Spectroscopic studies on Yb+ ions in liquid helium

An emission spectrum of the - (D1) transition and excitation spectra of the - (J=1/2 (D1) and 3/2 (D2)) transitions of Yb⁺ ions in liquid helium have been observed by means of a laser-induced fluorescence method. It has been found that all these spectra are blue-shifted from the ones in the free space, and also that the D2 excitation spectrum has double peaks. To understand these spectra, theoretical calculations have also been carried out based on a vibrating bubble model, in which the bubble surface is assumed to vibrate in the spherical (breathing), dipolar and quadrupolar modes. As a result, it has been found that the blue shifts are well understood with this bubble model, and also that the dynamic Jahn-Teller effect due to the quadrupole vibration of the bubble plays an important role for the double-peaked profile of the D2 excitation spectrum. Received 27 December 1999 and Received in final form 31 July 2000     

Bound states of an electron in an impurity potential on the surface of liquid helium

The energies and widths of the levels of an electron on impurity centers on the surface of liquid helium are calculated with allowance for the deformation of the surface. The level shift associated with the deformation effects is small and decreases very slowly with increasing level number. However, even a small shift of the energy levels relative to one another affects ripplon scattering, which makes the main contribution to the level width at low temperatures. It is predicted theoretically that this width depends very strongly on the external parameters and on the level number and that a maximum obtains at a clamping field E _⊥=51500 V/cm. The width of the levels of an electron in a bound state is found to be less than for free electrons. This makes it possible to perform a beautiful spectroscopic experiment. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 9, 599–604 (10 November 1997)     

Bound states of an electron and a macroscopic cluster at a liquid helium surface

In a strong electric field, there are bound states of an electron at the surface of liquid helium, interacting with a large cluster of atoms in the bulk of liquid. This phenomenon is related to long-range interaction between the electron and the dipole moment of the cluster. The electron, holding the cluster under the liquid surface, is localized at this surface. One electron is capable of binding a cluster of up to 10⁶ atoms. The value of the binding energy may reach up to several kelvins.     

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.     

Theory of electron escape from the surface of liquid helium

We have studied the properties of an electron bubble close to the surface of liquid³ He, by using a Density Functional approach. We find that up to an electron-surface distanced ₀ 23 Åthe bubble is stable, while at smaller distances it becomes unstable and bursts. A potential energy barrier /K _B 38°K for the thermal emission of electrons is obtained from our results, in agreement with experiments. Even when the electron-surface distance is larger thand ₀, however, tunneling through the surface layer dominates the electron escape probability. Large deviations of the electron potential energy from its ideal value are found close to the surface. These deviations have a profound effect on the calculated decay rates of the tunneling curent, which are much smaller than those obtained previously and in semi-quantitative agreement with experiments.     

Photogalvanic current in electron gas over a liquid helium surface

We study the stationary surface photocurrent in 2D electron gas near the helium surface. Electron gas is assumed to be attracted to the helium surface due to the image attracting force and an external stationary electric field. The alternating electric field has both vertical and in-plane components. The photogalvanic effect originates from the periodic transitions of electrons between quantum subbands in the vertical direction caused by a normal component of the alternating electric field accompanied by synchronous in-plane acceleration/deceleration due to the electric field in-plane component. The effect needs vertical asymmetry of the system. The problem is considered taking into account a friction caused by the electron-ripplon interaction. The photocurrent resonantly depends on the field frequency. The resonance occurs at field frequencies close to the distance between well subbands. The resonance is symmetric or antisymmetric depending on the kind (linear or circular) of polarization.     

Plasma modes in low-dimensional electron system over surface of liquid helium

The collective plasma modes in a quasi-two-dimensional (Q2D) electron system located over the free surface of liquid helium are studied theoretically within many-body dielectric formalism. The dispersion of modes is considered both over bulk liquid and over helium film where the essential modification of interelectron interaction occurs due to screening effects in the substrate with a large value of dielectric constant. It is shown that the plasma spectrum consists of longitudinal and transverse branches which dispersion laws depend on the values of the dielectric constant of helium and the film thickness. For the helium film covering metal, the longitudinal mode is acoustic differing of that for the surface electron (SE) system over bulk helium.     

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.     

Properties of surface state electrons on liquid helium

Electrons on the surface of liquid helium form a two-dimensional system that is of great interest for its own unique properties as well as being a probe of the helium surface. The spectroscopic evidence for the hydrogenic nature of the surface state is compared with predictions. Measurements of the electron mobility parallel to the surface in low and high electric fields are compared with theory. The lifetime in the surface state is discussed as well as the effect of the electron on the liquid surface. The possibility that the electrons crystallize to form a two-dimensional lattice and the properties of this lattice are also analyzed.     

Spectroscopic studies of a helium plasma generated near ceramic surface at pressures below 1000 hPa

Spatially resolved line intensity measurements from a plasma generated near ceramic surfaces have been performed. Disk-shaped helium plasmas of diameter 20 mm and thickness 0.9 mm have been studied in a pressure range of 2×10³–10⁵ Pa. On the basis of line intensity measurements and applying an appropriate collisional-radiative model for a helium plasma, the distributions of electron density and electron temperature have been evaluated.     

Self-consistent Debye-Waller factors of the electron solid on liquid helium

Based on the self-consistent field formalism we propose a shear-mode self-consistency for the high-frequency Debye-Waller factors (HFDWF) of the electron solid bound on a free surface of liquid helium. Our results are qualitatively in agreement with the empirical DW factor determined by Fisher, Halperin, and Platzman with the experiment of Grimes and Adams. We also report the analysis of the HFDWF according to the Lindemann law.     

Escape rate of two-dimensional electrons on liquid helium surface

Escape rate of two-dimensional electrons on liquid helium surface is measured. A strong dependence of the escape rate on the surface electron density suggests that the surface electron system is in a “liquid” state.     

A possible new electron state above liquid helium

Excited states of helium atoms have been observed in the bulk liquid. We discuss the properties of such states if created at the surface. For low quantum numbers the surface is a weak perturbation compared to the atomic excitation energy. At high quantum numbers (weakly bound states) the electron wave function is forced out of the interior of the fluid and the energy spectrum is Rydberg-like.     

Statistical description of the system electrons on the liquid helium surface

System of electrons on the liquid helium surface is considered. General methods for obtaining free energy functional for the systems in mean field approximation are developed. These methods applied for treating systems with particles arranged in a lattice. Thus obtained functional of free energy is analyzed. The localization distance for electron and conditions for existing square or triangular lattices as well as phase transition between them are obtained.     

Electron impact studies of autoionizing states in neon and helium

A study has been made of autoionizing states in neon in the energy region 42–48 eV by low energy electron impact. Observations have been made of stru     

Negative ions in liquid helium: Existence of new bound states

It is shown that fundamentally new bound states can be formed when a foreign negative ion is embedded in liquid helium. For such states the excess electron forms a bubble with a radius R ₀≃17–18 Å, and a foreign neutral atom is trapped inside this bubble because of the polarization interaction with the electric field of the excess electron, which has a maximum at a point r≃R ₀/2. The main properties of such structures are considered. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 454–458 (25 March 1997) Published in English in the original Russian journal. Edited by James Anderson and Steve Torstveit.