Rare Gas Systems Research Articles

Atom-Diatom Reactive Scattering Collisions in Protonated Rare Gas Systems.

The study of the dynamics of atom–diatom reactions involving two rare gas (Rg) atoms and protons is of crucial importance given the astrophysical relevance of these processes. In a series of previous studies, we have been investigating a number of such Rg RgH Rg RgH reactions by means of different numerical approaches. These investigations comprised the construction of accurate potential energy surfaces by means of ab initio calculations. In this work, we review the state-of-art of the study of these protonated Rg systems making special emphasis on the most relevant features regarding the dynamical mechanisms which govern these reactive collisions. The aim of this work therefore is to provide an as complete as possible description of the existing information regarding these processes.

Iterative combining rules for the van der Waals potentials of mixed rare gas systems

An iterative procedure is introduced to make the results of some simple combining rules compatible with the Tang-Toennies potential model. The method is used to calculate the well locations Re and the well depths De of the van der Waals potentials of the mixed rare gas systems from the corresponding values of the homo-nuclear dimers. When the “sizes” of the two interacting atoms are very different, several rounds of iteration are required for the results to converge. The converged results can be substantially different from the starting values obtained from the combining rules. However, if the sizes of the interacting atoms are close, only one or even no iteration is necessary for the results to converge. In either case, the converged results are the accurate descriptions of the interaction potentials of the hetero-nuclear dimers.

Erratum: Assessing the accuracy of the van der Waals density functionals for rare-gas and small molecular systems [Phys. Rev. B 91 , 195103 (2015)

Erratum: Assessing the accuracy of the van der Waals density functionals for rare-gas and small molecular systems [Phys. Rev. B <b>91</b> , 195103 (2015)

Assessing the accuracy of the van der Waals density functionals for rare-gas and small molecular systems

The precise description of chemical bonds with different natures is a prerequisite for an accurate electronic structure method. The van der Waals density functional is a promising approach that meets such a requirement. Nevertheless, the accuracy should be assessed for a variety of materials to test the robustness of the method. We present benchmark calculations for weakly interacting molecular complexes and rare-gas systems as well as covalently bound molecular systems, in order to assess the accuracy and applicability of rev-vdW-DF2, a recently proposed variant [I. Hamada, Phys. Rev. B 89, 121103 (2014)] of the van der Waals density functional. It is shown that although the calculated atomization energies for small molecules are less accurate rev-vdW-DF2 describes the interaction energy curves for the weakly interacting molecules and rare-gas complexes, as well as the bond lengths of diatomic molecules, reasonably well.

Collision-induced absorption (CIA) spectra and ground-state potentials of inert gas mixtures

Collision-induced absorption spectra of the rare gas systems He–Ne, He–Ar, He–Kr, He–Xe, Ne–Kr, Ne–Xe, Ar–Kr and Ar–Xe at different temperatures with the pressure second virial coefficients, viscosity and thermal conductivity have been used for deriving the empirical models of the induced dipole moment and the interaction potential. Theoretical zeroth, first and second moments of the binary spectra using various models for the induced dipole moment and interatomic potential are compared with the experimental values performed by the groups of Marteau, Bosomworth, Bucktoyarova, Bar-Ziv, Ryzhov and Frommhold. In addition, mixture diffusion coefficients and isotopic thermal factors calculated for these models are compared with experimental ones. The results show that these models are the most accurate models reported to date for these mixtures.

Argon Solid Response upon Rydberg Photoexcitation of the NO Chromosphore: Case of Using ab Initio Potential Energy Surfaces and Comparison to Similar Studied Systems

Molecular dynamics simulations of NO-doped Ar solid upon Rydberg photoexcitation of the impurity have been carried out taking into account angular dependent potential energy surfaces (PESs) in the ground and excited states. To go beyond isotropic potentials simulations, the effects of anisotropy of potentials on the structure, dynamics, and energetics are investigated by taking into account two cases, namely, the whole PESs and the isotropic parts. Results have been compared to those obtained in previous works for similar NO-doped rare gas systems. Radial distribution functions (RDF) for the ground and excited state indicate that for both cases the shell structure of the lattice is kept ordered and is characterized by well-defined bands. No influence of the anisotropy of potentials has been detected in the RDFs since the anisotropy is rather manifested at short distances. The well part, which has been proven to be unimportant for the dynamics in previous works, arises here to be important for the right simulation of the spectrum. In general, our results show a reasonable agreement with respect to the experimental values for the dynamics and energetics when ab initio potentials are used, although better results can be obtained if higher level ab initio PESs are used.

Carbondioxide rare-gas systems: Sensitivity of basis sets and double-hybrid density functionals

This study emphasizes on the performance of six newly developed double-hybrid density functionals (DHDF) in explaining the potential energy curves of different carbondioxide rare-gas systems. The basis set sensitivity has also been explored with the use of three basis sets. Our results suggest that for lighter He/Ne-CO(2) complexes, proper choice of DHDF and basis set lead to results those matches exactly with earlier calculations and also with the experiment. On the other hand, for heavier Ar/Kr-CO(2) complexes although the equilibrium separation distance matches exactly with earlier observations, the interaction energy values lie far apart. The overall investigation emphasizes on the fact that one has to tune the methods and basis sets properly to achieve good and satisfactory results.

Van der Waals Interactions in Density-Functional Theory: Rare-Gas Diatomics.

The application of conventional GGA and meta-GGA density functionals to van der Waals interactions is fraught with difficulties. Conventional functionals do not contain the physics of the dispersion interaction. To make matters worse, the exchange part alone can yield anything from severe overbinding to severe over-repulsion depending on the choice of functional. We have assessed a variety of exchange GGAs for their ability to reproduce exact Hartree-Fock repulsion energies in rare-gas systems, and we find that PW86 [ Phys. Rev. B 1986 , 33 , 8800 ] performs remarkably well. The addition of a dynamical correlation GGA and the nonempirical dispersion model of Becke and Johnson [ J. Chem. Phys. 2007 , 127 , 154108 ] to PW86 gives a simple GGA plus dispersion theory yielding excellent rare-gas interaction curves for pairs involving He through Kr, with only two adjustable parameters for damping of the dispersion terms.

Popular Kohn-Sham density functionals strongly overestimate many-body interactions in van der Waals systems

We find spuriously large repulsive many-body contributions to binding energies of rare gas systems for the first three rungs of Jacob's Ladder within Kohn-Sham density functional theory. While the description of van der Waals dimers is consistently improved by the pairwise London C-6/R-6 correction, inclusion of a corresponding three-body Axilrod-Teller C-9/R-9 term only increases the repulsive error. Our conclusions based on extensive solid state and molecular electronic structure calculations are particularly relevant for condensed phase van der Waals systems.

Beyond the Lennard-Jones model: a simple and accurate potential function probed by high resolution scattering data useful for molecular dynamics simulations

Scattering data, measured for rare gas-rare gas systems under high angular and energy resolution conditions, have been used to probe the reliability of a recently proposed interaction potential function, which involves only one additional parameter with respect to the venerable Lennard-Jones (LJ) model and is hence called Improved Lennard-Jones (ILJ). The ILJ potential eliminates most of the inadequacies at short- and long-range of the LJ model. Further reliability tests have been performed by comparing calculated vibrational spacings with experimental values and calculated interaction energies at short-range with those obtained from the inversion of gaseous transport properties. The analysis, extended also to systems involving ions, suggests that the ILJ potential model can be used to estimate the behavior of unknown systems and can help to assess the different role of the leading interaction components. Moreover, due to its simple formulation, the physically reliable ILJ model appears to be particularly useful for molecular dynamics simulations of both neutral and ionic systems.

The Hydrogen Peroxide−Rare Gas Systems: Quantum Chemical Calculations and Hyperspherical Harmonic Representation of the Potential Energy Surface for Atom−Floppy Molecule Interactions

A quantum chemical exploration is reported on the interaction potentials of H2O2 with the rare gases, He, Ne, Ar, Kr, and Xe. Hydrogen peroxide (the simplest example of chiral molecule in its equilibrium geometry) is modeled as rigid except for the torsional mode around the O-O bond. However, on the basis of previous work (Maciel, G. S.; et al. Chem. Phys. Lett. 2006 432, 383), the internal mode description is based, rather than on the vectors of the usual valence picture, on the orthogonal local representation, which was demonstrated useful for molecular dynamics simulations, because the torsion around the vector joining the center-of-mass of the two OH radicals mimics accurately the adiabatic reaction path for chirality changing isomerization, following the torsional potential energy profile from equilibrium through the barriers for the trans and cis geometries. The basic motivation of this work is the determination of potential energy surfaces for the interactions to be used in classical and quantum simulations of molecular collisions, specifically those leading to chirality changes of possible relevance in the modeling of prebiotic phenomena. Particular attention is devoted to the definition of coordinates and expansion formulas for the potentials, allowing for a faithful representation of geometrical and symmetry properties of these systems, prototypical of the interaction of an atom with a floppy molecule.

Quantum Chemistry Close to the Fermi Level: Reducing Clusters to Few Active Hole and/or Electron Systems

Various approximate models to describe the electronic properties of some families of clusters are reviewed. They correspond to specific elementary situations close to the Fermi level where one or few electrons are either removed from (or added to) a closed shell wavefunction. Simple hole-particule excitations are also considered. The models discussed involve diatomics-in-molecules schemes, use of pseudopotential framework extended to replace inert atoms, and finally combinations of both techniques for excited states. Applications to electronic structure of alkaline earth clusters, rare-gas systems or chromophores interacting with rare-gas systems are given also prospects for more complex molecular nanosystems or assemblies.

Thermodynamic properties of small rare gas clusters by path integral Monte Carlo simulations: a preliminary study

A strategy for reducing the risk of non-ergodic simulations in Monte Carlo calculations of the thermodynamic properties of clusters is discussed with the support of some examples. The results obtained attest the significance of the approach for the low-temperature regime, as non-ergodic sampling of potential energy surfaces is a particularly insidious occurrence. Fourier path integral Monte Carlo techniques for taking into account quantum effects are adopted, in conjunction with suitable tricks for improving the procedure reliability. Applications are restricted to Lennard-Jones clusters of rare-gas systems.

Molecular dynamics study of electron gas models for liquid water

The frozen density electron gas model proposed by Gordon and Kim for rare gas systems has been implemented in a molecular dynamics code. This code has been applied to investigate various options for extending this scheme to inter-molecular interactions in liquid water. We have compared a number of gradient corrections to the Thomas-Fermi kinetic energy. We also explored a more empirical approach based on adaptation of the frozen molecular electron density to the condensed phase environment. Consistent with experience from force field methods, enhancement of the molecular dipole moment proved to be necessary to reproduce the properties of the liquid. The best models we investigated are a gradient corrected expansion of the simple local density Hamiltonian applied in the original Gordon and Kim model. In addition, these models observed a modified molecular electron density carrying the same dipole moment of 2.95 D as has been observed by recent ab initio molecular dynamics studies based on fully self-consistent Kohn-Sham methods. Possible implications of this finding for force field models are discussed.

Potential energy surfaces for the benzene–rare gas systems

A harmonic expansion representation of the intermolecular interaction has been exploited to obtain the potential energy surface (PES) for the C 6H 6–He, –Ne, –Ar, –Kr and –Xe systems in an analytical form. Basic data employed are binding energy, equilibrium distance and long-range attraction predicted by a semi-empirical method for selected configurations of the complexes. For those favorable cases where additional information are available the proposed PESs exhibit features in good agreement with those derived from spectroscopy and scattering experiments and/or ab initio calculations. The availability of realistic PESs expressed in an analytical form opens new perspectives of calculations in molecular dynamics and spectroscopic simulations where the benzene molecule and rare gas atoms are involved.

Low lying electronic states of rare gas–oxide anions: Photoelectron spectroscopy of complexes of O− with Ar, Kr, Xe, and N2

The negative ion photoelectron spectra of the oxide anion complexes O−Rg, Rg=Ar, Kr, and Xe, and O−N2 have been recorded. In each spectrum, two partially resolved peaks were observed, their relative intensities varying with source conditions. These peaks were assigned to photodetachment transitions from the 2Σ ground state and unresolved 2Π3/2,1/2 low-lying excited states of the anion. From our data we find dissociation energies and bond lengths for the 2Σ and 2Π anion states. Periodic trends in the bond length and dissociation energy are examined and compared to those in the isoelectronic neutral halogen rare gas systems and the effect of anisotropy in the interatomic potential and relative interaction strength is examined. From our data we find that the dissociation energies in the anion system are much larger but that the 2Σ-2Π splitting is significantly lower. In addition to the diatomic clusters, we report the photoelectron spectra of the O−Krn=2–5 and O−Xen=2–3 clusters and tabulate the vertical detachment energies and peak widths. From a comparison of the energetics and peak broadening we are able to make a determination of the general structure of the n=2 and n=3 clusters.

Quasiclassical trajectory calculations of collisional energy transfer in propane systems: Multiple direct-encounter hard-sphere model

Quasiclassical trajectory calculations of collisional energy transfer from highly vibrationally excited propane + rare gas systems are reported. This work extends our hard-sphere model (A. Linhananta and K. F. Lim, Phys. Chem. Chem. Phys., 2000, 2, 1385) to examine the variation of the internal energy during collisions with a rare bath gas. This was accomplished by recording the vibrational and rotational energy of propane after each atom–atom encounter during trajectory simulations of propane + rare gas systems. This provides detailed information of the energy flow during a collision. It was found that collisions with small number of encounters transfer energy efficiently, whereas those with many encounters do not. Detailed analyses reveal that the former collisions arise from trajectories with high initial impact parameter, whereas the latter have small initial impact parameter. The reason behind this is the dependence of collision energy transfer (CET) of large polyatomic molecules on their shape. This is connected to the well-known role of rotational energy transfer (RET) as a gateway for CET.

Multiphoton spectroscopy of NO?Rg (Rg = rare gas) van der Waals systems

The spectroscopic characterization of the electronic ground and excited state interactions of NO with different rare gases is reviewed. In its electronic ground state, the NO molecule continues to serve as one of the most important benchmark systems for investigating van der Waals interactions involving open shell molecules. Combining high resolution infrared excitation with resonance enhanced multiphoton ionization (REMPI) detection, bound intermolecular bend-stretch levels have been detected for the complexes NO-Ar and NO-Ne for the first time. In parallel with these new experimental advances, new high-level ab-initio potential energy surfaces have been calculated very recently for several NO-Rg (Rg = rare gas) systems. Predictions for the bound states based on these potentials are in excellent agreement with the high resolution infrared spectra for NO-Ar and NO- Ne, demonstrating the great accuracy achieved with current ab initio methods. The large number of excited electronic states of NO provides ideal opportunities for studying molecular interactions in different excited states. The low lying Rydberg states are especially interesting because the Rydberg electron can reside in an orbital whose size is comparable with or larger than the van der Waals radius of the complexed atom. The existing set of experimental data on Rydberg states with n = 3(l = 0, 1, 2) and n = 4(l = 0) are reviewed critically. The excited states have been investigated employing mainly REMPI spectroscopy. The data provide detailed information about the bound levels of the associated intermolecular surfaces. Most of the work has concentrated on the complexes with Ar and Ne. Additional data for the states Ã2

Electronic band structures of low-dimensional adsorbate systems: Rare-gas adsorption on transition metals

Angle-resolved photoemission data are dis-cussed for five different Xe adlayers which exhibit electronic structures of different dimensionalities. Xe adsorption on Ni (110)-(1 × 2)-3Hand the (\(\)×\(\)) R30° Xe layer on Ru (001) reveal two-dimensional (2D) Xe-derived band structures that are characteristic for hexagonal rare-gas layers. Different Xe 5p dispersion widths on Ni and on Ru are found due to the difference in the Xe-Xe nearest-neighbor distance. For three rare-gas systems (two different Xe coverages on hydrogen-modified Pt (110)-(1 × 2)-H and Kr step decoration on a Pt (997) surface) true one-dimensional (1D) band structures are found. For Xe step adsorption on Pt (997), electronic localized (0D) behavior is observed due to an enlarged Xe-Xe separation. The qualitative differences of the band structures in the case of 2D, 1D and 0D rare-gas systems are demonstrated and are explained by the different dimensionalities of the various structures.

Energy flow and excimer yields in continuous wave rare gas–halogen systems

A stable, continuous wave (cw), electron beam at 14 keV has been used to study energy flow, reaction rates, and radiative decay in rare gas plus halogen systems at high pressure. Steady state solutions to rate equations were used to isolate parameters which affect both the transient and steady state yield of 193 nm radiation from ArF* and 157 nm radiation from F2*. The scaling of pumping power density to the inverse 4.25 power and cube of the pressure allowed a wide range of reaction times, from nanoseconds to seconds, to be considered. The spectra and yields as a function of partial pressures were used to obtain energy transfer efficiencies near 10% for 193 nm in a Ne:Ar:F2,1:0.008:0.0004 mixture and near 5% for 157 nm in a Ne:F2,1:0.002 mixture at 2–3 bar pressure. Scaling to high brightness lamps and near cw lasers is possible.