Rare Gas Systems Research Articles

Diffraction effects in the differential scattering of Li + by the rare gases

Relative elastic differential cross sections for Li+_rare gas systems have been measured over the energy range 300 to 1000 eV for scattering angles between 0 and 5 mrad. The results have been analysed using the uniform approximation and the potentials retrieved from these analyses have been compared with recent calculations.

Interionic potentials in alkali halides and their use in simulations of the molten salts

Abstract After an outline of work on rare-gas systems, which serves as a target for parallel work on alkali halides, and an initial brief survey of those parts of this parallel work for which results have been obtained, interionic potential models for alkali halides are considered in some detail. The rigid ion potentials of Fumi and Tosi are discussed and then a major part of the section is devoted to deriving a new set of polarizable ion potentials, which incorporate the ideas behind the lattice dynamical shell model. Extensions which include many-body terms in the potentials are considered briefly and finally the information which can be obtained from alkali halide diatomic molecules is discussed. In the third section methods of computer simulation for ionic liquids are outlined, concentrating on the molecular dynamics method, and some of the properties which can be obtained by analysing the ion trajectories are listed. Results from simulations, including some new work on LiF, NaCl and RbI, are reviewed.

Quasi-symmetry effects in outer-shell excitations of the Mg+-rare-gas systems

Collision spectroscopy of the Mg+-rare-gas systems has shown very different behaviour for Mg+-Ne compared with the other systems. This finding is discussed in the framework of the electron-promotion model. The importance of the quasi-u,g symmetry is pointed out and found to be responsible for the selectivity of the excitation processes.

Charge-transfer energy in closed-shell ion–atom interactions

The importance of charge-transfer energy in the interactions between closed-shell ions and atoms is investigated. Ab initio calculations on H+–He and Li+–He are used as a guide for the construction of approximate methods for the estimation of the charge-transfer energy for more complicated systems. For many alkali ion–rare gas systems the charge-transfer energy is comparable to the induction energy in the region of the potential minimum, although for doubly charged alkaline-earth ions in rare gases the induction energy always dominates. Surprisingly, an empirical combination of repulsion energy plus asymptotic induction energy plus asymptotic dispersion energy seems to give a fair representation of the total interaction, especially if the repulsion energy is parameterized, despite the omission of any explicit charge-transfer contribution. More refined interaction models should consider the charge-transfer energy contribution.

Fine structure transition cross sections for several alkali + rare gas systems

The energy dependence, E c.m. </ 0.2 eV, of the inelastic total cross sections for the 2P 1 2 → 2P 3 2 fine structure transition of the lowest excited states of the alkali atoms are calculated for the following systems: Na, K, Rb + He, Ne, Ar and Cs + He. Encouraging agreement between theory and experiment is obtained.

Scattering of Ions VI. Elastic Scattering of N+ on Ne, Ar, Kr, and Xe

In the elastic differential cross-section of the N+-rare gas systems two different types of oscillations have been observed. At an energy of a few eV a primary rainbow was detected for all systems. In the case of N+ - Ar, secondary rainbows were resolved too. From these experimental data the potential depths were determined to be: ε (N+ - Ne) = 0.4 eV, ε (N+ - Kr) = 0.29 eV, ε (N+ - Xe) = 0.92 eV. For the system N+ - Ar the shape of the reduced potential was obtained by evaluating the positions of the secondary rainbow extrema. The potential depth was found to be 2.16 eV. At higher energies and large scattering angles additional oscillations appeared which are presumed to be Stueckelberg oscillations. For N+ - Ne a single group of strongly marked oscillations appeared up to the highest energies used (EL = 220 eV). In the case of N+ - Ar the amplitudes of these undulations are strongly quenched. For N+ - Kr and N+ - Xe several groups of oscillations with different amplitudes and frequencies seem to be superimposed in the differential cross-section. They are attributed for all systems to crossings of the incoming state with the charge exchanged state.

Argon sensitized formation of Xe +2: A new mechanism for the Jesse effect

Photoexcitation of the argon resonance states in the presence of xenon leads to Xe + 2. Kinetic analysis indicates rapid near-resonant energy transfer between argon and xenon atoms. The possibility of an analogous mechanism in other rare gas systems is examined.

Calculation of Line Broadening Cross Sections for Level Crossing Experiments in Alkali-Rare Gas Systems

Using a simple approximation for the theoretical atomic interaction constants cross section data for level crossing and Hanle-effect line broadening experiments in Rb, Mg+ and Ca+ rare gas systems could be derived. It is shown that in the Rb system the interaction C6R-6 is predominant and that the results are little influenced by dipole-quadrupole and repulsive interactions in the frame work of the Lindholm-Foley-Hindmarsh theory.

Electon spectroscopy using exeted atoms and photons of Penning ionization of the rare gases

A study has been made of the Penning ionization electron spectra resulting from collisions between rare gas atoms and helium metastable atoms. The results are compared and contrasted with photoelectron spectra at 584 Å. Additional bands appear in the electron spectra and are explained on the basis of fast neutral-neutral collisions involving curve crossing mechanisms. Electron spectra from mixed rare gas systems at higher pressures are examined.

Applicability of Self-Consistent-Field Molecular-Orbital Methods to the Study of Hyperfine Shifts

The calculation of hyperfine pressure shifts (HPS) by a self-consistent-field molecular-orbital (SCF-MO) method is applicable to hydrogen---rare-gas systems only when helium is the rare gas. Calculations on the systems H-Ne and H-Ar are presented, and the discrepancy with experiment is analyzed. The SCF-MO method seems to be limited to cases with large positive HPS.

Energy trapping in solid He, Ne, Ar, Kr, and Xe by molecule formation - A theoretical interpretation

Trapping of electronic excitations in the rare gas solids is investigated from a dynamical point of view. A resonant transfer deexcitation mechanism is found to cause rapid vibrational relaxation of the R * 2 dimer, believed to be responsible for the trapping, in less than 6 × 10 −12 seconds for all the rare gas systems.

High-Energy Alkali–Rare-Gas Scattering

Absolute total-collision cross sections in the LAB energy range of 6–1000 eV were measured for several alkali-metal–rare-gas systems—Na + Ar, Xe; K + Ne, Ar, Kr, Xe; and Cs + Kr, Xe. The limiting repulsive part of the intermolecular potential was described accurately by an inverse-power law Cr−8, where s varied from 5.75 to 10.75 depending on the system. In the low-energy region the cross sections for the heavier systems show a pronounced rise due to the influence of the attractive part of the potential. In no cases could the cross sections be fitted by a two-term Lennard-Jones potential. A more abrupt transition is required between the inverse-power repulsion and the well region of the potential.

Genaue Potentialbestimmung aus Streumessungen: Alkali-Edelgas-Systeme

Recent improvements in precision and the increase in number of molecular beam scattering experiments demand a reanalysis of the experimental data. The commonly used potential functions show a serious lack of flexibility, and fitting procedures are not unambigious. A new potential ansatz (which is a continuous modification of the (12-6)-potential) has been used to fit all available data on the alkali-rare gas systems Li-Kr, Na-Ar, Na-Kr, Na-Xe, K-Ar, and K-Kr. Aχ 2-minimum method is used as the procedure to fit the data and to estimate the errors of the parameters obtained (see Table 1). It turns out, that the shape of the potential is similar for all systems evaluated here but probably different from that derived for rare gas-rare gas systems.

Pair and non-pair argon potential

This study constitutes an attempt to assess quantitatively the importance of many-body forces in dense rare-gas systems by deducing a more accurate pair potential from low density data. The preliminary conclusion is that quantitative effects of many-body forces on solid state properties are hidden within the experimental uncertainties and that for gas data (third virial coefficients) such effects are barely perceptible.

Lyman-Alpha Emission Cross Sections forH2+andD2+Collisions with the Rare Gases

Cross sections for the emission of Lyman-alpha radiation resulting from ${\mathrm{H}}_{2}^{+}$-and ${\mathrm{D}}_{2}^{+}$-rare-gas collisions have been measured in the energy range from 1 to 25 keV. Excited states of the H or D atoms which decay via the $2p\ensuremath{-}1s$ Lyman-alpha transition result from charge transfer to antibonding states of the ${\mathrm{H}}_{2}$ or ${\mathrm{D}}_{2}$ molecules or from direct excitation to antibonding states of the molecular-ion projectiles. The ${\mathrm{D}}_{2}^{+}$ cross sections are similar to those for ${\mathrm{H}}_{2}^{+}$ when plotted at equivalent projectile velocities. The results are compared with earlier measurements of the Lyman-alpha emission cross sections resulting from ${\mathrm{H}}^{+}$ and ${\mathrm{D}}^{+}$ collisions with rare gases and with the total charge-transfer cross sections for the ${\mathrm{H}}_{2}^{+}$-rare-gas systems.