Noble Gas Solids Research Articles

Self-consistentGWcalculations for semiconductors and insulators

We present $GW$ calculations for small and large gap systems comprising typical semiconductors (Si, SiC, GaAs, GaN, ZnO, ZnS, CdS, and AlP), small gap semiconductors (PbS, PbSe, and PbTe), insulators (C, BN, MgO, and LiF), and noble gas solids (Ar and Ne). It is shown that the ${G}_{0}{W}_{0}$ approximation always yields too small band gaps. To improve agreement with experiment, the eigenvalues in the Green's function $G$ $(G{W}_{0})$ and in the Green's function and the dielectric matrix $(GW)$ are updated until self-consistency is reached. The first approximation leads to excellent agreement with experiment, whereas an update of the eigenvalues in $G$ and $W$ gives too large band gaps for virtually all materials. From a pragmatic point of view, the $G{W}_{0}$ approximation thus seems to be an accurate and still reasonably fast method for predicting quasiparticle energies in simple $sp$-bonded systems. We furthermore observe that the band gaps in materials with shallow $d$ states (GaAs, GaN, and ZnO) are systematically underestimated. We propose that an inaccurate description of the static dielectric properties of these materials is responsible for the underestimation of the band gaps in $G{W}_{0}$, which is itself a result of the incomplete cancellation of the Hartree self-energy within the $d$ shell by local or gradient corrected density functionals.

UV Photolysis Products of Propiolic Acid in Noble-Gas Solids

Photolysis (193 nm) of propiolic acid (HCCCOOH) was studied with Fourier transform infrared spectroscopy in noble-gas (Ar, Kr, and Xe) solid matrixes. The photolysis products were assigned using ab initio quantum chemistry calculations. The novel higher-energy conformer of propiolic acid was efficiently formed upon UV irradiation, and it decayed back to the ground-state conformer on a time scale of approximately 10 min by tunneling of the hydrogen atom through the torsional energy barrier. In addition, the photolysis produced a number of matrix-isolated 1:1 molecular complexes such as HCCH...CO2, HCCOH...CO, and H2O...C3O. The HCCH...CO2 complex dominated among the photolysis products, and the computations suggested a parallel geometry of this complex characterized by an interaction energy of -9.6 kJ/mol. The HCCOH...CO complex also formed efficiently, but its concentration was strongly limited by its light-induced decomposition. In this complex, the most probable geometry was found to feature the interaction of carbon monoxide with the OH group via the carbon atom, and the computational interaction energy was determined to be -18.3 kJ/mol. The formation of the strong H2O...C3O complex (interaction energy -21 kJ/mol) was less efficient, which might be due to the inefficiency of the involved radical reaction.

Neutralization of solvated protons and formation of noble-gas hydride molecules: Matrix-isolation indications of tunneling mechanisms?

The (NgHNg)+ cations (Ng = Ar and Kr) produced via the photolysis of HFAr, HFKr, and HBrKr solid mixtures are studied, with emphasis on their decay mechanisms. The present experiments provide a large variety of parameters connected to this decay phenomenon, which allows us to reconsider various models for the decay of the (NgHNg)+ cations in noble-gas matrices. As a result, we propose that this phenomenon could be explained by the neutralization of the solvated protons by electrons. The mechanism of this neutralization reaction probably involves tunneling of an electron from an electronegative fragment or another trap to the (NgHNg)+ cation. The proposed electron-tunneling mechanism should be considered as a possible alternative to the literature models based on tunneling-assisted or radiation-induced diffusion of protons in noble-gas solids. As a novel experimental observation of this work, the efficient formation of HArF molecules occurs at 8 K in a photolyzed HFAr matrix. It is probable that the low-temperature formation of HArF involves local tunneling of the H atom to the Ar-F center, which in turn supports the locality of HF photolysis in solid Ar. In this model, the decay of (ArHAr)+ ions and the formation of HArF molecules observed at low temperatures are generally unconnected processes; however, the decaying (ArHAr)+ ions may contribute to some extent to the formation of HArF molecules.

Exact-exchange density-functional calculations for noble-gas solids

The electronic structure of noble-gas solids is calculated within density functional theory's exact-exchange method (EXX) and compared with the results from the local-density approximation (LDA). It is shown that the EXX method does not reproduce the fundamental energy gaps as well as has been reported for semiconductors. However, the EXX-Kohn-Sham energy gaps for these materials reproduce about 80 % of the experimental optical gaps. The structural properties of noble-gas solids are described by the EXX method as poorly as by the LDA one. This is due to missing Van der Waals interactions in both, LDA and EXX functionals.

Noble-gas solids at negative pressure

Noble-gas solids (Ne, Ar, Kr, and Xe) at negative pressure and temperatures below the triple-point temperature have been studied by path-integral Monte Carlo simulations in the isothermal-isobaric ensemble. This allows us to determine the metastability limits for these solids, and in particular the solid-gas spinodal line. The spinodal pressure ${P}_{s}$ is found to be negative in all cases, and becomes more negative as the atomic mass of the noble gas increases. The low-temperature limit for ${P}_{s}$ ranges from $\ensuremath{-}0.91\mathrm{kbar}$ for Ne to $\ensuremath{-}3.38\mathrm{kbar}$ for Xe. Quantum effects are found to affect appreciably the spinodal line at low temperatures, and make ${P}_{s}$ less negative than the classical expectancy.

Many-body effects on bandwidths in ionic, noble gas, and molecular solids

Many-body effects on bandwidths in ionic, noble gas, and molecular solids

Equations of state for solids under strong compression

The historical and theoretical background for the different mathematical forms commonly used to represent equations of state (EOS) for solids is reviewed and some criteria are discussed which allow to select some specific forms, which are not only convenient mathematical forms but bring out the physical meaning of the data. Some examples are discussed with emphasis on recent experimental and theoretical data including noble gas solids, simple metals and a few metals with special “anomalies” to illustrate the differences involved in the use of different EOS forms and to point out the specific physical reasons for differences in specific EOS forms.

Simulation of the response of the IPNS neutron time-of-flight chopper spectrometers to samples in the Compton limit

We describe the comprehensive biased Monte Carlo code SPEC_SIM used to obtain accurate estimates of instrument response functions for neutron Compton scattering studies of the atomic momentum distribution of noble gas solids and liquids. The code is tailored to simulate epithermal neutron transport in the Intense Pulsed Neutron Source (IPNS) chopper spectrometers. The simulation begins with the neutrons emitted from the face of the moderator. The neutron trajectories are traced through a curved slit Fermi rotor (chopper) and are allowed to scatter multiple times from the sample region which may contain a sample, sample cell, and set of thermal radiation shields in any of a complex set of geometries. The scattered neutrons are tracked to the detector bank where they are again allowed to scatter before being “absorbed” and converted into a time-of-flight (TOF) signal which is histogrammed. A similar simulation code calculates the TOF response of two beam monitors to the incident neutron pulse, including the effects of scattering in the walls of the monitors. All of the scattering in the simulation is treated within a generalization of the impulse approximation. The results of the Monte Carlo simulations compare very favorably with experiments on solid and liquid He, Ar, and Ne.

Defects in noble gas crystals

Direct measurements of thermal vacancy formation parameters, carried out for the whole family of noble gas crystals from xenon to the various solid heliums, reveal interesting patterns. A case study of vacancy measurements on solid neon by Schoknecht, not previously published, illustrates the conceptual advantages and experimental challenges of the direct X-ray and length method. Vacancy measurements on all noble gas solids together with related measurements of self-diffusion are systematically reviewed, the results are correlated, and comparisons are made to atomic and to bulk crystal properties. Interstitials and clusters are also mentioned. The relative importance of quantum effects and of multibody interactions is shown for point defects in each of the noble gas crystals.

On the tension of curved metal surface

The leading curvature correction δ to the surface tension of a spherical interface is defined by γ = γ (0) (1+ δC), where C is the surface curvature. In the framework of the density-functional-theory the values of δ are calculated for cesium and sodium. Semi-empirical estimations of δ are fulfilled for some simple, noble, transition metals and rare gas solids, using evaporation heats per atom and vacancy-formation energies. Numerical results indicate that δ > 0 is in a contradiction with measurements of γ for gold and lead droplets. An attempt of removal of this contradiction is carried out by re-definition of the dividing surface.

Single-Particle Momentum Distributions from Deep Inelastic Neutron Scattering (DINS)

Abstract Single-particle longitudinal momentum distributions in condensed matter are now accessible to direct measurement using eV neutrons. Some systems of particular interest include a) quantum solids and fluids, formed from 3Hie, 4He, and H2 , b) prototype molecular crystals, formed from noble gases, c) fluids such as H2 and Ne, which show deviations from classical behavior and d) mixed systems, in which guest-host interactions may be important. Quantum solids can be investigated at substantially different densities, for different structures, and further, can be compared to the corresponding fluids, with which they have significant similarities in their single-particle properties such as their momentum distributions. Noble gas solids comprise a graduated family with progressively varying importance of zero-point energy, of phonon anharmonicity, and of multi-body forces. They have also been very useful for matrix isolation studies, and eV neutron scattering can yield their center-of-mass motions, not previously seen directly. Finally, noble gas fluids can be used to investigate the nature and extent of final-state effects in the neutron scatttering. Such effects are defined to be the difference between the longitudinal "neutron Compton profile" and the longitudinal momentum distribution.

Band-structure calculations of noble-gas and alkali halide solids using accurate Kohn-Sham potentials with self-interaction correction.

The optimized-effective-potential (OEP) method and a method developed recently by Krieger, Li, and Iafrate (KLI) are applied to the band-structure calculations of noble-gas and alkali halide solids employing the self-interaction-corrected (SIC) local-spin-density (LSD) approximation for the exchange-correlation energy functional. The resulting band gaps from both calculations are found to be in fair agreement with the experimental values. The discrepancies are typically within a few percent with results that are nearly the same as those of previously published orbital-dependent multipotential SIC calculations, whereas the LSD results underestimate the band gaps by as much as 40%. As in the LSD---and it is believed to be the case even for the exact Kohn-Sham potential---both the OEP and KLI predict valence-band widths which are narrower than those of experiment. In all cases, the KLI method yields essentially the same results as the OEP.

Single-particle kinetic energies of noble-gas solids and liquids: Xenon, krypton, and argon

Direct measurements have been made on the average single-particle kinetic energies, 〈Ek〉, of a series of solid and liquid specimens of heavy noble gases near melting. To do this, the distributions of energy losses of incident eV neutrons, scattered at angles ranging from 87 to 136 deg, were analyzed on the assumptions that atoms in these specimens have a Gaussian momentum distribution n(p) and that for these experimental conditions the atoms scatter independently of one another. It is known that the solids, Ar, Kr, and Xe show considerable differences in such characteristics as (a) their respective ratios of triple point to Debye temperatures, (b) the extent of multibody force contributions to cohesion, and (c) phonon anharmonicity. Regardless, it is found for all three solids near their respective triple points that 〈Ek〉 is equal to the equipartition value (3/2)kT well within the various estimated experimental uncertainties of 3% to 7%. Moreover, liquid Kr near its triple point yields the same result, as does liquid Xe at temperature up to 1.8 times its triple point temperature. Because all these systems are expected to be classical at these temperatures, these results demonstrate applicability of this neutron recoil method to determine 〈Ek〉 even of heavy atoms in suitable cases where a value may not be known in advance.

Slow-proton reemission from noble-gas solids.

A 1-\ensuremath{\mu}sec pulsed proton beam is being used to study ${\mathrm{H}}^{+}$ thermalization and reemission from solid target surfaces in ultrahigh vacuum in order to help clarify analogous experiments using muon beams. Using a solid Ar target, vapor deposited on an \ensuremath{\approxeq}6-K Cu substrate, the reemission probability Y is 6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ at a proton implantation energy ${\mathit{E}}_{\mathrm{H}}^{+}$=1.4 keV and falls with increasing energy to 3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ at ${\mathit{E}}_{\mathrm{H}}^{+}$=5 keV and 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ at ${\mathit{E}}_{\mathrm{H}}^{+}$=15 keV. Ne exhibits a 25% larger yield, while the yield for Kr is a factor of 4 lower. The reemitted protons are slow, with kinetic energies of order 1 eV. The reemitted proton yield Y decreases with an \ensuremath{\approxeq}100-m time constant, presumably due to deposition of neutral contaminants associated with the incoming beam, and thus ruling out the possibility that the slow protons originate from surface contaminants. For Ar, the observed variation of Y with ${\mathit{E}}_{\mathrm{H}}^{+}$ is interpreted with the help of a Monte Carlo calculation of the stopping and backscattering of the incident protons.The observed magnitude of Y is significantly greater than the calculated backscattering yield at the higher values of ${\mathit{E}}_{\mathrm{H}}^{+}$. We therefore hypothesize that few-eV protons in the solid, which are considered ``stopped'' by the simulation, can diffuse a significant distance and escape into the vacuum. In our model, the diffusion length for few-eV protons in pristine solid Ar, ${\ensuremath{\lambda}}_{0}$, is found to be ${\ensuremath{\lambda}}_{0}$=(${50}_{\mathrm{\ensuremath{-}}20}^{+70}$) \AA{}. However, the diffusion length we deduce from our measurements and simulations varies with ${\mathit{E}}_{\mathrm{H}}^{+}$, possibly because of the interaction of the slow proton with its implantation trail. The vacancy density is computed to be too low for few-eV protons near the surface to be trapped at defects created by the energetic incoming particle. On the other hand, the proton neutralization probability could be dependent on the availability of free electrons in the ion trail of the implanted particle [O. E. Mogensen, J. Chem. Phys. 60, 998 (1974)]. Extension of our model to the case of positive muons suggests that an experiment to moderate 4-MeV ${\mathrm{\ensuremath{\mu}}}^{+}$ with a solid Ar target [Harshman et al., Phys. Rev. B 36, 8850 (1987)] may have underestimated ${\ensuremath{\lambda}}_{0}$ for ${\mathrm{\ensuremath{\mu}}}^{+}$ due to sample impurities. It appears that the prospects for making a slow ${\mathrm{\ensuremath{\mu}}}^{+}$ beam are better than we thought, but that remoderation of a few-keV ${\mathrm{\ensuremath{\mu}}}^{+}$ beam using an Ar surface might have an efficiency less than 1% due to the high muonium-formation probability.

Defect parameters obtained from positron-annihilation and self-diffusion experiments in silicon.

Positron-lifetime measurements have been recently conducted on silicon and led to values of 3.6\ifmmode\pm\else\textpm\fi{}0.2 eV for the vacancy-formation enthalpy and 6k\char21{}10k for the entropy. It is shown that these values are consistent with a model that has been recently suggested for metals and noble-gas solids. The same model applies to the activation enthalpy and entropy obtained from self-diffusion studies on silicon.

Infrared spectra of HF complexes in noble-gas solids

The very weak hydrogen-bonded complex of molecular hydrogen and HF has been prepared by condensing the neon-diluted reagents at 5 K. Infrared spectra of the H2⋯HF complex revealed a νs(HF) stretching mode at 3938 cm–1 and ν(H2) mode at 4155 cm–1, which are bathochromically shifted from the isolated molecule values of 15 and 6 cm–1, respectively. The very strong NH3⋯HF complex has been studied in solid neon and solid argon. The νs(HF) mode at 3215 cm–1 in the gas phase is shifted to 3106 cm–1 in solid neon and 3041 cm–1 at solid argon. The ν1(HF) librational mode appeared at 916 cm–1 with 2ν1 at 1679 cm–1, while ν2(NH3) was hypsochromically shifted to 1093 cm–1 in solid argon. The antihydrogen-bonded arrangement is the more stable structure for the HF⋯Cl2 complex in the gas phase; however, solid argon and neon samples reveal both antihydrogen-bonded and hydrogen-bonded complexes. In the Cl2⋯HCl case, evidence for only the hydrogen-bonded form was observed in solid argon

Erratum: Connection between the formation volume and formation Gibbs energy in noble-gas solids

In the past the authors advanced a macroscopic model that connects defect parameters of solids with the bulk modulus. Recently, the formation volume of vacancies was measured by Macrander in solid krypton. The value agrees with that calculated from the macroscopic model in conjunction with the experimental enthalpy measured by Losee and Simmons and thus provides a rigorous test of the model.

Connection between the formation volume and formation Gibbs energy in noble-gas solids

Connection between the formation volume and formation Gibbs energy in noble-gas solids

A universal thermal equation-of-state

Evidence has steadily accumulated to show that at high temperatures (above the Debye temperature, θ) the thermal pressure, P TH , of solids, is linear with T to a close approximation. This empirical finding yields a simple relationship between P, V, and T quite useful for the computation of the equation-of-state (EOS). For geophysical applications, the empirical data is, so far, limited to a few minerals, all of which are important to our geophysical models of the Earth. The same results have been found for a variety of types of solids, including alkali metals, noble gas solids, alkali halides and metals in addition to minerals. It is argued that the linearity between P TH and T is a general high-temperature property of solids. This includes minerals. Thus it is proposed that there exists a common thermal EOS which transcends the chemical bonding type and crystallographic class.

Orientational ordering, site structure, and dynamics for octahedral molecules in low temperature matrices: SF6 and SeF6 in rare gas solids

From polarization studies of high resolution IR spectra of SF6 and SeF6 trapped in noble gas solids we show that much of the structure observed for the stretching mode represents site symmetry split components for low symmetry trapping sites, the triply-degenerate ν3 mode being split into a doubly- and singly-degenerate mode. Most of the sites showing polarization are orientationally ordered with the singly-degenerate component perpendicular to the substrate. We attribute the driving force for ordering to guest–host interaction potentials which result in registry between the molecules and the (111) growth plane during deposition. The observed orientational ordering combined with high temperature annealing studies has allowed the identification of the symmetry of certain trapping sites and further analysis of vibrational dephasing dynamics. Several sites with the same nominal symmetry and structure can be tracked through the matrices discussed herein. The implications of the ordering of impurity structure in a host lattice formed by vapor deposition are discussed.