Neon Dimer Research Articles

A minimal distortion localization of occupied orbitals in supermolecule calculations of intermolecular interactions

A minimal distortion localization procedure is devised for defining subsystems in super-molecule calculations on weakly bonded complexes. When a set of orbitals for the isolated subsystem have been chosen, the localized supersystem orbitals are obtained by minimizing the sum of the least-square deviations from the isolated subsystem orbitals. Test calculations are presented for the beryllium complexes Be2, Be3 and Be4, and the neon dimer.

An unusual basis set superposition error in the electric field gradient of the neon dimer calculated with bond functions

In the course of studies on the longitudinal NMR relaxation time in liquid neon-21 the electric field gradient was calculated as a function of the neon-neon distance. A basis set including bond functions applied successfully in the calculation of a pair potential showed a strong and slowly decaying basis set superposition error. The full counterpoise correction eliminated this long tail nearly completely.

High resolution threshold photoelectron spectroscopy of rare gas dimers

Information on the structure of the ions of neon, argon, krypton and xenon dimers has been obtained using threshold photoelectron spectroscopy and synchrotron radiation under conditions of high resolution. Vibrational structure has been well resolved for the ground state of all these species thus allowing accurate values for the spectroscopic constants to be derived. Structure corresponding to excited states of the dimer ions dissociating to the 2P3/2,1/2 levels of the atomic ion have also been identified and their dissociation energies determined.

A new ab initio potential for the neon dimer and its application in molecular dynamics simulations of the condensed phase

An ab initio potential for the neon dimer is derived yielding about 92% of the experimentally estimated interaction energy. In addition to large basis sets on the nuclei, bond functions are utilized for polarization in Moller-Plesset MP4(SDTQ) calculations. The analytical potential was used to solve the nuclear Schrodinger equation for comparison with and prediction of spectroscopic properties and to perform classical molecular dynamics simulations in the condensed phase at high pressures. In the supercritical state at room temperature and pressures up to 1000 MPa the error in the predicted pressure is always less than 4%.

Thermodynamical and structural properties of neon in the liquid and supercritical states obtained from ab initio calculations and molecular dynamics simulations

Thermodynamical and structural properties including the equation of state, the second virial coefficient, the enthalpy and internal energy, the molar heat capacity, the speed of sound, the thermal expansion and pressure coefficients, the compressibility, and the pair distribution function are calculated in an ab initio approach for supercritical and liquid neon. The neon dimer potential energy curve has been obtained previously from ab initio calculations and is applied in classical molecular dynamics simulations. Care was taken to eliminate all possible errors thus reducing the remaining error in the supercritical state at higher temperatures to two sources, namely, the inaccuracies in the quantum chemical potential curve and the two particle approximation in the simulation. At lower temperatures, there is in addition an error due to the classical simulation. The calculated properties will be used as benchmarks in future work to investigate the influence of an improved potential curve and of an inclusion of the three particle potential in the simulation.

Ab initio calculation of the second virial coefficient of neon and the potential energy curve of Ne 2

For the ab initio calculation of properties of dense gases and liquids accurate potential energy curves are needed. Here we describe a method to obtain an “optimal” curve for a given number of ab initio calculations. Strictly speaking, we give a recipe, with which interatomic distances calculations should be performed to yield the second virial coefficient with a minimal loss in accuracy due to the limited number of points. With this technique we obtain the virial coefficient from about 15 points essentially without loss in accuracy. Experimental data for a direct test of the quality of the potential energy curve are scarce. For the neon dimer we have calculated the potential well depth, the equilibrium distance and the vibrational frequency for such a comparison with experiment. An indirect indication of the quality of the curve is given by the calculation of the second virial coefficient at different temperatures. The best calculations presented here use extended basis sets, correct for the basis set superposition error by the counterpoise method, and include electron correlation using Møller-Plesset perturbation theory on the MP4 (SDTQ) level.

Laser excitation spectroscopy of Ne2 n pπ Πg Rydberg states observed in the afterglow of a corona discharge

Several Rydberg series attributable to transitions originating from the a 3Σ+u metastable state of the neon dimer have been observed by laser excitation spectroscopy in the afterglow of a pulsed corona discharge. The n=4–9 members of an npπ Πg←a (0–0) series have been rotationally resolved and the upper states characterized. The probable multiplicity of the Πg states is discussed. An unusual loss of intensity in the Q branches for the higher n member transitions and a weak perturbation between the v=1 vibrational level of the 5fσ 3Σ+g state and the 6pπ Πg, v=0 level are also reported.

Dissociative recombination measurements at elevated temperatures in helium-neon mixtures

The temperature dependence of the electron-ion dissociative recombination (DR) rate of neon dimer ions has been measured in mixtures of helium and neon using the shock tube technique. The temperature range 500-3000 K was examined and the temperature dependence of the observed recombination was found to be independent of the composition of the helium-neon mixture and the different dimer ion formation mechanisms involved. The results obtained confirm earlier arguments about the fall-off in efficiency of DR for vibrationally excited rare-gas ions.

Emission spectrum of rare gas dimers in the vacuum UV regions. III. Ne2

Emission spectra of the neon dimer have been studied in the VUV region. Liquid nitrogen cooled neon was excited with a Tesla coil or transformer discharge. Three band systems I, II, and III were observed in the 735–1000 Å range and classified as produced by electronic transitions to the common ground state X0+g(1S0) from the three low lying excited states 1u(3P2), 0+u(3P1), and 0+u(1P1), respectively. Evidence for the existence of potential humps in the lowest two upper states were observed and the hump heights were calculated to be 142±10 and 670±50 cm−1, respectively. Observations of additional spectra, a newly observed emission continuum and two diffuse band groups, are presented.

Atomic interactions in neon and helium

The first two quantum corrections to the second virial coefficients of the Smith-Thakkar potential are calculated. Parameters for neon and helium, gases in which quantum effects are important, are then determined by fitting to semiempirical dispersion coefficients and experimental second virial coefficients. Viscosity coefficients for both gases and vibrational energy level spacings for the neon dimer are calculated as independent tests of the potentials. Overall agreement with experiment is excellent for neon and moderate for helium. The previously determined parameters for argon are found to be only very slightly perturbed by the inclusion of quantum corrections in the calculated second virial coefficients.

The determination of the intermolecular potential energy function of neon from spectroscopic, equilibrium and transport data

The vacuum u.v. absorption spectrum of the neon dimer is analysed using the Rydberg-Klein-Rees method to give information about the groundstate potential energy curve. The width of the potential energy well as a function of its depth and the dissociation energy of Ne2 are determined, and compared with the values of recently proposed potentials. New methods for the direct determination of potential energy functions from second virial coefficient and dilute gas viscosity measurements are applied to the data for neon. In conjunction with theoretical calculations and high energy molecular beam data, these results are used to characterize an intermolecular pair potential energy function for neon which is shown to be consistent with the properties of gaseous neon over a wide range of temperature.