Free electron gas spectrum parameters of noble gases in dc electric field

Free electron gas is present in every gas, whether it is of atomic or molecular structure. Since the Maxwell spectrum type is the consequence of only thermal motion of constitutive gas particles; therefore, the presence of electric field leads to change the spectrum of charged particles due to their directed motion. However, it has been shown that in the case of occurrence only of elastic interactions between electrons and neutral gas particles (a condition that has been met in the case of weakly ionized noble gases of a relatively huge volume) the deviation of the gas spectrum of free electrons in the electric field from the Maxwell type is negligible. In such a case, the gas spectrum of free electrons is either of Maxwell type (if the frequency collision value is energy-independent) or of Druyvesteyn type (if the mean free electron path value is energy-independent). The Maxwell and Druyvesteyn distribution types are very similar. The only noticeable difference is that the tail of the Maxwell distribution decreases with the energy exponent to the first degree of energy, and the tail of Druyvesteyn distribution with the energy exponent to the second degree of energy. The aim of this paper is to determine whether the gas spectrum of free electrons in weakly ionized noble gases at small values of the product pd (pressure and inter-electrode distance) follows either the Maxwell's or Druyvesteyn's type, as well as to determine the dependence of spectrum parameters on the product pd. It has been established that better results are obtained on the assumption that the mean value of collision frequency is energy-independent.     

Inelastic electron collisions with Rydberg atoms

The standard classical method of computer simulation is used for evaluation of the inelastic cross section in electron collisions with a highly excited (Rydberg) atom. In the course of collision, the incident and bound electrons move along classical trajectories in the Coulomb field of the nucleus, and the scattering parameters are averaged over many initial conditions. The reduced ionization cross section of a Rydberg atom by electron impact approximately corresponds to that of atoms in the ground states with valence s-electrons and coincides with the results of the previous Monte Carlo calculations. The cross section of an atom transition between Rydberg atom states as a result of electron impact is used for finding the stepwise ionization rate constant of atoms in collisions with electrons or the rate constant of three-body electron-ion recombination in a dense ionized gas because these processes are determined by kinetics of highly excited atom states. Surprisingly, the low-temperature limit of electron temperatures is realized when the electron thermal energy is lower than the atom ionization potential by about three orders of magnitude, as follows from the kinetics of excited atom states. The article is published in the original.     

Qualitative difference between the angular anisotropy parameters in fast electron scattering and photoionization

It is demonstrated for the first time that in spite of well known big similarities between atomic ionization by photons and fast electrons, a qualitative difference exists in angular anisotropy parameters of electrons knocked out in these processes. The difference is disclosed here and attributed to distinction between normal (transverse) and virtual (longitudinal) photons. Formulas are derived for dipole and non-dipole angular anisotropy parameters in fast electron-atom scattering. The ratio of quadrupole-to-dipole matrix elements is determined by the parameter ωR/v < 1, where ω is the transferred in collision energy, R is the ionized shell radius, and v is the speed of projectile. This factor can be much larger than that in the case of photoionization, where one has the speed of light c that is much higher than v. We illustrate general formulas by concrete results for outer s subshells of noble gas atoms Ar and Xe. Even for very low momentum transfer q, in the so-called optical limit, the deviation from photoionization case is prominent and instructive.     

Processes in condensed inert gases involving excess electrons

Drift of an excess electron in dense and condensed inert gases in external electric field and excitation of atoms by electron impact in these systems are analyzed. The effective potential energy surface for an excess electron at a given electric field strength consists of wells and hills, and the actions of neighboring atoms are therefore separated by saddles of the potential energy. At such atomic densities that the difference of interaction potentials for an excess electron between neighboring wells and hills of the potential energy surface becomes small, the electron mobility is large. This is realized for heavy inert gases (Ar, Kr, Xe) with a negative scattering length of an electron on individual atoms. In these cases, the average potential energy of the electron interaction with atoms corresponds to attraction at low atomic densities and to repulsion at high densities. The transition from attraction to repulsion at moderate atomic densities leads to a maximum of the electron mobility. A gas model for electron drift in condensed inert gases is constructed on the basis of this character of interaction. Due to high electron mobility, condensed inert gases provide high efficiency of transformation of the electric field energy into the energy of emitting photons through drifting electrons. It is shown that, although the role of formation of autodetaching states in the course of electron drift is more important for condensed inert gases than for rare gases, this effect acts weakly on exciton production at optimal atomic densities. The parameters of a self-maintained electric discharge in condensed inert gases as a source of ultraviolet radiation are discussed from the standpoint of electron drift processes.     

Stabilization of an excess electron on molecular surfaces by a pair of water molecules

This report presents the results of dimerized water hydration on several hypothetical cyclooctane and cyclohexane molecular surfaces. When these complexes have extensive OH groups on one side of a hydrocarbon surface (i.e. cyclohexane sheets), extended hydrogen bonded networks can form that increase the dipole moment of the system. At the same time, the hydrogen atoms on the opposite side form a pocket of positive charge that can attract excess electrons. In this work two orientations for water dimer hydration on eight molecular surfaces have been studied. All systems have been demonstrated to be stable towards electron detachment.     

Chemical structural effects on <Emphasis Type="Italic">γ</Emphasis>-ray spectra of positron annihilation in fluorobenzenes

Spectra of γ-ray Doppler shifts for positron annihilation in benzene and its fluoro-derivatives are simulated using low energy plane wave positron (LEPWP) approximation. The results are compared with available measurements. It is found that the Doppler shifts in these larger aromatic compounds are dominated by the contributions of the valence electrons and that the LEPWP model overestimates the measurements by approximately 30%, in agreement with previous findings in noble gases and small molecules. It is further revealed that the halogen atoms not only switch the sign of the charges on carbon atoms that they bond to, but that they also polarize other C-H bonds in the molecule leading to a redistribution of the molecular electrostatic potentials. As a result, it is likely that the halogen atoms contribute more significantly to the annihilation process. The present study also suggests that, while the Doppler shifts are sensitive to the number of valence electrons in the molecules, they are less sensitive to the chemical structures of isomers that have the same numbers and type of atoms and, hence, the same numbers of electrons. Further investigation of this effect is warranted.     

Bond breaking and temporary anion states in uracil and halouracils: implications for the DNA bases

Low energy electrons are capable of breaking bonds in gas phase DNA bases by means of the dissociative electron attachment process. With the aid of new total scattering data in the halouracils and input from quantum chemical calculations, we describe the dipole bound and valence anion states in these compounds and present assignments for the two types of structure appearing in the cross sections. A clear distinction between the two mechanisms for bond breaking is necessary for an understanding of electron induced damage to DNA.     

Barrier-free intermolecular proton transfer in the uracil-glycine complex induced by excess electron attachment

The photoelectron spectra (PES) of anions of uracil-glycine and uracil-phenylalanine complexes reveal broad features with maxima at 1.8 and 2.0 eV. The results of ab initio density functional B3LYP and second order M?ller-Plesset theory calculations indicate that the excess electron occupies a π^* orbital localized on uracil. The excess electron attachment to the complex can induce a barrier-free proton transfer (BFPT) from the carboxylic group of glycine to the O8 atom of uracil. As a result, the four most stable structures of the anion of uracil-glycine complex can be characterized as the neutral radical of hydrogenated uracil solvated by the anion of deprotonated glycine. The similarity between the PES spectra for the uracil complexes with glycine and phenylalanine suggests that the BFPT is also operative in the case of the latter anionic species. The BFPT to the O8 atom of uracil may be related to the damage of nucleic acid bases by low energy electrons because the O8 atom is involved in a hydrogen bond with adenine in the standard Watson-Crick pairing scheme. Received 6 April 2002 Published online 13 September 2002     

Are HBO− and BOH− electronically stable?

The binding of an excess electron to HBO and BOH was studied at the coupled cluster level of theory with single, double and non-iterative triple excitations and with extended basis sets to accommodate the loosely bound excess electron. The bent BOH molecule, with a dipole moment of 2.803 D, binds an electron by 39cm^?1, whereas the linear HBO tautomer possesses a similar dipole moment (2.796 D) yet binds the electron by less than 1 cm^?1. It is therefore likely that HBO^? is not stable when rotational energies are included whereas BOH^? is for low rotational quantum numbers.     

Anions of xenon clusters bound by long-range electron correlations

In contrast with the single atom, atomic van der Waals clusters can form stable anions where the excess electron is bound due to long-range correlations with the electrons of the cluster. We report on extensive all-electron many-body ab initio studies on Xe clusters. Three-dimensional, planar, and linear structures of the clusters are investigated and compared. In particular, we find that the minimal number of Xe atoms in the cluster required to form a stable anion is 5 independently of the dimensionality of the cluster. We provide electron affinities for clusters made of 5, 6, and 7 atoms in all dimensions and find that the planar clusters form the most stable anions. The Dyson orbitals of the excess electrons are computed and analyzed.     

Essai d'interprétation des déplacements et élargissements par la pression des raies du spectre de vibration-rotation des molécules diatomiques

An interpretation is given of the dependence on the rotational quantum number J of the pressure shift and broadening of the lines of the vibration-rotation band of a hydracide molecule perturbed by a noble gas. It is shown that, with the fundamental hypothesis of adiabatic collisions between the absorbing molecule and the noble gas atoms, the impact theory can describe the J dependence. This results from the fact that one introduces the average over the sine of the phase shift η, which is J dependent, and not the average of the phase shift itself which is vanishing. With a hard-sphere model good agreement is obtained between the calculated and the observed shifts. Further, a rough estimation of the breadth is made.     

Single-electron charging spectra: from natural to artificial atoms

We present a theoretical study of the charging spectra in natural and artificial atoms. We apply a model electrostatic potential created by a homogenously charged sphere. This model potential allows for a continuous passage from the Coulomb potential of the nucleus to parabolic confinement potential of quantum dots. We consider electron systems with N=1,…,10 electrons with the use of the Hartree–Fock method. We discuss the qualitative similarities and differences between the chemical potential spectrum of electron systems bound to nucleus and confined in quantum dots.     

Längsfeldstärke,Elektronentemperatur, mittlere Neutralgasdichte,Ionenstromdichte auf die Wand und mittlere Entladungsstromdichte in der Freifallsäule bei hohem Ionisationsgrad

On the basis of a foregoing paper new theoretical results for the positive column at low pressure and strong ionization, especially for discharges in noble gas ion lasers, are given. The mean velocity v_n0 of the neutral atoms reemitted from the wall is taken into account. The electric conductivity is calculated for an argon plasma. The formulas connecting the electron temperature, the mean neutral gas density, and the electric field strength are derived. The electron temperature, the axial electric field intensity, the degree of ionization, the axial electron drift velocity, the ion flux to the wall, and the force density causing the main part of gas pumping along the column are calculated as functions of the product of the mean current density and the tube radius, and of v_n0 for argon. The axial drift velocity of the electrons is still smaller than the mean thermal electron velocity for high discharge currents, except at very low gas pressures. In general, the ion flux to the wall is not directly proportional to the discharge current. The factor for the determination of the charged particle density by means of probe measurements at the wall is discussed. The self-magnetic field affects the discharge only at high electron temperature, high degree of ionization, and relatively large tube radius, i.e. at high current density and low gas pressure in not too narrow discharge channels.     

Rare gas dimers in a nozzle beam

Supersonic expansion from a nozzle produces noble gas clusters under various source conditions. The characteristics of atoms and dimers in a nozzle beam are examined. A study of the dimer collision cross-section suggests the existence of a temperature-dependent limiting oven pressure (P _L ) for the observation of pure dimers. This is further supported by the dependence of the beam intensity on stagnation pressure. The reduced pressure-temperature coordinates for noble gas dimers behave in accordance with the model of corresponding jets. The velocity distribution of atoms and dimers in a beam corresponding to stagnation pressure on either side ofP _L is measured to determine the effect of condensation on the distribution pattern.     

A new laser concept for isotopically selective analysis of noble gases

A new laser approach for the isotopically selective analysis of noble gases is presented. This approach uses noble gas atoms prepared in the 1s ⁵ metastable state. Hyperfine levels in the 1s ⁵ and 2p ⁹ states form two-level systems in Ne, Ar, Kr, and Xe which can be excited by a commercially available single-frequency laser system. Absorption of photons from such a laser, and the resulting momentum transfer, can be used to selectively deflect the desired isotope from a supersonic atomic beam into a detection area. Light from the same laser can then be used to selectively count atoms of the desired isotope using the photon-burst technique. Thus, enrichment and selective detection are accomplished with a single laser in a single pass through the apparatus.The problem of analyzing for⁸⁵Kr in a sample of noble gases extracted from the air is examined in detail. This is a stringent test of the selectivity of this approach because⁸⁵Kr has the same nuclear spin, and thus similar hyperfine splittings, as naturally occurring⁸³Kr. Calculations indicate that isotopic selectivity of the new approach is easily adequate to resolve⁸⁵Kr in a 10¹⁰ excess of⁸³Kr.     

Minimal dipole charge for a dipole-bound dianion

Full configuration interaction calculations for two electrons moving in the field of a fixed finite dipole (FFD) have been carried out, in order not only to determine the conditions for stability relative to one electron detachment, but also to check whether a true dipole-bound system, converse to a Stark-shifted atom-like system, is generated. The FFD model is constructed by placing two point charges of absolute value q, and opposite sign, separated by a distance d.

It has been found that, although dipole charges as small as q≈0.91 au can bind two electrons stronger than one for large enough values of d (thus giving stable systems relative to the detachment of one electron), it is only for q > 2.941 au that the length of the dipole is short enough for the size of the electronic cloud to actually exceed it, so that the system can really be regarded as a dipole-bound dianion, and not a Stark-shifted atom.     

Optical transitions for 2D electrons at the solid hydrogen-vapor interface

Details of the problem of “vertical” transitions between discrete levels of 2D electrons on the surface of liquid (solid) dielectrics in the presence of a gas over this surface (their own vapors or artificially produced combinations such as solid hydrogen-gaseous helium) are discussed. The structure of the interaction of an electron with a gaseous medium, where the so-called scattering length appears to depend on the gas density, is determined. The role of Van der Waals forces, which attract gas atoms to the dielectric surface, is shown. A notion of quasi-2D electron bound states on gas atoms is introduced. The experimental data concerning the effect of the gas on the frequencies of optical transitions for 2D electrons over the surface of solid hydrogen are discussed using this information.     

Stoletov constant and effective ionization potential of a diatomic gas molecule

Applying the dimension theory methods to the experimental data on static electric gas breakdown, we established the dependence of the empirical Stoletov constant on gas characteristics. This constant was shown to be 2.72 times larger than the energy expended by electrons to ionization of one gas particle (atom and molecule). Based on the analysis of elementary processes with participation of metastable levels of molecules and atoms under the optimum electron multiplication condition in a diatomic gas we concluded that the mean energy expended by the electrons to ionization of one molecule is the effective ionization potential of a gas molecule.     

Two-dimensional electron gas at noble-metal surfaces

The electrons of the surface states on the (111) surfaces of the noble metals Au, Ag, and Cu form a quasi-two-dimensional (2D) free electron gas which is confined to the first few atomic layers at the crystal surface. They are scattered by the potential associated with surface defects, e.g. impurity atoms, adatoms, or step edges, leading to quantum-interference patterns in the local density of states around these defects. We have used the quantum-interference phenomena to quantitatively measure the electron phase-relaxation length and to probe long-range adsorbate interactions. Received: 12 October 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002