Stimulation von ZnS-Phosphoren mit langwelligem Infrarot

Stimulation experiments with ZnS phosphors, using IR wavelengths from 2 to 15μ, were performed at the temperature of liquid helium. For this purpose a cryostat was constructed which allowed to keep the samples and the screening device at the temperature of liquid helium. Moreover glow curves after different decay times at 6 °K were taken. After excitation of ZnS phosphors a strong release of carriers from relatively deep traps is taking place although the phosphor is kept at the temperature of liquid helium. This emptying of traps is accompanied by a luminous recombination of the released electrons with the activator levels producing an intense afterglow which can be observed over a long time. This phenomenon cannot be explained by thermal release of trapped electrons into the conduction band, followed by recombination with the activator levels because of the depth of the emptied traps. The rate of thermal carrier release was calculated to be about 10^?29 sec^?1 for 0,05 eV deep traps, but the observed rate was of the order of 10^?4 sec^?1. An emptying of traps by IR-stimulation can be excluded because the phosphor was surrounded by a concentric screening device kept at 4.2 °K. The effect can be explained by luminous tunneling of trapped electrons to the activator levels. An estimate of the tunneling rate gives a value agreeing with the experimental results. If such a “tunnel afterglow” does exist a spectral shift to longer wavelengths compared to the usual fluorescence and phosphorescence is to be expected. This could be actually observed. Additional experiments included IR-stimulated emission after various decay times, glow curves taken after such stimulation, and studies of the influence of temperature. The results rule out the possibility that the carriers were released from the traps by IR light. Apparently, IR radiation affects the potential barrier between the traps and the activators so that the rate of recombination by tunneling increases.     

How the thickness of a liquid helium film influences the spectrum of electrons localized above its surface

We consider the problem of finding the energy spectrum of electrons localized above the surface of a liquid helium film under the electrostatic attraction of induced charges. We show that the forces of attraction to the charges induced on the surface of a conductor located beneath the film begin to additionally influence the forces of attraction of the electrons to the free liquid surface as the film thickness decreases. When the film thickness becomes less than 10μm, this influence causes a significant increase in the energy difference between the lower levels and a decrease in their depth. We suggest a numerical method for solving the problem and present the results of our calculations of the energy spectrum of electrons localized above the surfaces of liquid ³He, ⁴He, and neon films. The influence of the pressing electric field on the energy spectrum is considered.     

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)     

Electric-field dependence of muonium formation in liquid helium

The influence of electric fields on the formation of muonium in liquid helium (⁴He,³He, and mixture of⁴He + 0.2%³He) has been studied. It was found that the relative distribution of muon-electron pairs is anisotropic. The maximum muon density is shifted with respect to the electrons in the direction of the initial muon momentum. Due to the anisotropy the muonium asymmetry in normal liquid helium is enhanced by a factor of 3 in an electric fieldE=1 kV/cm.     

核力和重子相互作用

本文评述了核力和重子相互作用研究的历程,取得的成就和存在的问题。     

One-dimensional hydrogenic atom in an electric field with solid-state applications

A Hamiltonian describing a one-dimensional Coulomb field with an electric field in the same direction is useful for the discussion of electrons outside a free surface of liquid helium [1,2] and also for the study of far-infrared emission from Si inversion layers [3]. We present both a semi-classical and a WKB solution to the problem, which exhibits many of the features found experimentally.     

Influence of the atomic polarization potential on the tunneling rates of electrons between atoms and surfaces

The tunneling rate of electrons between a Li⁻ ion and a surface is studied using the coupled angular mode method and the complex scaling method. The comparison of the two sets of results obtained with various representations of the e⁻Li interaction and their analysis shows that the e⁻Li polarization potential plays an important role in determining the tunneling rates. This effect is attributed to a lowering of the potential barrier between the atom and the surface.     

Electron bound states on liquid helium

Using an exactly soluble model of image potential which has the form -Ze²/ (z+β), we have calculated the energy levels of electrons trapped on the surface of liquid helium as a function of the parameter β. Our results show that the experimental values of Grimes et al can be fit very well by locating the effective liquid helium-vacuum interface at β = 1.01 Å.     

Low-temperature conductance of the weak junction in InAs nanowire in the field of AFM scanning gate

We have investigated the conductance of an InAs nanowire in the presence of an electrical potential created by an AFM scanning gate at liquid helium temperature. The influence of the direction of a local electrical field on the tunneling rate through a weak junction in the InAs wire is clearly observed. To explain this behavior, the redistribution of the electrons among conductive channels in the wire must be taken into account. We have confirmed that the pattern of Coulomb blockade diamonds gives the same result for the ratio of quantum dot sizes as that revealed by scanning gate imaging.     

STM studies on quantum wire structures in air and liquid helium

In this work, low temperature scanning tunneling microscopy (STM) studies on quantum wires are reported, which were fabricated by laser holography and wet chemical etching. Inverted heterostructures with thin and highly doped cap layers were used as substrates in order to keep the total tunneling barrier as small as possible. Current—voltage curves were measured on the wires and in the depleted areas between them. Between the wires, significant current is only observed for electrons which tunnel from the GaAs valence band into the STM tip, whereas symmetric curren voltage curves are observed on the wires. This behavior is ascribed to the influence of surface depletion and thus, a comparison of current imaging spectroscopy data taken at 300 K and in liquid helium directly yields the edge depletion width of the quantum wires.     

Magnetization of an electron plasma by microwave pulses: Faraday induction

It has been argued theoretically that the recently proposed vacuum fieldB ⁽³⁾ is not accompanied by a real electric fieldE ⁽³⁾ . Experimental evidence for this interence is available in the data reported by Deschampset al. [10], using microwave magnetization of an electron plasma set up in helium gas. Faraday induction due toB ⁽³⁾ does not occur in the inert gas and is not observed experimentally in the absence of free electrons. WheneverB ⁽³⁾ interacts with free electrons, however, Faraday induction occurs through a pulse of induced magnetization (i.e., induced orbital electronic angular momentum).     

Novel room temperature ionic liquid for fluorescence enhancement of Eu and Tb

The newly prepared ionic liquid, 1-butyl-3-methylimidazolium benzoate, ([bmim][BA]), was found to enhance the fluorescence of Eu³⁺ and Tb³⁺. The fluorescence enhancement resulted from a sensitization of the lanthanide fluorescence by the benzoate anion of the ionic liquid, [bmim][BA], and a reduction in the non-radiative channels in the non-aqueous environment provided by the ionic liquid. However, the fluorescence enhancement of the lanthanides in the ionic liquid was limited due to the operation of the inner filter effect, which resulted from the strong absorption of the benzoate. The inner filter effect was minimized by observing the Eu³⁺ fluorescence using a front face geometry and also by diluting the lanthanide-[bmim][BA] system, using another ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf₂N]), as a solvent. In the case of Tb³⁺, the emission from the lanthanide was masked by the strong emission from the ionic liquid in the region 450-580 nm. The long lived Tb³⁺ emission was therefore observed using delayed gated detection, where an appropriate delay was used to discriminate against the short lived emission from the ionic liquid. The large fluorescence enhancement due to ligand sensitized fluorescence observed with [bmim][BA] diluted in [bmim][Tf₂N], leads to nanomolar detection of the lanthanides. This is, to the best of our knowledge, the first report of an ionic liquid being employed for ligand sensitized fluorescence enhancement of lanthanides.     

Crystal-Size Dependence of Replenishment Effect in BaFBr:Eu

Excitation spectra of photo-stimulated luminescence (PSL) in BaFBr:Eu and BaFBr:Eu ? Na crystals were studied. Replenishment of PSL at liquid helium temperature was investigated in crystals of various sizes including a commercially available X-ray storage phosphor plate (Image Plate). Although a weak replenishment effect was observed in the fine powder crystals, no replenishment effects were observed in Image Plate, while it showed strong PSL at liquid helium temperature. Results indicate that there are two types of PSL centers, one consists of spatially correlated electrons and holes and the other consists of electrons and holes trapped at isolated centers. Lack of the replenishment effect in Image Plate indicates that trapped electrons are stabilized by the surface and need thermal activation to recombine with holes which are possibly trapped at doped Eu^2?.     

Surface sensitivity of low energy electron Mössbauer spectroscopy (LEEMS) using57Fe

Very low energy electrons (LEE) (E _e ≤15 eV) are produced with high intensity directly by Mössbauerabsorption and conversion in the case of⁵⁷Fe [1, 4, 5]. These electrons should be very surface sensitive due to their very low attenuation length compared to the 7.3 keV K-Conversion electrons of⁵⁷Fe [5, 11]. We have examined the surface sensitivity of these resonant LEE, using nonresonant⁵⁶Fe metal and⁵⁶Fe stainless steel foils coated with about 20 Å and 50 Å⁵⁷Fe, respectively. They were exposed to air after evaporation: The 20 Å samples are found to be fully oxidized [5]. Depth Selective Conversion Electron Mössbauer Spectroscopy (DCEMS), performed with a high transmission orange type magnetic spectrometer [5, 6, 13] reveals a two layer structure of the 50 Å samples. Low Energy Electron Mössbauer Spectroscopy (LEEMS) [5] is found to be significantly more surface sensitive than conventional DCEMS, but not as surface sensitive as Auger Electron Mössbauer Spectroscopy (AEMS) using LMM-Auger electrons of 500–600 eV, as expected due to the different mean free path. But because of the very low intensity of these Auger electrons this mode appears to be not very useful for practical application.     

The energy distribution of secondary ion-electron emission

This study deals with the energy distribution of secondary electrons emitted when 3 keV He⁺ and Ne⁺ ions bombard the (100) surface of a copper single crystal and the (0001) surface of a zinc single crystal. A “long magnetic lens” was employed in the energy analysis. A group of high-energy electrons (having an energy of approximately 35 eV) is found to appear in the case of bombardment with helium ions. The results are discussed on the basis of the model of autoionization of the incoming ions (helium and neon) embedded in layers at near the surface. The effect of temperature and time on the energy distribution of secondary electrons is also examined.     

Energetics of Si(001) surfaces exposed to electric fields and charge injection

We perform density-functional calculations on the influence of external electric fields and electrons or holes injected into surface states on the relative stability of c(4x2) and p(2x2) reconstructed Si(001) surfaces. It is shown that an electric field parallel to the [001] direction or the insertion of electrons into surface states favors the formation of p(2x2) periodicities. Our results explain recent experimental studies reporting changes of surface reconstruction of Si and Ge(001) surfaces induced by the scanning tunneling microscope and the occurrence of p(2x2) reconstructions on (001) surfaces of n-doped Si.     

Image forces and electron spectrum at the surface of liquid helium

On the basis of the longitudinal field Green function, in a three-layer non-symmetrical system of media with spatial dispersion the image-force potential is calculated for surfaces of the semi-infinite helium bath and a liquid helium film, and also for a surface of the phase-separated ³He⁴He mixture. The dependence of the surface electron spectrum on the film thickness and on the pressing electric field strength is analyzed.     

Probing the adsorption of methylimidazole at ionic liquids/Cu electrode interface by surface‐enhanced Raman scattering spectroscopy

Two kinds of room‐temperature ionic liquids, 1‐butyl‐3‐methylimidazolium bromide ([BMIM]Br) and 1‐butyl‐3‐methylimidazolium tetrafluoroboride ([BMIM]BF₄), were used as solvent, and the adsorption of the ionic liquids themselves and of N‐methylimidazole (NMIM) were investigated by electrochemical surface‐enhanced Raman scattering (SERS) over a wide potential window. The results revealed that the cation of ionic liquid adsorbed onto Cu surface with different configurations in different potential ranges. When the potential was changed from the negative to the positive range, the orientation underwent a change from flat to vertical, and the onset potential for the orientation change was dependent on the types of anion of the ionic liquid. The ionic liquid in bulk solution exhibited a remarkable effect on the adsorption of NMIM. The electrode surface structure changed from adsorbing the ionic liquid at the negative potential to coadsorbing the ionic liquid and NMIM at relative positive potential for the [BMIM]BF₄ liquids, and formed films of NMIM at extremely positive potential. Due to the strong specific adsorption of Br⁻, the coadsorption of ionic liquid and NMIM was not observed in the system [BMIM]Br. By simulating the electrode surroundings, two surface complexes [Cu(NMIM)₄Br]Br·H₂O and [Cu(NMIM)₄](BF₄)₂ were synthesized by the electrochemical method in the corresponding ionic liquids for modeling the surface coordination chemistry of NMIM. The surface coordination configuration of NMIM and ionic liquids is proposed. Copyright © 2009 John Wiley & Sons, Ltd.     

Coupling Distant Spins of Surface-State Electrons by Manipulating Their Collective Vibrations

The system of electrons on liquid helium is an interesting candidate to implement quantum computation, due to the long coherence times of the qubits encoded by the electronic spins. In order to implement the quantum logic operations between the spins, we propose here a configuration, similarly to the cooled ions in a trap, to couple the distant electrons via manipulating their center of mass （CM） vibrations. First, we show that the electrons could be confined in a common harmonic oscillator potential by using an electrostatic field. Then, with a single current pulse （applied on the micro-electrode below the liquid helium） the distant electronic spins can be coupled simultaneously to the CM mode. Finally, by adiabatically eliminating the CM mode, effective interaction between the distant spins is induced for implementing the desired quantum computing.