E _e = 64 eV has been investigated in both desorption channels, with an additional cesium coating deposited on the already formed Cs/CsAu/Au/W layered system, as well as of the effect annealing produces on the yield and energy distributions of Cs atoms. It has been demonstrated that Cs atoms evaporated at 300 K on a layered coating with a cesium monolayer atop the CsAu layer on tungsten capped with a gold adlayer, rather than reflected from the cesium monolayer or adsorbing on it, penetrate through the cesium monolayer into the bulk of CsAu even with one CsAu layer present. The desorption yield does not vary with increasing cesium concentration at 300 K, but falls off gradually at 160 K. Annealing within the temperature range 320 K ≤ T _H ≤ 400 K destroys the cesium monolayer and the one-layer CsAu coating, but the multilayer CsAu compound does not break up in this temperature range even after evaporation of the cesium monolayer. It is shown that Cs atoms escape from the multilayer CsAu compound primarily out of the top CsAu layer.     

Low temperature diffusion of oxygen in titanium and titanium oxide films

Work in our laboratory and elsewhere indicates that the thermal ignition characteristics of Ti-based pyrotechnics are controlled by diffusion of oxygen from a surface coating into the bulk of the metal. Diffusion of oxygen in Ti at temperatures greater than 700 K has previously been modeled using Fick's law. No work has been reported at lower temperature, and the results from previous high temperature oxidation studies are both inconsistent and insufficient to define diffusion behavior for the temperature range 400 to 800 K. We have conducted Ti oxidation experiments in oxygen ambients of 3 to 700 Torr, at temperatures from 400 to 800 K, for periods from 1 to 100 h. Oxygen concentration profiles were determined by Auger analysis combined with sputter depth profiling. Calibration of the Auger atomic concentrations were confirmed by Rutherford backscattering spectroscopic measurements. Results show growth of four distinct oxide regions which are consistent with the titanium-oxygen phase diagram. Growth of the oxide regions is independent of the oxygen partial pressure and appears to be diffusion controlled. Fick's law with a constant diffusion coefficient was assumed, and an Arrhenius expression for the oxygen diffusion coefficient in each region was evaluated from the experimental data. Activation energies for oxygen diffusion range from about 9 to 43 kcal/mol, and indicate that the thermal ignition mechanism is controlled by growth of a TiO₂ region adjacent to the gas-solid interface.     

Kinetics of reduction of bacteriochlorophyll dimer in reaction centers of photosynthetic bacteria

Kinetic curves of reduction of the oxidized bacteriochlorophyll dimer by the proximal heme of cytochrome in seven mutated reaction centers from Rps. viridis was measured in the temperature range of 295-10 K under an ambient potential when two high-potential hemes were reduced. The data are analyzed in frames of a model which accounts for slow medium dynamics (the diffusion-reaction equation) and the quantum effect in the modes responsible for reorganization of the cofactors and the medium. The model reproduces the observed nonexponential kinetics down to the lowest temperature where the fast kinetic component still survives. The modeling results in determination of an average characteristic time for re-orientation of dipoles of the protein matrix as a function of temperature. At temperatures above the glass-transition temperature, this function shows Arrhenius behavior with the activation energy varying between the values of 0.5 eV for wild type Rps. viridis and 0 eV for Rps. sulfoviridis found previously. Possible reasons for disappearance of the fast kinetic component in some reaction centers under study at low temperatures are discussed.     

Investigation of the molecular mobility of peptin macromolecules using photon-correlation spectroscopy

The pH dependence of the intermolecular interaction coefficient of pepsin in an aqueous solution is obtained by using photon-correlation spectroscopy. The dependence shows a minimum near the isoelectric point (pH 1.6). The dynamic parameters of pepsin macromolecules in solutions containing various metal ions, including potassium, cobalt, lead, cesium, and rubidium are studied. It is shown that the addition of these ions causes a decrease of the translational diffusion coefficients. It is found that an increase in temperature leads to the structural transition of a pure solution of pepsin in water. The activation energy of this transition is calculated using the Arrhenius law.     

超低频调制光缔合光谱的实验研究

通过激光冷却技术在磁光阱中俘获原子数约107,温度约200 μK,直径约400 μm的超冷铯原子,利用超冷铯原子光缔合方法制备了激发态的超冷铯分子。实验研究了光缔合光不同扫描速率对铯分子振转光谱分辨率的影响,发现光缔合光扫描速率较慢时,铯分子振转光谱分辨率较高。通过高灵敏的雪崩光电探测器探测冷原子荧光,获得了超冷铯分子第一激发态6S_1/2+6P_3/2离解限0-_g长程态高分辨振转光谱。为了实现受控拉曼光缔合制备超冷基态分子,光缔合激光频率需要锁定在原子-分子共振跃迁线,对超冷原子光缔合光谱进行了超低频波长调制,通过改变调制幅度和调制频率获得最优化的一阶微分信号,将该信号反馈回激光器,实现闭合环路稳频,满足了受控拉曼光缔合制备振转能级可控的基态分子的实验要求,该工作对研究受限空间中的超冷原子分子具有很重要的意义。     

Hopping charge-transfer mechanism in quasi-crystals of boron and of its compounds

A large array of experimental data on the electrical conductivity of boron and high-boron com-pounds obtained within a broad temperature range is analyzed. It is shown that there is no need to describe the temperature dependence of the conductivity, as is usually done, by summing numerous exponentials corresponding to different charge-transfer mechanisms from conduction in extended states at high temperatures to hopping between localized states near the Fermi level at low temperatures. The conductivity obeys, within a broad temperature range, either a power law or the inverse Arrhenius relation. The difference is associated with specific structural features of the materials, the depth of the spatial potential relief, and with whether polarons participate in the charge transport or not. A number of compounds exhibit a crossover from the power-to inverse-Arrhenius law dependence with increasing temperature.     

Melting Behaviour of Shell-symmetric Aluminum Nanoparticles: Molecular Dynamics Simulation

用基于镶嵌原子方法势能的分子动力学模拟研究了含有561个原子的铝纳米粒子. 利用总势能和比热来计算铝纳米粒子的熔点：二十面体、十面体、切去顶端的八面体铝纳米粒子的熔点分别是540±10、500±10和520±10 K. 均方位移、键参数和回转半径的变化趋势与势能和比热的变化一致. 通过拟合均方位移得到了Kohlraush-William-Watts弛豫法则中的弛豫时间和伸缩参数,计算表明在高温区域弛豫时间和温度之间遵循标准阿伦尼乌斯关系.     

Anomalous behavior of silver atoms in intercalation under a two-dimensional graphite film

It is shown that silver atoms, the only ones of many atoms studied previously (Si, C, Mo, Pt, Cu, Ir, Ni, Au, Cs, K, Na, Ba,...), do not intercalate, that is, do not penetrate under a two-dimensional graphite film (2DGF) on a metal either upon direct depositing in the temperature range 300–2000 K or annealing of a previously deposited silver film. Intercalation becomes possible if silver is deposited on a 2DGF with previously intercalated cesium; in this case, silver atoms displace Cs atoms from under the 2DGF upon heating up to 1100 K.     

Quantum phase diffusion in a small underdamped Josephson junction

Quantum phase diffusion in a small underdamped Nb/AlO(x)/Nb junction (～0.4 μm(2)) is demonstrated in a wide temperature range of 25-140 mK where macroscopic quantum tunneling (MQT) is the dominant escape mechanism. We propose a two-step transition model to describe the switching process in which the escape rate out of the potential well and the transition rate from phase diffusion to the running state are considered. The transition rate extracted from the experimental switching current distribution follows the predicted Arrhenius law in the thermal regime but is greatly enhanced when MQT becomes dominant.     

An Experimental Study of the Fluorescence Spectrum of Cesium Atoms in the Presence of a Buffer Gas

A direct experiment is performed to determine the quantum efficiency of a cesium fluorescence filter. The fluorescence spectra of cesium atoms are recorded under excitation of the upper states of the second resonance doublet with a Bell–Bloom cesium lamp. Introduction of different noble gases into the cell with cesium leads to the appearance of additional fluorescence photons. It is found that a fluorescence filter based on atomic cesium vapor with addition of helium in the working cell has the highest efficiency and response rate of all known fluorescence filters based on alkali-metal atomic vapors.     

Low temperature surface chemistry and nanostructures

The new scientific field of low temperature surface chemistry, which combines the low temperature chemistry (cryochemistry) and surface chemistry approaches, is reviewed in this paper. One of the most exciting achievements in this field of science is the development of methods to create highly ordered hybrid nanosized structures on different organic and inorganic surfaces and to encapsulate nanosized metal particles in organic and polymer matrices. We consider physical and chemical behaviour for the systems obtained by co-condensation of the components vapours on the surfaces cooled down to 4–10 and 70–100 K. In particular the size effect of both types, the number of atoms in the reactive species structure and the thickness of growing co-condensate film, on the chemical activity of the system is analysed in detail. The effect of the internal mechanical stresses on the growing interfacial co-condensate film formation and on the generation of fast (explosive) spontaneous reactions at low temperatures is discussed. The examples of unusual chemical interactions of metal atoms, clusters and nanosized particles, obtained in co-condensate films on the cooled surfaces under different conditions, are presented. The examples of highly ordered surface and volume hybrid nanostructures formation are analysed.     

Three-dimensional cooling of cesium atoms in a reflecting copper cylinder

We present what is to our knowledge the first observation of spin-polarized atoms cooled within a reflecting cylinder in a high-power medium. A low-pressure vapor of cesium atoms is stored in a glass cell whose volume is 58 cm(3). Cooling laser light (lambda=852 nm) is injected into the cell by optical fibers and is recycled by multiple reflections from the walls of the cylinder. The technique used in this experiment greatly simplifies the generation of laser-cooled atoms. A maximum of 2.5 x 10(8) cold atoms was detected by a time-of-flight technique. The damping of atomic motion has lead to temperatures as low as 3.5muK .     

热迁移对Cu/Sn/Cu焊点液-固界面Cu6Sn5生长动力学的影响

电子封装技术中, 微互连焊点在一定温度梯度下将发生金属原子的热迁移现象, 显著影响界面金属间化合物的生长和基体金属的溶解行为. 采用Cu/Sn/Cu焊点在250℃和280℃下进行等温时效和热台回流, 对比研究了热迁移对液-固界面Cu₆Sn₅生长动力学的影响. 等温时效条件下, 界面Cu₆Sn₅生长服从抛物线规律, 由体扩散控制. 温度梯度作用下, 焊点冷、热端界面Cu₆Sn₅表现出非对称性生长, 冷端界面Cu₆Sn₅生长受到促进并服从直线规律, 由反应控制, 而热端界面Cu₆Sn₅生长受到抑制并服从抛物线规律, 由晶界扩散控制. 热端Cu 基体溶解到液态Sn中的Cu原子在温度梯度作用下不断向冷端热迁移, 为冷端界面Cu₆Sn₅的快速生长提供Cu 原子通量. 计算获得250℃和280℃下Cu原子在液态Sn中的摩尔传递热Q^*分别为14.11和14.44 kJ/mol, 热迁移驱动力F_L分别为1.62×10^-19和1.70×10^-19 N.     

Influence of magnetic field on ferrite transformation in a Fe-C-Mn alloy

The kinetics of ferrite transformation in a Fe-0.10mass%C-2.94mass%Mn alloy in a strong magnetic field of 8 T were studied with regard to alloying element-partitioned and partitionless growth. According to the theory of diffusion-controlled growth, the slow Mn diffusion dictates partitioned growth that occurs at a low undercooling, whereas partitionless growth at a larger undercooling is rate-controlled by fast carbon diffusion. The alloy was austenitized and isothermally reacted at temperatures that encompass the two growth modes. The nucleation and growth rates of ferrite increased at all temperatures in the magnetic field, whereas the amount of increase was somewhat greater at lower temperatures. In the region of slow growth, besides its sluggish diffusion Mn possibly destabilizes the ferrite phase due to the influence on the magnetic moment and the Curie temperature of bcc Fe solid solution, and partially offsets the accelerating effect of transformation. The temperature of transition from the slow to the fast growth is predicted to increase, due to the shift in the ferrite/austenite phase boundaries in the presence of magnetic field.     

Preparation and characterization of boron nitride coatings on carbon fibers from borazine by chemical vapor deposition

Boron nitride (BN) coatings were deposited on carbon fibers by chemical vapor deposition (CVD) using borazine as single source precursor. The deposited coatings were characterized by scanning electron microscopy (SEM), Auger electron spectroscopy (AES), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The effect of temperatures on growth kinetics, morphology, composition and structure of the coatings was investigated. In the low temperature range of 900 °C-1000 °C, the growth rate increased with increasing temperature complying with Arrhenius law, and an apparent active energy of 72 kJ/mol was calculated. The coating surface was smooth and compact, and the coatings uniformly deposited on individual fibers of carbon fiber bundles. The growth was controlled by surface reaction. At 1000 °C, the deposition rate reached a maximum (2.5 μm/h). At the same time, the limiting step of the growth translated to be mass-transportation. Above 1100 °C, the growth rate decreased drastically due to the occurrence of gas-phase nucleation. Moreover, the coating surface became loose and rough. Composition and structure examinations revealed that stoichiometric BN coatings with turbostratic structure were obtained below 1000 °C, while hexagonal BN coatings were deposited above 1100 °C. A penetration of carbon element from the fibers to the coatings was observed.     

Influence of cesium atoms on the field electron emission from multi-walled carbon nanotubes

It has been shown that the deposition of cesium atoms on multi-wall carbon nanotubes abruptly increases the current of the field electron emission, decreases the threshold electric field by a factor of three (to 0.8 V/m), and decreases the work function to 2.1–2.3 eV. It has been found that the flowing of the large emission current I ≥ 2 × 10^?6 A leads to a change in the current-voltage characteristics and a decrease in the emission current. This effect has been explained by escape of cesium atoms from the tips of most nanotubes into the nanotube depth due to desorption or intercalation. At the same time, the low work function is retained for some nanotubes, probably, due to the stronger bonding of Cs atoms with these nanotubes.     

Desorption of Rb and Cs from PDMS induced by non resonant light scattering

Simultaneous light-induced desorption of rubidium and cesium atoms has been observed in polydimethylsiloxane (PDMS) coated Pyrex cells at room temperature and at low light intensity. The two alkali atoms show the same dynamics and the same dependence on the desorbing light frequency. No competition in the free sites occupancy exists. An interpretation of the experimental results in terms of non-resonant light scattering from the PDMS coating is discussed.     

Effect of the cesium and potassium doping of multiwalled carbon nanotubes grown in an electrical arc on their emission characteristics

The effect of cesium and potassium atoms deposited onto multiwalled carbon nanotubes grown in an electrical arc on their emission characteristics was studied. The current–voltage characteristics of the field electron emission of specimens with cesium or potassium doped multiwalled carbon nanotubes of this type were revealed to retain their linear character in the Fowler–Nordheim coordinates within several orders of magnitude of change in the emission current. The deposition of cesium and potassium atoms was shown to lead to a considerable increase in the emission current and a decrease in the work function φ of studied emitters with multiwalled nanotubes. The work function was established to decrease to φ ~ 3.1 eV at an optimal thickness of coating with cesium atoms and to φ ~ 2.9 eV in the case of doping with potassium atoms. Cesium and potassium deposition conditions optimal for the attainment of a maximum emission current were found.