Rare Gas Systems Research Articles

Discharge stability of electrically excited rare-gas halide laser systems

An explanation of the cause of discharge constriction in an electrically excited XeCl* laser system is presented. Our analysis, carried out for a specific system, is more generally applicable to other rare-gas halide lasers and represents a contribution to the theory of discharge stability in high-pressure, strongly electron-attaching gas mixture. The conclusions obtained in various theories of the growth of instabilities in high-pressure laser discharges, thus far appeared in the literature, are briefly resumed and compared with our experimental and theoretical findings. Finally, prospects for obtaining a better discharge stability in rare-gas halide systems are discussed.

Theoretical studies of tunneling processes in three-body exchange reactions of van der Waals rare gas dimers

Rare gas atom–diatom collisions of Ar+Ar2, Xe+Ar2, Kr+Xe2, Kr+Ne2, and Kr+NeAr have been investigated to determine the importance of tunneling processes in exchange and dissociation reactions involving van der Waals molecules. Reaction cross sections, angular distributions, and product-energy distributions have been computed using Monte Carlo quasiclassical trajectories. The effect of tunneling through the rotational barrier upon these quantities has been computed using WKB methods. The results show that metastable diatomic products with energies above the classical dissociation limit, but below the rotational barrier, play a significant role in the dynamics of both exchange and dissociation reactions. Lifetime distributions of such metastable dimers illustrate their importance in crossed molecular beam studies of rare gas systems. The WKB calculations indicate that a significant, and possibly measurable, number of the metastables dissociate by tunneling before they would reach the detector in a molecular beam experiment. Close agreement has been found between these calculations and statistical state-counting calculations of metastable product dimer lifetimes. Experiments are suggested that might permit the direct observation of tunneling in these systems.

Collision-induced dipoles of rare gas mixtures

New ab initio calculations of the collision-induced dipole moment of the rare gas systems He–Ar, Ne–Ar, Ne–Kr, and Ar–Kr are obtained on the basis of a molecular Hartree–Fock treatment. With these and recent potential functions the spectral moments and line shapes of collision-induced absorption spectra are computed. Reasonable agreement with existing measurements is observed for the first time for the system Ne–Ar and Ne–Kr.

Interaction potentials for Li +—rare-gas systems

The accuracies of proposed interaction potentials for the Li +—rare-gas systems are tested by comparing the transport coefficients calculated from the potentials with the experimental values. The agreement is generally good for theoretical potentials that take electron correlation into account and for potentials inferred from ion-beam measurements of high accuracy, except where the transport data are primarily influenced by the long-range tail of the potential. The transport data are also used to directly determine the Li +—rare-gas interaction potentials, with an estimated accuracy of 10% over wide ranges of ion—atom separation.

Accurate Ne-heavier rare gas interatomic potentials

Accurate interatomic potential curves for Ne-heavier rare gas systems are obtained by a multiproperty analysis. The curves are given via a parametric function which consists of a modified Dunham expansion connected at long range with the van der Waals expansion. The experimental properties considered in the analysis are the differential scattering cross sections at two different collision energies, the integral cross sections in the glory energy range and the second virial coefficients. The transport properties are considered indirectly by using the potential energy values recently obtained by inversion of the transport coefficients.

Nonlinear excitation of rare-gas liquid mixtures in the ultraviolet

Studies of the electronic properties of condensed rare-gas liquid systems have been conducted with the use of nonlinear excitation in the ultraviolet range. Analysis is furnished for several media, including binary (Ar-Kr, Ar-Xe, and Kr-Xe) and tertiary (Ar-Kr-Xe) systems. Evidence is reported for the selective excitation of impurity species, molecule-molecule electronic energy transfer in the liquid phase, and the identification of the 162.5-nm emission in Xe-Kr with the KrXe excimer.

Atom–molecule interactions from multiproperty analysis. An integrated study of the dynamics for oxygen–rare-gas systems

The interaction potential between the ground electronic state of the oxygen molecule and the helium atom is obtained through a numerical fit of data from collision experiments and is given via an effective, spherically symmetric analytic form. The latter expression, together with similar ones obtained previously for the O2–Ar and O2–O2 systems, is then used within a quantum-mechanical model to treat vibrationally inelastic processes and to generate various partial integral cross-sections at several energies. A comparison with experimental relaxation data is then carried out for the three cases examined; the results show generally good agreement for the He–O2 and Ar–O2 systems, suggest a greater influence of rotational inelasticity for the O2–O2 case and allow us to assess the importance of Van der Waals complex formation on the relaxation mechanism.

Collision spectroscopy of ion-diatom systems:He+-N2and -O2at medium energies

Energy-loss measurements of direct-excitation and electron-capture processes are performed for the He/sup +/-N/sub 2/, -O/sub 2/ collisional systems in the 0.2--4-keV energy range and in the 0/sup 0/--3/sup 0/ angular range. Pure vibrational excitation and vibrational excitation in excited electronic states are studied using a high-resolution spectrometer (100 meV). Pure vibrational excitation is always found to be a weak process. The analysis of vibrational distributions in excited states strongly supports an interpretation in terms of the Franck-Condon principle. Relative probabilities as functions of energy and angle are reported for the identified excitation and electron-capture processes. The results show the importance of electron-capture processes among which quasiresonant and endothermic processes dominate exothermic processes. The striking resemblance of the present data with those available on the He/sup +/--rare-gas systems conveys a similar quasimolecular interpretation of the collision mechanisms.

Excitation of K and Zn+in collisions with He, Ne and Ar: measurements of K I and Zn II resonance-line emission and polarisation

Total emission cross sections and polarisations have been measured for the K I and Zn II 42S-42P multiplet in collisions of 0.7-80 keV K atoms and 1-500 keV Zn+ ions with He, Ne and Ar. The excitation of the projectile resonance multiplet is found to be the dominant inelastic process for all collision systems. Several of the cross sections show structures more complex than those found previously for the similar but lighter Li, Be+, Na, Mg+-rare-gas systems. A high-energy maximum is located as predicted by the Massey criterion. Secondary maxima at lower energies may be large and in some cases exceed the high-energy maximum. For the K-Ar system, the impact-parameter dependence of the excitation probability has been measured for selected impact energies by means of the scattered-particle-photon-coincidence technique. The origin of the observed structures is discussed.

Inhomogeneity corrections to the Thomas-Fermi-Dirac model of rare-gas diatomic systems

Total energies and intermolecular potentials are calculated for the homonuclear diatomic Ne-Ne and Ar-Ar systems according to the Thomas-Fermi-Dirac formulae with a modified Weizsacker inhomogeneity correction. Using the electron densities of the Hartree-Fock molecular orbital calculations of Gilbert and Wahl as inputs, the inhomogeneity corrections are small compared to the total energies but are comparable to the Dirac exchange contributions at all the available distances. The errors in the total energies using the Thomas-Fermi-Dirac-Weizsacker formulation are approximately -0·4 per cent for Ne-Ne and -0·01 per cent for Ar-Ar molecules respectively. However, the values of the intermolecular potentials thus calculated are in general too negative and not in agreement with the corresponding Hartree-Fock values. Using instead the electron-gas approximations of Gordon and Kim as inputs, where the superpositions of the atomic Hartree-Fock densities are used, the inhomogeneity contributions are also too negative.

The effect of elastic and reorientation collisions on vibration-rotation lineshapes: A semi-empirical approach

Collisional broadening of rotation-vibration spectral lines is investigated. The contributions from inelastic and elastic-reorientation collisions are separated using a semi-empirical inversion procedure. The basis for this deconvolution is the derivation of a relationship between the enelastic, W in, and elastic-reorientation, W el−r, contributions to the width, which depends upon a single unknown parameter. The experimental linewidth, W exp = W el− r + W in , suffices to determine this parameter uniquely, which is then used to calculate the individual components W el−r and W in. Results of applications to the CO-, HF-, and HCI- rare gas systems are presented.

Crossed molecular beam studies on the interaction potentials for F(2P) + Ne,Ar,Kr(1S)

Angular distributions are presented of F(2P3/2,1/2) scattered off Ne,Ar, and Kr in the thermal energy range measured in crossed molecular beams experiments. The data are analyzed as previously done for the F–Xe system to obtain the interaction potential curves for the three relevant states (X 1/2, I 3/2 II 1/2)governing the scattering. The resulting X 1/2 curves for the fluorine– rare gas systems show no simple trend and the origin of these interactions is discussed. The I 3/2 and II 1/2 states resemble those of the ground state Ne–Ne,Ar,Kr systems.

Rare gas interactions using an improved statistical method

New density functionals are employed to represent the correlation and exchange energies (per electron) in the calculation of rare gas interactions from the electron gas model. The correlation energy density functional is a rational function involving two parameters which were optimized to reproduce the correlation energy of He atom. Our results indicate that these parameters are ’’universal’’, i.e., they are accurate for all rare gas atoms. The exchange energy density functional is that recommended by Handler. Rare gas systems X2 and XY are investigated, where X and Y are He, Ne, Ar, and Kr. The results are a significant improvement over those available from competing electron gas models.

Calculated long-range interactions and low energy scattering in He+H, Ne+H, Ar+H, Kr+H, and Xe+H

The van der Waals (VDW) forces in rare-gas hydride systems are evaluated through the use of the multiconfiguration self-consistent field (MCSCF) method. For Kr–H and Xe–H, a pseudopotential variant of the MCSCF method is used. For each system, a potential energy curve is fit to the calculated potential energies, the VDW well depth and equilibrium position are determined, and the low energy total elastic cross sections are computed as a function of relative velocity. The calculated cross sections are in good to excellent agreement, for He–H, Ar–H and Xe–H, and in fair agreement, for Ne–H and Kr–H, with the most recent experimental measurements.

Bands associated with Cs(6 2S1/2) – Cs(7 2S1/2 and 5 2D5/2) transitions perturbed by helium

New bands are observed in the absorption and emission spectra of a cesium helium mixture by using a laser induced fluorescence technique. The comparison between these spectra and those already obtained with the other cesium–rare gas systems allows us to associate these bands with transitions between the ground state and the 7 2S1/2 and 5 2D5/2 excited states of cesium perturbed by helium atoms. The analysis of the bands profile gives information about the shape of the adiabatic potential energy curves.

Scaling theoretic deconvolution of bulk relaxation data: State-to-state rates from pressure-broadened linewidths

Inversion of certain types of relaxation data is shown to be possible by application of two new developments. First, a recently developed scaling theory of rotationally inelastic cross sections is utilized in deriving the corresponding formulas for the state-to-state rate constants kjj′. Second, practical means are presented for assessing the number of independent pieces of information contained in the experimental data. Application of this work to simple relaxation times (i.e., T1 or T2) provides an essential reduction in the number of unknowns and allows for the determination of the rates k0Δ by inversion of a set of algebraic equations subject to the constraints k0Δ?0. The dimensionality of this set of equations is related to the number of independent pieces of information contained in the experimental data. A stable and fast method for solving the equations is given. The inversion of pressure broadening data to yield state-to-state rate constants is illustrated for the CO–, O2–, N2–rare gas systems.

A quasiclassical trajectory study of the energy transfer in CO2–rare gas systems

Computational methods are presented for the study of collisions between a linear, symmetric triatomic molecule and an atom by three-dimensional quasiclassical trajectory calculations. Application is made to the investigation of translational to rotational and translational to vibrational energy transfer in the systems CO2–Kr, CO2–Ar, and CO2–Ne. Potential-energy surfaces based on spectroscopic and molecular beam scattering data are used. In most of the calculations, the CO2 molecule is initially in the quantum mechanical zero-point vibrational state and in a rotational state picked from a Boltzmann distribution at 300°K. The energy transfer processes are investigated for translational energies ranging from 0.1 to 10 eV. Translational to rotational energy transfer is found to be the major process for CO2–rare gas collisions at these energies. Below 1 eV there is very little translational to vibrational energy transfer. The effects of changes in the internal energy of the molecule, in the masses of the collidants, and in the potential-energy parameters are studied in an attempt to gain understanding of the energy transfer processes.

Radiative and kinetic mechanisms in bound-free excimer lasers

An interactive kinetic model for bound-free excimer lasers is described, with particular emphasis on the electron beam pumped rare gas systems. Important kinetic processes and the interaction of the photon field with excited states are examined in detail. Application of the model to the xenon and krypton oscillators yields results which are in good agreement with experimentally observed quantities. It is found that the collisional coupling of the excimer singlet and triplet manifolds in combination with the losses arising from photoionization play an important role in determining the performance. Numerical values of the parameters used in the computational model are given.

Oscillator strengths for S–S and S–D induced dipole transitions in alkali–rare gas systems

Electronic oscillator strengths for induced dipole transitions in alkali–rare gas systems have been calculated as a function of internuclear distance using adiabatic potential energy curves and molecular wavefunctions previously obtained from a pseudopotential model. The molecular transitions considered are those associated, in the separated atom limit, with S–S and S–D electric dipole forbidden transitions in the alkali atom. The oscillator strengths are found to be relatively large (about 10−2–10−4) at small and intermediate internuclear distances (R∼3–12 a.u.).

Thomas–Fermi–Dirac interaction potentials for rare gas systems

The energy components and the Thomas–Fermi–Dirac interatomic interaction potentials of the Ar–Ar, Kr–Kr, and Xe–Xe systems are calculated for several internuclear distances, solving exactly the Thomas–Fermi–Dirac equation for the diatomic systems. Comparisons are made to results from other density functional approaches. The dependence of the energy components on the nuclear charge is discussed, and it is suggested that (EαTFD −EαTF)/EαTF, the relative deviation of a TFD energy component from that calculated from the Thomas–Fermi density, is proportional to Z−2/3. A possible analytic form of the interatomic potential is proposed.