Neutral Rare Gas Atoms Research Articles

Measurement of absolute detection efficiencies of a microchannel plate using the charge transfer reaction

A new method for determining the absolute detection efficiencies of a microchannel plate (MCP) detector is presented. This method uses a charge transfer reaction and can be applied to determine not only the detection efficiencies for ions but also those for neutral atoms. The measurements were carried out for Ar+ and neutral rare gas atoms (Ne, Ar, Kr and Xe) in the 0.5–4.8 keV energy range. The detection efficiencies for all species increase with increasing impact energy and approach the open area ratio of the MCP used (about 50%). The measured detection efficiencies were found to scale with the Lindhard, Scharff and Schiott formula for electronic stopping power at keV energies.

Alkali-metal ion in rare gas clusters: global minima

Structural optimization for heteronuclear clusters consisting of one alkali-metal ion and of up to 79 neutral rare gas atoms has been carried out. The basin-hopping Monte Carlo minimization method of Wales and Doye is used. Rare gas atoms interact with the Lennard-Jones potential, whereas the interaction between a neutral atom and an ion impurity is given by the Mason–Schamp potential. Starting from eight rare gas atoms the alkali-metal ion is always inside a cluster.

Novel Rare Gas Ions BXe+, BKr+, and BAr+ Formed in a Halogen/Rare Gas Exchange Reaction

Singly charged rare gas borynium ions were prepared for the first time by making use of mass-selected ion/molecule reactions in a triple-quadrupole mass spectrometer. Halogen for rare-gas exchange reactions leading to the formation of the novel species BAr+, BKr+, and BXe+ were carried out by allowing the radical cation BBr+• to react with neutral rare gas atoms in the second quadrupole under conditions typically used for collision-induced dissociation (CID) in tandem mass spectrometry experiments. The ions formed contain the bonds B−Ar, B−Kr, and B−Xe, the lengths of which were calculated at the B3-LYP/LANL2DZ level of theory to be 2.590, 2.635, and 2.713 A, respectively. Natural bond orbital (NBO) population analysis revealed that the bond order increases with increasing size of the rare gas atom: Ar < Kr < Xe. The reactions leading to the formation of the singly charged boron cations of Ar, Kr, and Xe are endothermic by 50.1, 45.8, and 38.7 kcal/mol, respectively. The energy required to drive these re...

Electron-atom bremsstrahlung and the sonoluminescence of rare gas bubbles

The bremsstrahlung spectra of electrons interacting with neutral rare gas atoms are calculated for temperatures from 5000 to 40 000 K. The calculations were previously shown to be in agreement with measurements of such spectra recorded under a variety of conditions. The computational results are compared with measured sonoluminescence (SL) spectra of rare gas bubbles. For the heavier rare gases, computed intensities and spectral line shapes compare favorably with the measurement at electron ``temperatures'' of roughly 20 000 K. The agreement suggests that electron--neutral-atom bremsstrahlung may be a principal mechanism of the light emission of rare gas SL bubbles if a weakly ionized SL environment may be assumed.

Relativistic many-body perturbation theory based on the no-pair Dirac-Coulomb-Breit Hamiltonian: Relativistic correlation energies for the noble-gas sequence through Rn (Z=86), the group-IIB atoms through Hg, and the ions of Ne isoelectronic sequence.

Relativistic correlation energies have been computed for the neutral rare-gas atoms Ne, Ar, Kr, Xe, and Rn, the group-IIB atoms Zn, Cd, and Hg, and the ions of the Ne isoelectronic sequence with up to Z=100 by means of a recently developed relativistic many-body perturbation theory based on the no-pair Dirac-Coulomb-Breit Hamiltonian. Analytic basis sets of Gaussian-type functions are employed to expand the upper and lower components of the Dirac four-spinors in the matrix Dirac-Fock self-consistent-field and relativistic many-body perturbation procedures. The effects of the low-frequency Breit interaction on relativistic many-body effects for high-Z neutral atoms and Ne-like highly ionized ions are examined.

Molecular dynamics simulation of Xe + ion collisions with Ar 29 and Ne 29 clusters

The attachment of Xe + ions with thermal energies ( T=500 K) to Ar 29 and Ne 29 clusters has been studied using the classical molecular dynamics (MD) simulation procedure. The dynamics and kinetics of this process is different from that of the collisions of neutral rare gas atoms with van der Waals clusters because of the long-range ion-induced dipole electrostatic forces causing the projectile to interact simultaneously with all cluster atoms. The fragmentation pattern of (Ar 29Xe +) * cluster ions produced by Xe + ion attachment was compared with that of (Ar + 29) * cluster ions produced by direct electron impact ionization and it was found that the ion attachment is a “softer” ionization process than electron impact ionization. This agrees with previous experimental results. Cross sections estimated from the MD simulations are found to be in good agreement with a modified Langevin-type cross section.

Empirical and semiempirical interaction potentials for rare gas–rare gas and rare gas–halide systems

The Tang–Toennies (TT) semiempirical model potentials for ion–atom systems is applied to the rare gas–halide negative ion exciplexes. The coefficients defining the repulsive Born–Mayer term in the TT semiempirical potentials are determined from the equilibrium bond length Re and dissociation energy De taken from ab initio calculations and from transport studies of these molecular ions. The damped dispersion and induction energy terms in the TT potentials are obtained from coupled Hartree–Fock calculations for the neutral rare gas atoms and F− and Cl− ions. The multipole polarizabilities for the heavier halogen atomic negative ions are estimated from a knowledge of polarizability ratios across isoelectronic sequences. The resultant semiempirical ionic potentials are compared to available ab initio calculations and the results of inversion of transport theory. To facilitate the comparison of the (sparse) ab initio data with the semiempirical potentials, a simple fitting procedure is presented for determining empirical potentials for diatomic molecules from a set of three constraint equations. The fitting procedure is applied to a total of 22 rare gas excimers and rare gas–halide exciplexes (both neutral and ionic) of interest to a variety of applications in gaseous discharges and excimer lasers. A three-term representation of the empirical potentials generated is accomplished with the use of a minimal data set which include the ‘‘geometric’’ parameters {R0,Re,De} and the additional parameters {αd, I.P., E.A.} needed for the dispersion and induction energy terms. A novel feature of the empirical procedure is the formulation of the constraint equations at two nuclear displacements (one constraint at R0, wherein the potential passes through zero, and two constraints at Re the equilibrium separation) which yields an accurate fit to available ab initio data and greatly extends the range of internuclear separations R for which an accurate piecewise analytical empirical potential can be generated. To test the relative importance of the different terms in the fitted three-term empirical representations, the classical orbiting cross section Qorbit(E) is computed using the full empirical potential and compared against the standard Langevin orbiting cross section Qpol(E) for a pure polarization interaction.

Charge transfer and collision-induced dissociation reactions of CO++ with the rare gases at Elab=49 eV

Multiple product channels are observed for the reaction of 13CO++ with each of the rare gases (Rg) at Elab=49±1 eV. A beam of 13CO++ is produced by electron impact ionization and is mass selected using a quadrupole mass spectrometer. The ion beam is focused into a collision region and the reaction products are monitored using time-of-flight mass spectrometry. Relative yields for the production of 13C+, O+, and 13CO+ are measured directly. Absolute charge transfer reaction cross sections for collisions of 13CO++ with He, Ne, Ar, and Kr are estimated by comparing the Rg+ production with that for the charge transfer reactions of doubly charged rare gas ions with neutral rare gas atoms. The cross sections are found to range from 0.9−0.9+1.5 Å2 for collisions of 13CO++ with He to 37.5±19.6 Å2 for collisions with Kr. The reaction of 13CO++ with He proceeds almost exclusively into the collision-induced dissociation channel. The branching fraction for collision-induced dissociation is smaller for reactions with Ne and almost disappears for Ar, Kr, and Xe. As the relative importance of the collision-induced dissociation process decreases, branching into the charge transfer channel increases. The charge transfer reactions of 13CO++ with Ar, Kr, and Xe are shown to populate excited, dissociative electronic states of 13CO+ selectively. These effects are modeled successfully using Landau–Zener theory in conjunction with reaction window theory.

Semiempirical study of rare gas and rare gas–hydrogen ionic clusters: R+n, (RnH)+, and (RnH2)+ for R≡Ar, Xe

The ionic rare gas clusters Ar+n and Xe+n and rare gas–hydrogen clusters (ArnH)+, (ArnH2)+, (XenH)+ and (XenH2)+ are studied by the semiempirical diatomics-in-ionic-systems (DIIS) method. The Ar+n clusters (n&amp;gt;3) are seen to have a structure of a linear Ar+3 core surrounded by n−3 neutral or almost neutral Ar atoms. For Xe+n (n&amp;gt;3), a symmetrical Xe+4 ionic core with the geometry of regular pyramid is formed. The rare gas–hydrogen clusters with one H atom have a simple Rk(RH)+ structure with k neutral rare gas atoms attracted to the (RH)+ molecule by polarization forces. Two H atoms can bind with Ar atoms to form quasistable clusters ArnH+2 which dissociate to (n−1)Ar+H+(ArH)+ through a high barrier of roughly 0.75 eV. Two H atoms and one Xe+ ion are shown to form a collinear valence-bound (XeHH)+ cluster whose dissociation energy is 0.46 eV.

Recent results from photon stimulated desorption of rare gas solids excited with synchrotron radiation

Desorption of neutral rare gas atoms following selective excitonic excitation of rare gas solids (Ne, Ar, Kr, Xe) by VUV photons underlines the crucial role of excitons concerning (i) transport of excitation energy from the bulk to the surface, and (ii) the microscopic desorption mechanism itself. All rare gas solids yield desorption of ground state atoms which stem from relaxation of molecular-type excitons after their radiative decay. In addition, solid Ne and Ar yield desorption of 3P1, 1P1, and metastable (3P2, 3P0) atoms. Total desorption yields as well as partial yields of excited atoms will be presented. The dominating role of primary excitation of surface excitons is obvious for excited atoms. Primary creation of free electron-hole pairs favours desorption of ground state atoms. The kinetic energy distributions of metastable Ne and Ar atoms were determined by a time-of-flight technique. Maxima were found at ≅ 0.2 eV (Ne) and 0.04 eV (Ar) which establish a new desorption mechanism of excitonic origin.

Optical emission in magnetrons: Nonlinear aspects

Optical emission measurements have been made on the plasmas generated in conventional, planar magnetrons used for high-rate sputtering of metals. The measurements were made as a function of distance from the 20-cm-diam cathode for Al, Cu, and Ti cathodes in Ne, Ar, and Kr plasmas. The emission from the plasma was coupled through a movable collimator and window assembly to a monochromator and diode array detector. The emission intensity was strongly peaked near the sheath, a few millimeters from the cathode. The intensity of the emission of the neutral rare-gas atoms was approximately linear with discharge current at low discharge power, and proportional to the square root of current at high power. This is consistent with previously observed gas rarefaction effects caused by heating by the sputtered atoms. The emission intensity from the sputtered atoms depended on discharge current to approximately the second power at low discharge current, falling to the 1.4 power in some cases at high power levels. The emission intensity for the ionized, sputtered atoms was observed to depend on the discharge power to the 2.6–3.0 power. These latter observations are consistent with a simple model of excitation dependent on the species density and the electron density, coupled with predicted changes in the electron density and temperature as a function of increasing discharge power.

Dynamic effects of surface plasmons on the lifetimes of neutral excited rare-gas atoms physisorbed on metal surfaces

Numerical calculations are presented for the optical spectra of neutral rare-gas atoms physisorbed on metal surfaces. The excitation spectra of adsorbates on metal surfaces have been calculated at low coverage and when the nonradiative processes due to the damping of the surface plasmons dominate. Numerical results indicate that the peaks of the symmetric and antisymmetric modes arising from two nearby excited states of the physisorbed atom on the metal surface cancel each other out provided that the frequency profiles of the peaks in question overlap. The resulting excited geometrical configurations arising from the cancellation process between the spectra of the symmetric and antisymmetric modes have been computed as a function of the distance R from the atom to the metal surface, and they are presented graphically. The vanishing computed spectra of Xe and Al and Au and the persistence of the spectra of Xe on Mg, Cu and Ar on Au and Mg are compatible with experimental observations. Predictions have been...

Optical excitation spectra of rare-gas atoms physisorbed on metal surfaces

Numerical computations are presented for the optical excitation spectra of neutral rare-gas atoms physisorbed on various metal surfaces. At low coverage and when the damping of the surface plasmons is much greater than the effective radiative damping, the occasional disappearance of the spectral lines of the optical absorption is due to a cancellation process, which occurs between the frequency profiles arising from two nearby excited states of the adsorbed atom. The red (blue) shifted peak of the symmetric mode of the higher (lower) excited state and the blue (red) shifted peak of the antisymmetric mode of the lower (higher) excited state of the atom cancel each other out provided that the frequency profiles of their lineshapes nearly coincide. Results of numerical calculations indicate that at low coverage the peaks of excited Xe on Ti and Au and excited Kr on Mg, in addition to those of Xe on Al and Kr on Au, vanish; this is compatible with the experimental observations. Predictions have been made for the adsorption spectra as a function of the distance from the atom to the surface of excited Xe and Kr, which are physisorbed on the surfaces of W, Ni, and Rh, respectively.

Dynamic effects of surface plasmons on the lifetimes of neutral excited rare-gas atoms physisorbed on metal surfaces Radiative and nonradiative processes

We have considered the excitation spectra of neutral rare-gas atoms physisorbed on metal surfaces. The adsorbed atom and its image interact through the dipole-dipole interaction and radiate to each other as well. The charge of the image atom is screened by the dielectric function of the surface plasmons. Due to the common radiation field between the atom and its image, the excitation spectra consist of the symmetric and antisymmetric modes, respectively. Each of them splits into two excitations: the atomic-like and the surface plasmon-like excitations, which arise because of the presence of the surface plasmons. The surface plasmon-like excitations appear near the surface plasmon frequencies and consist of broad spectral lines, which have large radiative widths and small relative intensities in comparison with those of the atomic like excitations, that emerge near the atomic frequencies. The spectral functions describing the symmetric and antisymmetric modes have been calculated in the presence of the plasmon damping and consist of asymmetric Lorentzian lines, where the extent of the asymmetry depends on the strength of the surface plasmons. Competition between the cooperative radiative and non-radiative processes takes place. In the absence of plasmon damping or when the effective radiative damping is greater than the damping of the surface plasmons, the largest enhancement of the relative intensities per atom occurs for the spectra of the symmetric modes of the excited Xe, Kr and Ar when they are physisorbed on Mg with K and Li holding the second and third place, respectively. The relative intensities per atom for the spectra of Xe, Kr and Ar on the surfaces of A1, Cu, Ag and Au are much less than the corresponding ones for the single free atoms in question, respectively. The enhancement or the decrease of the maximum relative intensity per atom is due to the dynamic effect arising from the presence of the surface plasmons. In the opposite limit, when the nonradiative process due to the damping of the surface plasmons dominates, the relative intensities per atom for the peaks of the symmetric and anti-symmetric modes take positive and negative values describing the physical processes of absorption and stimulated emission, respectively. Hence, a cancellation is expected to occur between the peaks of the symmetric and anti-symmetric modes, which arise from two nearby excited states of the physisorbed atom on the metal surface, provided that an overlap exists between the frequency profiles of the two peaks in question. The red (blue) shifted peak of the symmetric mode of the higher (lower) excited state and the blue (red) shifted peak of the antisymmetric mode of the lower (higher) excited state of the atom cancel each other out provided that the frequency profiles of their lineshapes nearly coincide. This may be a possible explanation of the persistence-extinction phenomenon that has been observed for a number of rare-gas metal substrate systems in the low coverage limit, where it has been proposed that a charge-transfer instability exists. The computed spectra of the symmetric and antisymmetric modes for excited Xe, Kr and Ar physisorbed on the metal surfaces of K, Li, Mg, Al, Cu, Ag, Au and Ti are presented graphically and discussed. Results of numerical calculations indicate that at low coverage the peaks of excited Xe on Al, Ti and Au and excited Kr on Au, vanish; this is compatible with the experimental observations. At low coverage, the height of the peaks of the spectral lines is shown to be proportional to the number of atoms at the surface of the metal N and, hence, the total spectral function describing the exctiation spectra varies as N2; this is compatible with the observed data.

Adsorbate spectra of rare-gas atoms on metal surfaces

The optical excitation spectra of neutral rare-gas atoms physisorbed on metal surfaces have been considered. Emphasis has been given to the dynamic effects of the surface plasmons on the lifetimes of the adsorbed atoms. At low coverage and when the damping of the surface plasmons is much greater than the effective radiative damping, the spectral functions of the symmetric and antisymmetric modes consist of asymmetric Lorentzian lines, whose asymmetry depends on the strength of the surface plasmons. At this limit the relative intensities of the symmetric and antisymmetric modes take positive and negative values describing the physical processes of absorption (attenuation) and stimulated emission (amplification), respectively. Hence, the occasional disappearance of the spectral lines of the optical absorption is due to a cancellation process, which takes place between the frequency profiles arising from two nearby excited states of the adsorbed atom. The red shifted peak of the symmetric mode of the higher excited state and the blue shifted peak of the antisymmetric mode of the lower excited state of the atom cancel each other out provided that their frequency profiles nearly coincide. This may be a possible explanation of the persistence-extinction phenomenon that has been observed for a number of rare-gas substrate systems in the low coverage limit, where it has been proposed that a charge-transfer instability exists. Numerical results indicate that the peaks of excited Xe on Al and excited Kr on Au vanish in the low coverage limit.

Regular Two-Component Pauli-Like Effective Hamiltonians in Dirac Theory

The standard Pauli Hamiltonian is highly singular for Coulomb potentials near the nuclei of the atoms. It is shown that the theory of effective Hamiltonians allows the determination of Pauli-like Hamiltonians that are regular enough to be used in variational calculations. A numerical illustration is given for hydrogenic atoms with atomic number varying from Z = 1 to Z = 86. These calculations yield relativistic correction potentials which are used for studying the series of neutral rare gas atoms. Our approach opens the way for accurate two-component calculations for molecules.

Atomic beam spectrometer for surface investigations

An atomic beam spectrometer for the study of the interactions of neutral rare gas atoms with solid surfaces is described. These atoms, preferably neon atoms, have a very low kinetic energy, in the order of only a few meV, so that energy gains or energy losses by surface phonon processes can be detected. The change of the momentum of these particles is given by the preset scattering angle.

Optical excitation in collision of 500–5000 eV H2+, He+, Ne+, Ar+, and N2+ ions with CO2

Electronic excitation of the products of collisions of 500–5000 eV H2+, He+, Ne+, Ar+, and N2+ ions with CO2 has been investigated by a spectroscopic study of the optical emissions from the collision products at wavelengths from 3000 to 6500 Å. For H2+ and N2+ projectile ions,the most important contribution to the observed emissions was from the A 2Πu excited state of CO2+. For He+ and Ne+ impact, however, strong emissions from Oi, Oii, Ci, and Cii in high Rydberg states produced by dissociative ionization of CO2 were observed. Emission from excited molecular dissociation fragments (CO and CO+) was weak in the latter case, but it was possible to identify fluorescence definitely from the d 3Δi and a′ 3Σ+ states of CO and the A 2Π state of CO+ as well as to identify tentatively emissions from the e 3Σ− and b 1Σ+ states of CO. Emissions from the projectile ions or fast neutral atoms produced by charge transfer were also observed. The most intense lines produced with He+, Ne+, and Ar+ projectiles originated from the neutral rare-gas atoms in Rydberg states as high as n=9. Analysis of the results shows that charge transfer, dissociative charge transfer, and direct excitation channels all contribute to the observed emissions, but the relative importance of the various channels is different for different projectile ions.

Repulsive Interaction Potentials between Rare-Gas Atoms. Homonuclear Two-Center Systems

Using a previously derived theoretical expression based on the Thomas-Fermi-Dirac statistical model of the atom, the repulsive interaction energies $U(R)$ between a homonuclear pair of neutral rare-gas atoms have been calculated at internuclear distances $R$ ranging from $0.01{a}_{0}$ to $8.0{a}_{0}$. (${a}_{0}=0.529$ \AA{}.) The present calculations show that agreement with experiment is both closer as well as more extensive than was previously estimated. Specifically, for the He-He interaction, $U(R)$ nearly coincides with other theoretical and experimental results up to $R\ensuremath{\sim}5{a}_{0}$, excepting Amdur's measurements at $R\ensuremath{\lesssim}2{a}_{0}$. Similar and progressively closer accord with experiment is obtained for the Ne-Ne and Ar-Ar interactions. For Kr-Kr and Xe-Xe, agreement is reasonable up to separations of $\ensuremath{\sim}6{a}_{0}$ and $\ensuremath{\sim}7{a}_{0}$, respectively. Curves for the Rn-Rn interaction have also been obtained. Comparison of the experimental data with Bohr's screened Coulomb potential, and with a potential proposed by Firsov, shows that for $R\ensuremath{\gtrsim}1{a}_{0}$, the former decreases far too rapidly with increasing $R$, while for $R\ensuremath{\gtrsim}3{a}_{0}$, the latter falls off much too slowly. For extrapolations towards small $R$, the (12-6) potential is found to rise rapidly to values exceeding the experimental ones by several 100%, whereas the (exp-6) potential or its repulsive part consistently remains within an order of magnitude of the empirical values and often considerably closer. Whether this latter behavior is fortuitous or not is not determined.

Ionization of Helium, Neon and Argon under Impact of their own Atoms and Positive Ions

The number of electrons liberated from neutral rare gas atoms under impact of their own atoms and positive ions was measured as a function of the kinetic energy of the impinging particles. When the impinging beam is composed mostly of neutral atoms, it is found that helium is ionized at about 60 equivalent volts, neon at about 50 volts and argon at about 40 volts. If the impinging beam is composed of atoms and positive ions in about equal proportions, additional ionization is found to set in in argon at around 330 volts. No ionization by positive ions was observed in either helium or neon up to 500 volts. Curves are shown which indicate the efficiency of ionization of the rare gases under impacts of their own atoms and positive ions.