Energy levels of the electrons localized over the surface of an inert film with address electrodes

The problem of searching for the potential energy and the energy spectrum of the electrons localized over the surface of a thin liquid or solid inert film due to address electrodes placed under the film is considered.     

Effect of electron cooling on the electron lifetime on liquid 4He

Electron trapped in image potential induced surface states on liquid ⁴He can escape into the gas by thermal ionization. We have calculated the rate of ionization on the assumption that the electrons are strongly coupled to each other and weakly coupled to the liquid and gas. The first electrons to leave the surface extract their ionization energy from the kinetic energy of the remaining electrons. This adiabatic cooling effect rapidly lowers the electron temperature to a small fraction of the temperature of the surrounding gas and liquid.     

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.     

Interaction of negative ions with the surface of inert liquids

The interaction potential of negative ions (electron bubbles) with the surface of liquid ⁴He, ³He, and Ne has been found. In addition to the electrostatic repulsion, the contribution of the long-range Van der Waals attraction of the electron bubble to the liquid surface has been also taken into account. Competition of these repulsion and attraction forces results in the formation of a potential barrier that prevents the motion of a negative ion from the liquid to the vacuum. The temperature and electric-field dependences of the lifetime of the bubble have been determined. The theory has been compared with the experiments with negative ions in liquid ⁴He. In contrast to the conventional idea based on the hypothesis of the quantum tunneling of an electron from a bubble to a vacuum, our theory is based on the Kramers’ diffusion model of the classical escape of the bubble over the potential barrier. In this model, a low-dynamic-friction approximation is applicable to liquid ⁴He owing to a high mobility of negative ions in the superfluid.     

The influence of ions and radiation on tunneling of electrons from the surface of liquid helium

In work previously reported by Saville et al. [1] and Saville [2], we found that electrons tunneling from their shallow potential well above the surface of liquid helium behave as an almost ideal 1D hydrogen atom. Computations [3], without approximation or adjustable parameters, using the time dependent Schrödinger equation are in excellent agreement with measured tunneling rates. The same computation method was used to compute the influence of AC fields on the tunneling rates [3] as preparation for planned experiments. In this paper I discuss phenomena which appear to set upper and lower limits on the observable tunneling rates. Lower limits were determined by photo assisted escape due to thermal radiation and by cosmic ray generation of ions in the bulk liquid helium. Upper limits were set by the creation of excitations in the superfluid film on the top of the sample chamber when electrons impacted with energy greater than 21.6 eV. Through its influence on the tunneling rate, we were also able to measure very small changes in the electron potential at the surface due to a sub-monolayer coverage of³He.     

Structure of the dimple lattice on liquid 4He

Electrons on the surface of liquid ⁴He become localized in macroscopic dimples when the electric field perpendicular to the surface exceeds a critical value. These dimples, each holding about 10⁷ electrons, form a two-dimensional hexagonal lattice.     

Convoy electrons produced at glancing angle scattering of MeV HeH+ ions from a crystal surface

Abstract

Convoy electrons produced at glancing angle scattering of MeV HeH⁺ ions from an atomically clean (001) surface of SnTe crystal are observed. Energy spectrum of the convoy electrons shows a peak broader than that at scattering of atomic projectiles and the most probable energy of convoy electrons at HeH⁺ scattering is larger than those at scattering of isotachic He ions. This acceleration of convoy electrons is qualitatively explained by the force due to surface wake induced by Coulomb exploding fragment He²⁺ and H⁺.     

Magnetism induced by capping of non-magnetic ZnO nanoparticles

Magnetism of 10 nm size capped nanoparticles, NPs, of non-magnetic ZnO is analysed in terms of the surface band since, as magnetic dichroism analysis has pointed out, impurity atoms bonded to the surface act as donor or acceptor of electrons that occupy the surface states. Due to the nanometric scale of the particles the kinetic energy spectrum of the surface states can be considered as discrete. Therefore, the magnetic polarisation cannot be easily induced by pumping electrons to energy levels above the Fermi energy. It is in the Fermi level itself, generally unfilled, that develops a spontaneous magnetic moment similarly to that induced by Hund rules in unfilled atomic orbitals. It is shown, however, that the total magnetic moment of the surface originated at the unfilled Fermi level can reach values as large as 10² or 10³ Bohr magnetons.     

Bose-Einstein condensation and thermodynamic functions of 4He film

A monolayer of ⁴He atoms is treated as a system of hard-sphere bosons in a thin film geometry, with a finite thickness. The method of pseudopotential is used to calculate first the energy spectrum, and then the Helmholtz free energy and other thermodynamic functions of the system. It is found that Bose-Einstein condensation exists below a definite temperature. Much like a liquid-gas transition, the boson system displays a high temperature normal phase, a low temperature condensed superfluid phase and coexistence region. In the present treatment, the minimum momentum associated with the finite thickness of monolayer is used as a parameter. We find that the transition temperature is linearly proportional to the density of the ⁴He film. After performing double-tangent construction of the Helmholtz free energy curve we find for the specific heat a rounded peak at the transition temperature, in agreement with recent experiments. The ratio of the superfluid density at the transition point to the transition temperature is found to be essentially a constant.     

Surface modification of polytetrafluoroethylene film using single liquid electrode atmospheric- pressure glow discharge

Polytetrafluoroethylene films are treated by room temperature helium atmospheric pressure plasma plumes, which are generated with a home-made single liquid electrode plasma device. After plasma treatment, the water contact angle of polytetrafluoroethylene film drops from 114°to 46°and the surface free energy increases from 22.0 mJ/m2 to 59.1 mJ/m2. The optical emission spectrum indicates that there are reactive species such as O2+ , O and He in the plasma plume. After plasma treatment, a highly crosslinking structure is formed on the film surface and the oxygen element is incorporated into the film surface in the forms of -C-O-C-, -C=O, and -O-C=O groups. Over a period of 10 days, the contact angle of the treated film is recovered by only about 10 , which indicates that the plasma surface modification is stable with time.     

Energy distribution of plasma-assisted electron and ion emission from TGS single crystals

Electron and ion emission accompanying non-thermal plasma processes, produced at the surface of TGS single crystals under driving ac electric field exceeding 10³ V/cm, have been carried out. These plasma-assisted emission of electrons and ions were examined by means of time and energy distribution measurements. The intensity of registered charges (electrons and ions) displayed on the 2 ms time scale are represented by two distinct peaks. Time dependent energy spectrum of charges, detected under our experimental conditions, involves electrons and ions with maximum energy up to 30-40 eV for first peaks and up to 70-80 eV for second one. Additionally, the energy of electrons is focused at about 10-15 eV for first and second peaks and about 60-70 eV for second ones; the ion energy spectrum for both peaks exhibits only distinct low energy maximum focused at about 5-15 eV.     

Comparison of the efficiency of 63Ni beta-radiation detectors made from silicon and wide-gap semiconductors

The current induced by the radiation from a ⁶³Ni film of variable thickness is simulated taking into account the real spectrum of emitted electrons and their angular distribution for GaN. The efficiency of β-radiation detectors made from Si and SiC is estimated based on the results obtained in this paper and previously. Using a scanning electron microscope the efficiency of β-radiation detectors made from Si and SiC under conditions corresponding to β radiation from a Ni film with a thickness of 3 μm and activity of 10 mCi/cm² is analyzed. It is shown that the efficiency of real Si-based structures is virtually as good as the efficiency of SiC-based structures.     

Hydrogen and helium films as model systems of wetting

Optical experiments on the wetting properties of liquid ⁴He and molecular hydrogen are reviewed. Hydrogen films on noble metal surfaces serve as model systems for studying triple point wetting, a continuous transition between wetting and non-wetting. By means of optically excited surface plasmons, the adsorbed film thickness for temperatures around, and far below, the bulk melting temperature is measured, and the physical mechanisms responsible for the transition are elucidated. Possible applications for other experiments in pure and applied research are discussed. Thin films and droplets of liquid helium are studied on cesium surfaces, on which there is a first order wetting transition. Our studies concentrate on dynamical observations via surface plasmon microscopy, which provide insight into the morphology of liquid helium droplets spreading at different temperatures. Features corresponding to pinning forces, the prewetting line, and the Kosterlitz-Thouless transition are clearly observed.     

Electrostatic Nature of Size Dependences of Adsorption Properties of Ytterbium Nanofilms Grown on the Surface of Silicon: the CO–Yb–Si(111) System

The adsorption of CO molecules onto ytterbium nanofilms with their thickness varying from 1 to 16 monolayers is studied. The dependences of the number of adsorbed CO molecules (adsorption isotherms) and the work function of ytterbium films on the dose of carbon monoxide are examined. It is demonstrated that both the number of adsorbed molecules and the work function depend (under equal conditions) on the nanofilm thickness; in other words, a size effect is revealed. It is found that this size effect is induced by the electrostatic interaction between the conduction electrons of ytterbium and the electrons localized on the nanofilm surface, which establish bonding between the surface and CO molecules. This interaction depends on the film thickness and limits the number of CO molecules that may be adsorbed onto the surface of a film with a given thickness.

     

磁场中液氦薄膜表面电子涟波子系统的性质

研究了磁场中液氦薄膜表面电子与涟波子强耦合和弱耦合的性质。采用线性组合算符方法导出磁场中液氦薄膜表面电子 涟波子系统的振动频率和基态能量。讨论磁场对表面电子 涟波子系统的振动频率和基态能量的影响。     

Simulation of the current induced by 63Ni beta radiation

The current generated by radiation from a ⁶³Ni layer of variable thickness is simulated with the actual spectrum of emitted electrons and with their distribution over the angles for Si and SiC taken into account. The dependences of the generation rate for nonequilibrium charge carriers on the depth are obtained for the cases of several Ni film thicknesses for both materials. The results are compared with the simulation results for a monoenergetic electron beam that is perpendicular to a semiconductor detector. It is shown that, for both Si and SiC, it is possible to choose an energy value of the electron beam in a SEM such that the ratio of the currents induced by the SEM beam and beta radiation from ⁶³Ni is essentially independent of the diffusion length.     

Nanofabrication using liquid helium films

We propose here a technique for precision deposition of single atoms or ordered arrays of atoms onto a substrate by first suspending the atoms as ions within a superfluid ⁴He film adhering to the substrate via van der Waals forces. The ions are held within the film by a combination of image forces and an externally applied electric field. Turning off the electric field causes the ions to fall toward the substrate and bond to the substrate surface. If the applied electric field comes from a scanned probe above the film, individual atoms can be deposited with atomic precision. A uniform applied electric field can be used to deposit a crystal of ions onto the substrate.     

Dimple‐assisted dewetting: heterogeneous nucleation in undercooled wetting films

Undercooled wetting films near a first‐order wetting transition exhibit an unusually long lifetime: the thermal nucleation barrier for formation of a critical hole in a film of thickness F diverges according to Γ ∼ exp (ℰ_c/k_BT) where the excess free energy ℰ_c ∼ F^ζ with ζ ≥ 2. Localized perturbations of the liquid‐vapor interface (‘dimples’) are shown to be a useful tool in reducing Γ in a controlled way: they act as heterogeneous nucleation centers for thermal critical nuclei. For ⁴He wetting films on weak‐binding alkali substrates (Cs, Rb) dimples can be generated either by vortices in a superfluid film or by surface electrons. The theory of the heterogeneous nucleation process initiated by the presence of surface dimples (‘dimple‐assisted dewetting’) is developed, accompanied by quantitative predictions for experiment.     

Electron spectroscopy of liquid mercury under impact of metastable He(23 S) atoms

We have performed electron spectroscopy of a liquid mercury surface under impact of HeI photons and of metastable He(2³ S) atoms. The latter data are interpreted in terms of Auger neutralization. This process involves two electrons from the metal surface and thereby yields information about the mutual interaction between two metal electrons. Evaluation of our data shows that the interaction is specific to the band from which the electrons come and whether they originate from the same or from different bands. Further, we find no support for the suggestion that a few per cent of the surface sites of liquid surface are occupied by nonmetallic mercury atoms.     

Supercooling molecular hydrogen down through the superfluid transition

Recent calculations by Vorobev and Malyshenko [JETP Lett. 71, 39 (2000)] show that molecular hydrogen may stay liquid and superfluid in strong electric fields of the order of 4x10(7) V/cm. I demonstrate that strong local electric fields of similar magnitude exist beneath a two-dimensional layer of electrons localized in the image potential above the surface of solid hydrogen. Even stronger local fields exist around charged particles (ions or electrons) if the surface or bulk of a solid hydrogen crystal is statically charged. Measurements of the frequency shift of the 1 --> 2 photoresonance transition in the spectrum of a two-dimensional layer of electrons above a positively or negatively charged solid hydrogen surface performed in the temperature range 7-13.8 K support the prediction of electric field induced surface melting. The range of surface charge density necessary to stabilize the liquid phase of molecular hydrogen at the temperature of superfluid transition is estimated.