Improved techniques for the calculation of ab initio ion-neutral interaction potentials: application to coinage metal ions interacting with rare gas atoms

Ab initio values for the potential energy functions for ion–neutral interactions can be tested by comparison with gaseous ion transport coefficients, but only if special care is taken to compute the interaction potentials accurately over wide ranges of internuclear separation. This is illustrated here by a reanalysis of the ab initio values for the coinage metal ions interacting with rare gas atoms, precise calculations of the transport cross sections over extremely wide ranges of energy, and similarly precise calculations of the zero-field ion mobilities as functions of gas temperature and the field-dependent ion mobilities at various fixed temperatures. The calculations indicate that the mobilities for Ag⁺(¹S) moving in Ne or Ar can distinguish between the existing, very similar ab initio potentials. They also show that substantial differences exist among the mobilities of the coinage metal anions and the ground and excited states of the cations. The techniques implemented are recommended for future ab initio calculations.     

Interaction potentials,spectroscopy and transport properties of RG+–He (RG=Ar–Rn)

High-level ab initio potential energy curves are calculated for the RG⁺–He complexes (RG=Ar–Rn). RCCSD(T) calculations are employed with large basis sets, and taking account of spin–orbit coupling. The calculated spectroscopic parameters are compared with experimentally determined values, with other high-level ab initio results, and with results from potentials that were obtained by fitting to experimental data. The gas-phase mobilities of RG⁺ ions in He are calculated from our potentials and compared, graphically and statistically, with the experimental mobilities as a function of E/n ₀ at several temperatures. We conclude that more precise experimental data are required in order to discriminate between potentials with more certainty. In addition, we discuss previously reported, unexpectedly large drops in experimental mobility values for RG⁺ in He at 4.35 K as E/n ₀ → 0.     

Fragmentation of fast positive and negative C ions in collisions with rare gas atoms

The fragmentation of anions and cations resulting from 50 keV collisions with rare gas targets is studied. Positive ion fragment patterns are recorded, and dramatic changes in these patterns are observed as a function of target atom number. The fragment pattern dependence on the target atom size is investigated within a simple model, normally used for stopping power calculations. Fair agreement is obtained between calculated and experimental spectra. From these comparisons we conclude that the range of the screened atomic potentials, as e.g., the Thomas-Fermi potential, is an essential parameter in the collisional induced fragmentation process. Received: 13 February 1998 / Revised: 27 October 1998 / Accepted: 29 October 1998     

A theoretical study of the non-adiabatic charge transfer process Ar2+ (3 P) + He(1 S) → Ar+(2 P) + He+(2 S)

CI calculation with a large basis have been used to calculate the two lowest ³Π adiabatic potential energy curves for the title reaction. These potentials have been transformed to diabatic potentials by employing a recipe based on the CI coefficients. Quantum mechanical close coupling calculations in the diabatic basis have produced total and differential cross sections which are in good agreement with experimental data. Full quantum mechanical and Landau-Zener calculations of the total cross section are in fair agreement with recent experimental measures and by small changes to the diabatic potentials can be brought into essentially exact agreement.     

Influence of the spin-orbit coupling on the optical collision for Ba atom perturbed by rare gases

The quantum non-adiabatic theory of atomic collisions in the presence of a weak radiation field is applied to describe the Ba-rare-gas (RG) optical collision. The absorption coefficient as well as linear and circular polarizations of collisionally redistributed fluorescence light are calculated for a range of detuning around the barium (λ₃ = 5535Å) resonance line. Closecoupling calculations involving the relevant X¹Σ, ¹Σ,¹II, ³Σ and ³II states for each Ba-RG pair are carried out based on the new theoretical potential curves obtained by Czuchaj within the valence ab initio scheme. Our attention is mainly paid to the role of spin-changing transitions between the (6s6p) ¹P and (6s6p) ³P states in the process investigated. The numerical results show that the singlet-triplet mixing is important in the red-wing part of the spectra. The calculated absorption coefficients and polarizations are thermally averaged over the collision energy and compared with the experimental results of Alford et al. [1984, Phys. Rev. A, 30, 2366] and Ni et al. [1996, Z. Phys. D, 38, 303].     

Accurate potential energy curves for Tl+–Rg (Rg=He–Rn): spectroscopy and transport coefficients

High-quality ab initio potential energy curves are presented for the Tl⁺–Rg series (Rg=He–Rn). Calculations are performed at the CCSD(T) level of theory, employing aug-cc-pV5Z quality basis sets, with ‘small core' relativistic effective core potentials being used for Tl⁺ and Kr–Rn. The curves are shown to be in excellent agreement with experimental mobility data for the systems Tl⁺–Rg (Rg=He–Xe), and generally excellent agreement is also obtained with longitudinal diffusion data. An exception to the latter is Tl⁺ in He, which is attributed to the experimental data not being obtained under steady state conditions. Spectroscopic information is also presented for the titular species, derived from potential energy curves, and the results are compared with previous potentials inferred from the ion transport data.     

Study of the broadening of the rotational Raman lines of CO2 perturbed by rare gases

This paper presents the results of an experimental and theoretical study of the broadening of the rotational Raman lines of the linear molecule CO₂ perturbed by rare gases: helium, neon and argon. In the first part, the experimental set-up and the method to deduce linewidths from the spectra are presented. This method is similar to that used by Welsh et al. although we take into account the contribution of the molecules in the (01¹0) vibrational state for which the rotational quantum number J can be odd. The results for the pressure broadening coefficient are then given for several values of J. We then briefly recall how one can derive collision cross sections from the measured linewidths. The second part is devoted to an attempt to interpret the experimental results in terms of the theory of the Raman linewidths developed by Van Kranendonk. After recalling briefly the assumptions used in that theory and discussing the intermolecular potentials that are used, we present the results of numerical calculations performed with several types of anisotropic interaction potentials between CO₂ and the atom of rare gas. We reach the conclusion that the approximate methods used by Van Kranendonk (matrix elements of the evolution operator S computed by second order perturbation theory) are probably inadequate to calculate the effect of elastic collisions that disorient the molecule. It is suggested that it might be advantageous to consider anisotropic forces of shorter range than the anisotropic London dispersion forces derived from an r^-6 potential.     

Ab initio pair potential energy curve for the argon atom pair and thermophysical properties for the dilute argon gas. II. Thermophysical properties for low-density argon

A recent argon–argon interatomic potential energy curve determined from quantum-mechanical ab initio calculations and described with an analytical representation [B. Jäger, R. Hellmann, E. Bich, and E. Vogel, Mol. Phys. 107, 2181 (2009); 108, 105 (2010)] was used to calculate the most important thermophysical properties of argon governed by two-body interactions. Second pressure, acoustic, and dielectric virial coefficients as well as viscosity and thermal conductivity in the limit of zero density were computed for natural argon from 83 to 10,000?K. The calculated values for the different thermophysical properties are compared with available experimental data and values computed for other argon–argon potentials. This extensive analysis shows that the proposed potential is superior to all previous ones and that the calculated thermophysical property values are accurate enough to be applied as standard values for the complete temperature range of the calculations.     

Theoretical calculation for the high pressure properties of alkali fluorides

Abstract

The overlap repulsive potentials of ion pairs (Li ⁺?F^?, Na⁺?F^? and K⁺?F^?) are calculated by means of generalized Heitler-London method and the numerical results are fitted in the Born-Mayer form, i.e. Dexp (—αR), Then the parameter D for each pair potential is modified to satisfy the crystal equilibrium condition at the experimentally known lattice constant. The redetermined repulsive potentials are applied to calculate the cohesive energies, bulk moduli, equations of state and NaC1–to-CsC1 structural phase transition pressures of LiF, NaF and KF crystals. The results obtained are compared with some other theoretical values and the available experimental data, and good agreement is reached.     

Computer modelling of BaY2F8: defect structure, rare earth doping and optical behaviour

BaY₂F₈, when doped with rare earth elements, is a material of interest in the development of solid-state laser systems, especially for use in the infrared region. This paper presents the application of a computational technique, which combines atomistic modelling and crystal field calculations, in a study of rare earth doping of the material. Atomistic modelling is used to calculate the intrinsic defect structure and the symmetry and detailed geometry of the dopant ion-host lattice system, and this information is then used to calculate the crystal field parameters, which are an important indicator in assessing the optical behaviour of the dopant-crystal system. Energy levels are then calculated for the Dy³⁺-substituted material, and comparisons with the results of recent experimental work are made. PACS 61.50.Ah; 61.72.Bb; 78.20.Bh     

Spectroscopic properties of the molecular ions BeX+ (X=Na,K, Rb): forming cold molecular ions from an ion–atom mixture by stimulated Raman adiabatic process

In this theoretical work, we calculate potential energy curves, spectroscopic parameters and transition dipole moments of molecular ions BeX⁺ (X=Na, K, Rb) composed of alkaline ion Be and alkali atom X with a quantum chemistry approach based on the pseudopotential model, Gaussian basis sets, effective core polarisation potentials and full configuration interaction. We study in detail collisions of the alkaline ion and alkali atom in quantum regime. Besides, we study the possibility of the formation of molecular ions from the ion–atom colliding systems by stimulated Raman adiabatic process and discuss the parameters regime under which the population transfer is feasible. Our results are important for ion–atom cold collisions and experimental realisation of cold molecular ion formation.     

Unified calculations of spin-Hamiltonian parameters and optical band positions for Ni+ ion in AgGaSe2 crystal

The complete diagonalisation (of energy matrix) method based on the two-spin-orbit-parameter model is applied to unifiedly calculate the spin-Hamiltonian parameters (g factors g_//, g_⊥ and hyperfine structure constants A_//, A_⊥) and optical band positions for Ni⁺ ion in silver gallium selenide (AgGaSe₂) crystal. In the model, besides the contribution due to the spin-orbit parameter of central dⁿ ion (i.e., the one-spin-orbit-parameter model in the traditional crystal-field theory), that of ligand ions are taken into account. The calculated results are reasonably consistent with the experimental values. The local structure of Ni⁺ centre in AgGaSe₂ is estimated through the calculation. The complete diagonalisation method based on the one-spin-orbit-parameter model is also applied to calculate these electron paramagnetic resonance and optical data. It is found that although the calculated optical band positions are close to those based on the two-spin-orbit-parameter model and hence to the experimental values, the calculated spin-Hamiltonian parameters (in particular, the g factors) are in disagreement with the experimental values. The latter point is further confirmed from the calculations with the perturbation method. So, for the rational calculations of spin-Hamiltonian parameters of dⁿ clusters with ligand having large spin-orbit parameter, the contributions due to spin-orbit parameters of both the central dⁿ ion and ligand ion should be contained.     

Ionization of helium in positron collisions

A new Coulomb distorted-wave method with coupled-channel target functions is used to calculate total ionization cross-sections for helium in positron collisions. Besides Slater-like orbitals we use regular Coulomb wave packets in our configurational interaction basis to describe the target continuum. The incident positron energy was varied between the ionization threshold and 500 a.u. The results are in good agreement with experimental data and other theoretical calculations. Comparing to other sophisticated distorted wave methods our model is much easier to implement and gives accurate results. As a new feature we present ionization cross-sections where the He ⁺ ion remains in the 1s ground state or excited to the 2s or 2p state. As we know there are no experimental work done to determine such cross-sections. In the case of ionization followed by 2s or 2p excitation we compared our results with other calculations.Received: 18 February 2004, Published online: 15 April 2004PACS: 34.85. + x Positron scattering     

Theoretical study of the A 3 0 + ← X 1 0 + and B 3 1 ← X 1 0 + transitions in the Cd-rare gas van der Waals molecules

Excitation spectra arising from A ³ 0 ⁺ ← X ¹ 0 ⁺ and B ³ 1 ← X ¹ 0 ⁺ electronic transitions in the Cd-rare gas (RG) van der Waals molecules are calculated using newly obtained theoretical potential curves for these species. In the molecular structure calculations, Cd²⁰⁺ and RG⁸⁺ cores are simulated by energy-consistent pseudopotentials which also account for scalar-relativistic effects and spin-orbit (SO) interaction within the valence shell. Potential energies in the Λ S coupling scheme have been obtained by means of ab initio complete-active-space multiconfiguration self consistent-field (CASSCF)/complete-active-space multireference second-order perturbation theory (CASPT2) calculations with a total 28 correlated electrons, while the SO matrix has been computed in a reduced CI space restricted to the CASSCF level. The final Ω potential curves are obtained by diagonalization of the modified SO matrix (its diagonal elements before diagonalization substituted for the corresponding CASPT2 eigen-energies). The spectroscopic parameters for the ground and several excited states of the Cd-RG complexes deduced from the calculated potential curves are in quite reasonable agreement with available experimental data. In addition, the radial Schr?dinger equation for nuclear motion was solved numerically with the calculated potentials to evaluate the corresponding vibrational levels and radial wavefunctions. The latter have been used in the calculation of the appropriate Franck-Condon factors to yield information on relative intensities of the vibrational bands of the Cd-RG complexes. The theoretical vibrational progressions are discussed in the context of experimental spectra. Received 10 August 2000 and Received in final form 7 November 2000     

On the interaction of excited alkali atoms with rare gas targets in scattering processes

A model potential calculation has been applied to evaluate scattering experiments for Na and K in the groundstate and the resonance state interacting with Ar. The model potential has only two free parameters which are determined by a best fit of the interatomic potentials to experimental results. Satisfactory agreement between calculated and experimental results is found for the differential cross sections in the groundstate and the excited state, for satellites in theK(4P),K(5P) and Na(3P) line profile, for the van der Waals constantsC ⁽⁶⁾ andC ⁽⁸⁾, the alkali ion-rare gas interaction and the vibrational energy levels of the Na-Ar molecule. As maior advantages we point out that for a given pair of atoms all these calculated data are given with one single pair of values for the free parameters and that with this set also the interatomic potentials for the higher alkali states (specifically up to 5f for Na and K) are obtained.     

Pressure-volume calculation in bcc metals using Born stability criteria

A simple analytical two-body potential φ(r) = −Ar ⁻ⁿ + B exp(−pr ^m) is considered for P-V calculations in bcc metals using Born stability criteria. It is shown that the stability of bcc metals can be expressed uniquely as a function of a parameter q (discussed in the text). The P-V calculations are done in ten bcc metals. The calculations are compared with the experimental data of shock-wave measurements and also with other potential available. It is found out that the present potential is better than the other two-body potentials in case of bcc metals. Further, the calculations done for TOEC and the first pressure derivative of SOEC are found in good agreement with the reported results.     

HNC neutron-matter calculations with the Paris potential

A formalism is developed for handling momentum-dependent potentials within the framework of hypernetted chain calculations of dense matter. Some new distribution functions are required and appropriate expressions are obtained for calculating them. The formalism is applied to neutron-matter calculations with the Paris potential. As a test, the first application is to the ¹S₀ acting in all partial waves. A more realistic calculation which includes the singlet and triplet potentials in the isospin one channels was used to calculate the equation of state of dense neutron matter and to obtain neutron-star models. LOCV calculations were performed including all partial waves, but attempts to handle state dependence in the correlation function met with complications like those encountered with momentum independent potentials. Significant features of the results with the Paris potential are discussed.     

Ab initio potential energy curve for the helium atom pair and thermophysical properties of the dilute helium gas. II. Thermophysical standard values for low-density helium

A helium–helium interatomic potential energy curve determined from quantum-mechanical ab initio calculations and described with an analytical representation considering relativistic retardation effects (R. Hellmann, E. Bich, and E. Vogel, Molec. Phys. (in press)) was used in the framework of the quantum-statistical mechanics and of the corresponding kinetic theory to calculate the most important thermophysical properties of helium governed by two-body and three-body interactions. The second pressure virial coefficient as well as the viscosity and thermal conductivity coefficients, the last two in the so-called limit of zero density, were calculated for ³He and ⁴He from 1 to 10 000 K and the third pressure virial coefficient for ⁴He from 20 to 10 000 K. The transport property values can be applied as standard values for the complete temperature range of the calculations characterized by an uncertainty of ±0.02% for temperatures above 15 K. This uncertainty is superior to the best experimental measurements at ambient temperature.     

Theoretical study of the MgAr molecule and its ion Mg<Superscript>+</Superscript>Ar: potential energy curves and spectroscopic constants

The adiabatic potential energy curves of the low-lying electronic states of the MgAr molecule dissociating into Mg (3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p)+Ar have been investigated. The electronic structure of the Mg-Ar molecule is calculated using [Mg²⁺] and [Ar] core pseudopotentials complemented by the core polarization operators for both atoms, considering the molecule to be a two-electron system. The derived spectroscopic constants of the ground state and lower excited states are in good agreement with available experimental and theoretical work. In addition, for the purpose of checking the pseudopotential accuracy on a simpler related system, low lying potential energy curves of the single active electron Mg⁺Ar ion are also reported and the corresponding molecular constants are compared with those in the existing literature.