Rare Gas Atoms Research Articles

Simulation of oriented collision dynamics of simple chiral molecules

AbstractWith the aim of illustrating computationally the mechanism of chiral discrimination induced by molecular collisions, we present results of extensive classical dynamics simulations of oriented collisions of rare gas atoms with prototypical chiral molecules, such as hydrogen peroxide and hydrogen persulfide. Models for the intermolecular and intramolecular interactions are based on previous quantum chemical calculations. The phenomenon of right‐left asymmetry in scattering directions is documented both by exemplary single trajectories and by angular distributions obtained by statistical averagings. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Coupled cluster study of structural properties of RgI and RgI− (Rg = He, Ne, Ar) weakly bound molecules

Structural properties, spectroscopic constants and the interaction between rare-gas atom and open and closed-shell iodine atom of weakly bound van der Waals molecules have been studied in details using CCSD(T) method. The Lennard-Jones potential is used to study the interaction and spectroscopic constants of the neutral and anionic systems. The spectroscopic constants and the interacting force between the two atoms of RgI and RgI− (Rg = He, Ne and Ar) have been calculated using our method developed recently for the weakly bound molecule in Lennard-Jones potential. The results have been compared with the theoretical and experimental data wherever available. Most of the spectroscopic constants are first reported.

Infrared spectroscopy of mass-selected metal carbonyl cations

Metal carbonyl cations of the form M ( CO ) n + are produced in a molecular beam by laser vaporization in a pulsed nozzle source. These ions, and their corresponding rare gas atom “tagged” analogs, M ( CO ) n ( RG ) m + , are studied with mass-selected infrared photodissociation spectroscopy in the carbonyl stretching region and with density functional theory computations. The number of infrared-active bands, their frequency positions, and their relative intensities provide distinctive patterns allowing determination of the geometries and electronic structures of these complexes. Cobalt penta carbonyl and manganese hexacarbonyl cations are compared to isoelectronic iron pentacarbonyl and chromium hexacarbonyl neutrals. Gold and copper provide examples of “non-classical” carbonyls. Seven-coordinate carbonyls are explored for the vanadium group metal cations (V +, Nb + and Ta +), while uranium cations provide an example of an eight-coordinate carbonyl.

Adsorption of Rare-Gas Atoms and Water on Graphite and Graphene by van der Waals-Corrected Density Functional Theory

The recently developed, efficient DFT/vdW-WF method for the inclusion of the van der Waals interactions in density functional theory (DFT), based on the use of the maximally localized Wannier functions, is applied to the study of the adsorption of rare-gas atoms (He, Ar, and Xe) and a water molecule on graphite and graphene surfaces, at three high-symmetry sites, by evaluating the equilibrium binding energies and distances. The results, compared to previous theoretical calculations and experimental estimates, are promising, particularly considering that the method is free from system-dependent fitted parameters. The crucial role of the chosen reference DFT functional is discussed in detail, and possible strategies to improve the method and overcome some present limitations are proposed.

Laser-Enabled Auger Decay in Rare-Gas Atoms

In rare-gas atoms, Auger decay in which an inner-valence shell ns hole is filled is not energetically allowed. However, in the presence of a strong laser field, a new laser-enabled Auger decay channel can open up to increase the double-ionization yield. This process is efficient at high laser intensities, where an ns hole can be filled within a few femtoseconds of its creation. This novel laser-enabled Auger decay process is of fundamental importance for controlling electron dynamics in atoms, molecules, and materials.

Infrared spectroscopy of Sc+(H2O) and Sc2+(H2O) via argon complex predissociation: The charge dependence of cation hydration

Singly and doubly charged scandium-water ion-molecule complexes are produced in a supersonic molecular beam by laser vaporization. These ions are mass analyzed and size selected in a specially designed reflectron time-of-flight spectrometer. To probe their structure, vibrational spectroscopy is measured for these complexes in the O-H stretching region using infrared laser photodissociation and the method of rare gas atom predissociation, also known as "tagging." The O-H stretches in these systems are shifted to lower frequency than those for the free water molecule, and the intensity of the symmetric stretch band is strongly enhanced relative to the asymmetric stretch. These effects are more prominent for the doubly charged ions. Partially resolved rotational structure for the Sc(+)(H(2)O)Ar complex shows that the H-O-H bond angle is larger than it is in the free water molecule. Fragmentation and spectral patterns indicate that the coordination of the Sc(2+) ion is filled with six ligands (one water and five argons).

On the accuracy of DFT-SAPT, MP2, SCS-MP2, MP2C, and DFT+Disp methods for the interaction energies of endohedral complexes of the C60fullerene with a rare gas atom

Selected points on the potential energy surface for the complexes Rg@C(60) (Rg = He, Ne, Ar, Kr) are calculated with various theoretical methods, like symmetry-adapted perturbation theory with monomers described by density functional theory (DFT-SAPT), supermolecular Møller-Plesset theory truncated on the second order (MP2), spin-component-scaled MP2 (SCS-MP2), supermolecular density functional theory with empirical dispersion correction (DFT+Disp), and the recently developed MP2C method that improves the MP2 method for long-range electron correlation effects. A stabilization of the endohedral complex is predicted by all methods, but the depth of the potential energy well is overestimated by the DFT+Disp and MP2 approaches. On the other hand, the MP2C model agrees well with DFT-SAPT, which serves as the reference. The performance of SCS-MP2 is mixed: it produces too low interaction energies for the two heavier guests, while its accuracy for He@C(60) and Ne@C(60) is similar to that of MP2C. Fitting formulas for the main interaction energy components, i.e. the dispersion and first-order repulsion energies are proposed, which are applicable for both endo- and exohedral cases. For all examined methods density fitting is used to evaluate two-electron repulsion integrals, which is indispensable to allow studies of noncovalent complexes of this size. It has been found that density-fitting auxiliary basis sets cannot be used in a black-box fashion for the calculation of the first-order SAPT electrostatic energy, and that the quality of these basis sets should be always carefully examined in order to avoid an unphysical long-range behavior.

Low energy positron interactions with rare gas atoms: threshold features and benchmark cross sections

Using a high resolution (~60 meV FWHM) positron beam from a Surko buffer-gas trap, we have measured grand total cross sections as well as total elastic, positronium formation and electronic excitation cross sections in the rare gases. Strong threshold (Wigner) cusps are observed in the elastic scattering channel in all the rare gases at the opening of the positronium formation channel. We present low energy elastic scattering measurements for neon which show excellent agreement with recent theory and potentially establish this system as a 'benchmark'.

Annealing Effects of Microstructure in Thin-film Silicon Solar Cell Materials Measured by Effusion of Implanted Rare Gas Atoms

ABSTRACTIn thin film silicon solar cell technology, annealing (heat treatment) effects are of interest since (i) annealing of underlying films often cannot be avoided during deposition and (ii) heat treatment (e.g. by laser) may be actively used for improvement of as-deposited material. Changes in the microstructure of several thin film silicon solar cell materials like hydrogenated amorphous silicon, microcrystalline silicon and zinc oxide by heat treatment were investigated by effusion measurements of hydrogen and implanted helium. Densification is observed for all materials studied, i.e. interconnected voids disappear or are transformed to isolated voids. We attribute the observed annealing effects primarily to an incomplete polymerization during growth. Important for solar cell processing is the result that the annealing effects involving structural changes set in at temperatures close to the temperature of deposition.

A stochastic, local mode study of neon–liquid surface collision dynamics

Equations of motion for a fast, light rare gas atom passing over a liquid surface are derived and used to infer the dynamics of neon collisions with squalane and perfluorinated polyether surfaces from experimental data. The equations incorporate the local mode model of a liquid surface via a stochastic process and explicitly account for impulsive collisional energy loss to the surface. The equations predict angular distributions for scattering of neon that are in good quantitative agreement with experimental data. Our key dynamical conclusions are that experimental angular distributions derive mainly from local mode surface topography rather than from structural features of individual surface molecules, and that the available data for these systems can be accounted for almost exclusively by single collisions between neon atoms and the liquid surface.

Quadrupole polarizabilities of the rare-gas homonuclear diatoms and methane molecules

The contribution of the quadrupole polarizability C to the long-range polarizability of two interacting rare-gas atoms and methane molecules is specified by fitting the trace and anisotropy models to the intensity of the isotropic and anisotropic interaction-induced light scattering spectra. Good agreement with ab initio results in the literature is obtained and the first few even moments of these profiles are in excellent agreement with experiments.

Dynamics and mechanism of the non-adiabatic transitions from the ungerade I2(D0+u) state induced by collisions with rare gas atoms

The stepwise three-photon two-color laser excitation scheme is used for selective population of the first-tier ion-pair D0(+)(u) state of molecular iodine. Collection and analysis of the luminescence after the excitation of the v(D) = 6, 8, 13 and 18 vibronic levels of the D state in the pure iodine vapor and the gas-phase mixtures with He, Ar and Xe provide the total and, whenever possible, partial rate constants for the collision-induced non-adiabatic transitions to the other ion-pair states of the first tier. Comparison with the analogous data obtained previously for the non-adiabatic transitions from the E0(+)(g) state reveals the similarity between two cases. For He, the D ↔ E transitions are preferable, whereas for Ar and Xe transitions to the D' and β states dominate at v(D) = 6, 8 and 13, in accord with the statistical considerations. Efficient population of the δ state at v(D) = 18 in Ar and Xe is the most prominent non-statistical feature observed. The vibrational product state distributions for the D → E transitions are also obtained. In contrast to the previously studied E → D transition, they show significant positive vibronic energy transfer. The measurements for He and Ar are accompanied by the quantum scattering calculations that reproduce well the main qualitative features of the experimental results.

Recombination-pumped triatomic hydrogen infrared lasers

Mid-infrared laser lines observed in hydrogen/rare gas discharges are assigned to three-body recombination processes involving an electron, a rare gas (He or Ne) atom, and the triatomic hydrogen ion (H(3)(+)). Calculations of radiative transitions between neutral H(3) Rydberg states support this interpretation, and link it to recent results for hydrogenic∕rare gas afterglow plasmas. A mechanism for the population inversion is proposed, and the potential generality and astrophysical implications of such molecular recombination laser systems are briefly discussed.

Threshold photoelectron source for the study of low-energy electron scattering: Total cross section for electron scattering from krypton in the energy range from 14 meV to 20 eV

An experimental technique for the measurement of the total cross section for electron scattering from atoms and molecules at high resolution is described. The total cross sections for electron scattering from Kr in the energy range from 14 meV to 20 eV obtained with the technique are also reported. The present technique employs a combination of the penetrating field technique and the threshold photoionization of rare-gas atoms using synchrotron radiation as an electron source in order to produce a high-resolution electron beam at very low energy. The characteristics of the electron sources were determined by measuring the ionizing photon energy dependence of photoelectron yield. Absolute total cross sections for electron scattering are obtained by the attenuation method. The measured absolute values of the total cross sections for electron scattering from Kr agree with those obtained by other groups down to 175 meV, above which several experimental works have been reported. Below 175 meV, the present results generally agree with theoretical cross sections down to 14 meV. The resonant structures in the total cross sections due to Kr${}^{\ensuremath{-}}$ ($4{p}^{5}5{s}^{2 2}{P}_{3/2}$) and the Kr${}^{\ensuremath{-}}$ ($4{p}^{5}5{s}^{2 2}{P}_{1/2}$) Feshbach resonances are also reported. The resolution of the present setup has been estimated from a fit of the measured profile of the Kr${}^{\ensuremath{-}}$ ($4{p}^{5}5{s}^{2 2}{P}_{3/2}$) resonance by the theoretical curve obtained from the resonant scattering theory.

Dispersion Interactions between Rare Gas Atoms: Testing the London Equation Using ab Initio Methods

A computational chemistry experiment is described in which students can use advanced ab initio quantum mechanical methods to test the ability of the London equation to account quantitatively for the attractive (dispersion) interactions between rare gas atoms. Using readily available electronic structure applications, students can calculate the interaction energies of the four rare gas dimers (He2, Ne2, Ar2, and Kr2) and the six heterodimers at internuclear distances large enough to warrant neglecting repulsive interactions. The London C6 coefficients, obtained from experimental polarizability volumes and ionization energies, are used with the calculated interaction energies to test the London equation. The role of the higher-order C8 term in accounting for dispersion interactions in the dimers is examined. In addition, students calculate a portion of the argon dimer interaction potential at large internuclear separations and compare this potential with that predicted by the London equation.

Enhancements of rescattered electron yields in above-threshold ionization of molecules

In above-threshold ionization of rare-gas atoms, photoelectron spectra recorded in the ${10}^{13}$--${10}^{14}$ W cm${}^{\ensuremath{-}2}$ range exhibit enhancements in the rescattering plateaus that do not have a unified theoretical interpretation yet. Here an experimental search for such enhancements is reported in simple molecules with ionization potentials near those of rare-gas atoms such as ${\mathrm{H}}_{2}$ and ${\mathrm{N}}_{2}$ for argon and ${\mathrm{O}}_{2}$ for xenon, and in other molecules such as CO${}_{2}$ and ${\mathrm{N}}_{2}$O. Only ${\mathrm{H}}_{2}$ exhibits the enhancements previously observed in atoms. The ${\mathrm{H}}_{2}$ particularity is interpreted in terms of its simpler ion structure and associated ionization paths compared with other molecules.

Theory of below-threshold kinetic electron emission

We investigate the electron emission from clean monotonically flat crystalline aluminum surfaces due to the impact of rare gas atoms at the surface under grazing angles of incidence. Recent improvements of experimental methods allowed for the first time the measurement of electron yields as small as γ ≈ 0.01 electrons/projectile. Using these methods kinetic electron emission (KE) was found for projectile velocities well below the classical threshold vth for KE, the minimum velocity a projectile has to have in order to transfer in a binary collision enough momentum to an electron at the Fermi edge (with k = kF) to overcome the surface potential. Quantum mechanical effects smear out the Fermi edge creating electrons with momenta above kF. We present a calculation of γ in the below threshold region within the framework of a classical trajectory Monte Carlo simulation in which special emphasis is put on the description of the projectile trajectory and on an accurate determination of the momentum distribution, i.e. the Compton profile of the surface electronic structure. We employ different methods for calculating the electronic structure and study its influence on. We find that realistic momentum distributions can account for kinetic electron emission in the below threshold region.

Direct Monte Carlo simulation of the sympathetic cooling of trapped molecules by ultracold argon atoms

We demonstrate that cold molecules (H2 and benzene) can be created at temperatures below 1 mK by sympathetic cooling with laser-cooled rare gas atoms on timescales of seconds. The thermalization process is studied using the direct simulation Monte Carlo (DSMC) method, which allows a detailed analysis of the atomic and molecular spatial and energy distributions as a function of time. As part of this study, ultracold elastic cross sections for Ar–Ar and Ar–C6H6 are also calculated.

Direct Test of Laser Tunneling with Electron Momentum Imaging

Tunneling is often used to describe multiphoton ionization of rare gas atoms in infrared fields. We test the tunneling approximation and its nonadiabatic extension by measuring the unperturbed momentum distribution along the κ direction of a circularly polarized light pulse. We find substantial, but not total, agreement between our results and the predictions of the model. As predicted, the κ direction momentum distribution is Gaussian and its width increases with the square root of electric field strength. However, the width is 15% too large and we find no evidence of nonadiabatic effects as we approach the expected limits of the approximation.

Multiphoton ionization of atoms with soft x-ray pulses

The new soft x-ray free-electron laser in Hamburg (FLASH) has opened the doorway to totally new experiments of materials research on nanometre and femtosecond scales. However, the mechanisms of photon–matter interaction are not well understood under the conditions of ultra-high photon intensities in conjunction with short wavelengths. In this context, we have quantitatively investigated nonlinear photoionization of rare gas atoms at FLASH by ion mass-to-charge spectroscopy and, thus, also the limits for the application of gas-ionization detectors for the characterization of x-ray lasers. By strong beam focusing, we have achieved irradiance levels beyond 1013 W cm−2 at about 40 eV photon energy and up to 1016 W cm−2 in the extreme ultraviolet at about 90 eV. Here, surprisingly high degrees of photoionization were observed on Xe atoms. By comparison with other rare gas targets, it emerged that the excitation of inner-shell resonances might play a significant role in the degree of atomic perturbation by the radiation field.