Rare Gas Atoms Research Articles

Анализ эффективностей процессов захвата электронов ионами в ридберговские состояния и неупругих переходов n n-=SUP=--=/SUP=- в плазме смесей инертных газов

We have carried out the comparative analysis of the efficiencies of resonant and non-resonant mechanisms of electron capture by ions into Rydberg states of atoms, Xe(n), and the inelastic n → n′ transitions between the highly-excited states in the plasmas of rare gas mixtures, Rg/Xe, containing atomic, Xe$^{+}$, and molecular, RgXe$^{+}$ and Xe$_{2}^{+}$, ions (Rg = Ne, Ar and Kr, [Xe]≪[Rg]). The rate constants of resonant electron capture by Xe$^{+}$ ions in ternary collisions with rare gas atoms, Rg($^{1}S_{0}$), and dissociative recombination of heteronuclear, RgXe$^{+}$, and homonuclear, Xe$_{2}^{+}$, ions were evaluated on the basis of the original approach within the framework of the theory of non-adiabatic transitions between electronic terms of RgXe$^{+}+e$ system. For the alternative mechanism of the three-body electron capture by Xe$^{+}$ ions in collisions with Ne, Ar and Kr atoms the calculations of the rate constants were carried out in the impulse approximation by taking into account both short-range and polarization electron-atom interactions. The rate constants of the three-body electron capture by ions and n → n′ transitions in collisions with electrons were calculated on the basis of the well-known theoretical models. We have determined the ranges of plasma ionization degree and electronic and gas temperatures as well as the principal quantum number of Xe atom which correspond to situations when the resonant free-bound and bound-bound transitions of electron play a key role.

A thermodynamic view on the microsolvation of ions by rare gas: application to Li+ with argon.

We present a thermodynamic perspective of the microsolvation of ions by rare gas atoms, which is based on parallel tempering Monte Carlo (PTMC) simulations. This allows the establishment of a clear relationship between the structure of the solvation shells and the heat capacity (CV) as a function of the number of individual solvent species. The dependence of CV on the temperature allows the identification of the internal structure rearrangements and the onset of partial or total melting of the clusters. As an application, we have employed the PTMC technique to study the thermodynamic properties of clusters resulting from the microsolvation of Li+ by argon atoms. Specifically, calculations have been carried out for the clusters Li+Arn (n = 4-18, 33, 34, and 38) by applying two different potential energy surfaces (PESs): one includes only two-body interactions, while the other also incorporates three-body contributions. Whenever possible, we compare the present thermodynamic results with global optimization studies carried out previously (F. V. Prudente, J. M. C. Marques and F. B. Pereira, Phys. Chem. Chem. Phys., 2017, 19, 25707; W. S. Jesus et al., Int. J. Quantum Chem., 2019, 119, e25860). We conclude that the melting process arises for lower temperatures when the model PES accounts for three-body interactions. Additionally, we characterize the melting processes of the first and second solvation shells. For some specific clusters, structural rearrangements of the most external argon atoms are observed at very low temperatures.

Transversely optically pumped Ar:He laser with a pulsed-periodic discharge.

Optically pumped rare gas lasers have the potential for scaling to high-power cw systems with good beam quality. Metastable atoms of heavier rare gases that are the lasing species are produced in an electric discharge at near atmospheric pressure. The key problem for this class of lasers at present is the development of a suitable discharge system. In this paper, we present the results of optimization of a pulsed discharge system with the goal of minimizing cathode sputtering and peak discharge current. The first demonstration of a transversely pumped system and measurements of the optical pumping threshold for the Ar:He laser are also presented.

High-temperature continuous molecular beam source for aggressive elements: An example of zinc.

Expansion of Zn2 or ZnRg (Rg = rare gas atom) in a molecular supersonic beam constitutes a considerable technical challenge due to the high zinc melting point and high zinc reactivity with stainless steel at high temperatures. In order to overcome these difficulties and meet the requirements for spectroscopy of van der Waals molecules containing zinc, a high-temperature source-module of the supersonic molecular beam for aggressive elements was designed. The module was tested in the laser-induced fluorescence excitation spectroscopy experiment using the b30u +43P1←X10g +(41S0) bound ← bound transitions in Zn2. The new source-module can be used for other aggressive elements for which a laser-vaporization technique has been used to date.

Pulse-length effects in strong-field ionization of atoms by co-rotating and counter-rotating bicircular laser pulses

We study strong-field ionization of H and selected rare-gas atoms by solving the three-dimensional time-dependent Schr\odinger equation in the single-active-electron approximation with particular attention to the effect of pulse length on the total ionization yields (TIYs) produced by counter-rotating bicircular (CRBC) lasers compared to co-rotating bicircular (COBC) lasers. To this end, TIYs, photoelectron momentum distributions, and above-threshold ionization spectra are presented for various pulse durations and peak intensities. For pulses containing five or more cycles of the fundamental near-infrared laser field, the CRBC/COBC ratio of TIYs is affected due to ionization via excited states and multiphoton ionization channels. For pulses containing less than five cycles, the CRBC/COBC ratio of TIYs depends on both the frequency combination and number of optical cycles: With the $1\ensuremath{\omega}\text{\ensuremath{-}}2\ensuremath{\omega}$ frequency combination, the ratio of TIYs is enhanced (reduced) for pulses with an odd (even) number of optical cycles. The $1\ensuremath{\omega}\text{\ensuremath{-}}3\ensuremath{\omega}$ frequency combination was considered for pulses containing two or three optical cycles, and for both pulse lengths the CRBC/COBC ratio of TIYs is less than unity.

Imaging a chemisorption process

Surface Chemistry At low temperatures, a molecule may adsorb to a surface only through weak forces (physisorption), and only upon heating and overcoming an energetic barrier does it form a strong covalent bond (chemisorption). Huber et al. imaged this transition for an atomic force microscopy tip terminating in a carbon monoxide molecule. Although the oxygen atom of the tip is normally considered to act like a rare gas atom, interacting only through van der Waals interactions, at short distances directly above a transition metal atom, it transitions to a strongly interacting chemisorption state. Science , this issue p. [235][1] [1]: /lookup/doi/10.1126/science.aay3444

Resonantly enhanced harmonic generation via dressed states with large Autler–Townes splittings

We investigate harmonic generation of ultrashort, picosecond pump laser pulses toward the vacuum-ultraviolet spectral regime, enhanced by preparation of dressed states in media of rare gas atoms. A control laser, with appropriately matched frequency and/or peak intensity serves to generate dressed states with large, tunable Autler–Townes splittings in the THz range. The energetic position of the dressed states is laser-controlled to match multiple pump laser photons. This enables resonance enhancement of harmonic generation also for typically fixed-frequency and off-resonant pump laser pulses. Moreover, the tunable Autler–Townes splittings also serve to compensate and even exploit inevitable Stark shifts of atomic resonances, which otherwise reduce the nonlinear response of the medium. We provide convincing experimental data to demonstrate the concept in argon and xenon, yielding enhancements of harmonic generation by 1–2 orders of magnitude.

Chemical bond formation showing a transition from physisorption to chemisorption.

Surface molecules can transition from physisorption through weak van der Waals forces to a strongly bound chemisorption state by overcoming an energy barrier. We show that a carbon monoxide (CO) molecule adsorbed to the tip of an atomic force microscope enables a controlled observation of bond formation, including its potential transition from physisorption to chemisorption. During imaging of copper (Cu) and iron (Fe) adatoms on a Cu(111) surface, the CO was not chemically inert but transited through a physisorbed local energy minimum into a chemisorbed global minimum, and an energy barrier was seen for the Fe adatom. Density functional theory reveals that the transition occurs through a hybridization of the electronic states of the CO molecule mainly with s-, p z -, and d z 2-type states of the Fe and Cu adatoms, leading to chemical bonding.

Thermoluminescence of nitrogen–neon and nitrogen–argon nanoclusters immersed in superfluid helium

Ensembles of nanoclusters created by injection of nitrogen atoms and molecules as well as rare gas (RG) atoms (Ne and Ar) into superfluid 4He have been studied via optical and electron spin resonance (ESR) spectroscopies. We studied the dynamics of thermoluminescence spectra emitted during the warming of porous structures formed by nitrogen–neon and nitrogen–argon nanoclusters inside superfluid helium. We show experimental evidence that quantum vortices initiate chemical reactions in porous ensembles of nanoclusters. Using this experimental approach, it is possible to study chemical reactions of heavy atoms and molecules at very low temperatures where normally their diffusion and quantum tunneling in solid matrices are completely suppressed.

A concept to generate ultrashort ion pulses for pump-probe experiments in the keV energy range

The impact of an energetic particle onto a solid surface generates a strongly perturbed and extremely localized non-equilibrium state, which relaxes on extremely fast time scales. In order to facilitate a time-resolved observation of the relaxation dynamics using established ultrafast pump-probe techniques, it is necessary to pinpoint the projectile impact in time with sufficient accuracy. In this paper, we propose a concept to generate ultrashort ion pulses via femtosecond photoionization of rare gas atoms entrained in a supersonic jet, combined with ion optical bunching of the resulting ion package. We calculate the photoion cloud generated by an intense focused laser pulse and show that Arq+ ions with q = 1–5 can be generated with a standard table-top laser system, which are then accelerated to energies in the keV range over a very short distance and bunched to impinge onto the target surface in a time-focused manner. Detailed ion trajectory simulations show that single ion pulses of sub-picosecond duration can be generated this way. The influence of space charge broadening is included in the simulations, which reveal that flight time broadening is insignificant for pulses containing up to 10–20 ions and starts to increase the pulse width above ∼50 ions/pulse.

ZMP-SAPT: DFT-SAPT using ab initio densities.

Symmetry Adapted Perturbation Theory (SAPT) has become an important tool when predicting and analyzing intermolecular interactions. Unfortunately, Density Functional Theory (DFT)-SAPT, which uses DFT for the underlying monomers, has some arbitrariness concerning the exchange-correlation potential and the exchange-correlation kernel involved. By using ab initio Brueckner Doubles densities and constructing Kohn-Sham orbitals via the Zhao-Morrison-Parr (ZMP) method, we are able to lift the dependence of DFT-SAPT on DFT exchange-correlation potential models in first order. This way, we can compute the monomers at the coupled-cluster level of theory and utilize SAPT for the intermolecular interaction energy. The resulting ZMP-SAPT approach is tested for small dimer systems involving rare gas atoms, cations, and anions and shown to compare well with the Tang-Toennies model and coupled cluster results.

Bonding, Reactivity, and Dynamics in Confined Systems.

Confined systems often exhibit unusual behavior regarding their structure, stability, reactivity, bonding, interactions, and dynamics. Quantization is a direct consequence of confinement. Confinement modifies the electronic energy levels, orbitals, electronic shell filling, etc. of a system, thereby affecting its reactivity as well as various response properties as compared to the cases of corresponding unconfined systems. Confinement may force two rare gas atoms to form a partly covalent bond. Gas storage is facilitated through confinement and unprecedented optoelectronic properties are observed in certain cases. Some slow reactions get highly accelerated in an appropriate confined environment. In the current Feature Article we analyze these aspects with a special emphasis on the work done by our research group.

Influence of translational motion of rare gas atoms on electron-atom Compton scattering

We report an atomic momentum spectroscopy study about how translational motion of gaseous targets affects electron-atom Compton scattering. A series of experiments on rare gas atoms in an effusive beam reveals that forward-backward asymmetry exists in energy loss for He with respect to the target beam direction, but its degree rapidly decreases for heavier atoms. Furthermore, it is shown that the results are reproduced by a gas beam model and supported by a reference experiment on He in a gas cell. We believe the present work forms a basis toward investigating intramolecular atomic motion.

Phonon spectra and vibrational heat capacity of quasi-one-dimensional structures formed by rare gas atoms on the surface of carbon nanotube bundles

The features of phonon spectra and their effect on the vibrational heat capacity of linear chains of inert gas atoms adsorbed onto a substrate, which is the surface of nanotubes bound to a nanobundle. The influence of the substrate results both in a shift of the lower limit of the chain spectrum from zero, and in mechanical stress in the chain (its extension or compression) also. It is shown that in the case of a compressed chain, the non-central interaction between atoms is negative (repulsive), it results in a shift of the lower boundary of the spectrum of transverse vibrations to low frequencies and to a shortening of the part of the specific heat temperature dependence in which this dependence is close to exponential. Heterogeneity of the nanobundle structure can cause a change in the distances between atoms of the chain. It is shown both and analytically and numerically, that as a result of it, discrete levels with frequencies both above and below the quasi-continuous spectrum band can appear in the phonon spectrum of the chain. The discrete levels with frequencies below the quasi-continuous spectrum band lead to a further shortening of the temperature interval at which the temperature dependence of the specific heat is close to the exponential one.

Vibrational analysis of methyl cation-Rare gas atom complexes: CH3 +-Rg (Rg = He, Ne, Ar, Kr).

The vibrational spectra of simple CH3 +-Rg (Rg = He, Ne, Ar, Kr) complexes have been studied by vibrational configuration interaction theory relying on multidimensional potential energy surfaces (PESs) obtained from explicitly correlated coupled cluster calculations, CCSD(T)-F12a. In agreement with experimental results, the series of rare gas atoms leads to rather unsystematic results and indicates huge zero point vibrational energy effects for the helium complex. In order to study these sensitive complexes more consistently, we also introduce configuration averaged vibrational self-consistent field theory, which is a generalization of standard vibrational self-consistent field theory to several configurations. The vibrational spectra of the complexes are compared to that of the methyl cation, for which corrections due to scalar-relativistic effects, high-order coupled-cluster terms, e.g., quadruple excitations, and core-valence correlation have explicitly been accounted for. The occurrence of tunneling splittings for the vibrational ground-state of CH3 +-He has been investigated on the basis of semiclassical instanton theory. These calculations and a direct comparison of the energy profiles along the intrinsic reaction coordinates with that of the hydronium cation, H3O+, suggest that tunneling effects for vibrationally excited states should be very small.

Role of high ponderomotive energy in laser-induced nonsequential double ionization

The laser-induced nonsequential double ionization (NSDI) of rare gas atoms in the near and mid-IR laser fields is studied experimentally and theoretically. We investigate electron-electron correlation at high recollision energies, experimentally achieving ponderomotive energies (${U}_{p}$) above 80 eV. The contribution of the two dominant channels of NSDI in the photoelectron momentum distribution, impact, and excitation ionization, are both shown to scale with ponderomotive energy and are well reproduced by theory. Surprisingly, for a large ${U}_{p}$ in mid-IR fields, a noticeable electron-electron anticorrelation signal emerges at low photoelectron momenta, which cannot be explained by these mechanisms within state-of-the-art theoretical approaches.

Ab initio interaction potentials of the Ba, Ba+ complexes with Ar, Kr, and Xe in the lowest excited states.

The complexes of the Ba atom and Ba+ cation with the rare gas atoms Ar, Kr, and Xe in the states associated with the 6s → 5d, 6p excitations are investigated by means of the multireference configuration interaction techniques. Scalar relativistic potentials are obtained by the complete basis limit extrapolation through the sequence of aug-cc-pwCVnZ basis sets with the cardinal numbers n = Q, T, 5, combined with the suitable effective core potentials and benchmarked against the coupled cluster with singles, doubles, and non-iterative triples calculations and the literature data available for selected electronic states. Spin-orbit coupling is taken into account by means of the state-interacting multireference configuration interaction calculations performed for the Breit-Pauli spin-orbit Hamiltonian. The results show weak spin-orbit coupling between the states belonging to distinct atomic multiplets. General trends in the interaction strength and long-range anisotropy along the rare gas series are discussed. Vibronic spectra of the Ba and Ba+ complexes in the vicinity of the 1S → 1P° and 2S → 2P° atomic transitions and diffusion cross sections of the Ba(1S0, 3DJ) atom in high-temperature rare gases are calculated. Comparison with available experimental data shows that multireference calculations tend to underestimate the interaction strength for excited complexes.

An empirical potential energy function for the interactions between rare gas atoms

Abstract A new potential model was developed in this work by the modification of the repulsive part of the Tang-Toennies model. The proposed potential function was used in this work to investigate the interactions between both like and unlike rare gas atoms. The considered systems are He2, Ne2, Ar2, and Kr2 for the like interactions and HeNe, HeAr, and HeKr for the unlike interactions. The potential parameters in the repulsive part of each system were determined from the recent literature values of the equilibrium distance R e , the well depth D e , and the dispersion coefficients C 6 - C 10 . Accurate potential energies available in the literature were compared with the modified potential model and the original Tang-Toennies model. Although both potential energy functions show the same performance in the attractive region and the well-depth region, the proposed potential model can provide a more realistic repulsive wall than the Tang-Toennies model. Since the transport properties of low-density gases are sensitive to the potential energies at small intermolecular distances, the viscosity and thermal conductivity computed in this work were compared with the recommended experimental data in the literature to further verify the quality of the modified potential energy function.

Two-stage excitation model of diode pumped rare gas atoms lasers.

Diode pumped rare gas atoms lasers (DPRGLs) are potential candidates of the high-energy lasers, due to the advantages of high laser power and high optical conversion efficiency. In this paper, a two-stage excitation model of DPRGLs is established including gas discharge excitation and semiconductor laser pump to study energy loss mechanism and obtain total efficiency. The results of numerical simulation agree well with those of Rawlins et al.'s experiment. Through parameter optimization, the total efficiency and optical conversion efficiency reach 51.5% and 62.7% respectively, at pump intensity of 50 kW/cm2 and reduced electric field of 8 Td. Parameter optimization of two-stage excitation lasers is theoretically studied, which is significant for the DPRGLs design with high total efficiency.

Asymmetry flip in photoelectron angular distribution of rare gas atoms in intense circularly-polarized few-cycle laser fields

An analytical formula representing the photoelectron kinetic energy at which the ejection direction of photoelectrons generated by an intense circularly-polarized few-cycle laser pulse flips was derived and was used for determining the laser pulse duration.