States Of Xe Research Articles

HXeSH, the First Example of a Xenon−Sulfur Bond

Quite recently, we have reported a number of novel rare-gas compounds of type HXY (X ) Xe or Kr, and Y is a fragment with a large electron affinity). The compounds obtained so far include HXeH, HXeCl, HXeBr, HXeI, HXeCN, HXeNC, HKrCl, and HKrCN.1-4 They are prepared in rare-gas matrices at low temperatures (<50 K) by photolyzing a suitable precursor HY, which yields isolated hydrogen atoms and Y-fragments. After photolysis, thermal mobilization of hydrogen atoms leads to their reactions with the rare-gas atoms surrounding the Y-fragments. The resulting novel HXY rare-gas compounds are detected by IR spectroscopy mainly via extremely intense X-H stretching absorptions. In this report we introduce a new member of this family, HXeSH. This molecule expands the number of elements capable of making chemical bonds with xenon and represents the first compound with a xenon-sulfur bond. The electronic structure calculations for HXeSH were carried out with the Gaussian 94 package of computer codes.5 The relativistic effective core potential (ECP) by LaJohn et al. was used on Xe.6 This ECP includes the d-subshell in the valence space resulting in 18 valence electrons and was used in a decontracted form. The standard split-valence 6-311++G(2d,2p) basis set was used for hydrogen and sulfur. The calculated structural parameters of HXeSH at different computational levels are given in Table 1. Like H2S, HXeSH is also a bent molecule with an H-S-Xe angle near 90 degrees. The calculated S-H bond distance of 1.34 A in HXeSH is essentially the same as the experimental value in H2S. The Xe-H distance (1.84 A at the CCSD(T)-level) is somewhat larger than the value of free XeH+ (1.60 A)8 but much smaller than the van der Waals distance of neutral Xe-H (3.95 A).9 The calculated length of the Xe-H bond compares well with XeH2 and HXeI molecules, which both have experimental Xe-H stretching absorptions in the 1200 cm-1 region of the spectrum.1,2,10 The calculated Xe-S bond distance of 2.77 A can be compared with the recently calculated values for the excited states of Xe-S. For the 11Σ+ state (Xe+ S(1D)) and the lowest chargetransfer (Xe+S-) state 23Π, 2.62 and 3.32 A were obtained for the Re-values, respectively.11 Also, our Xe-S bond distance is near the I-S bond distance of 2.67 A for the linear S-I-S unit in [(S7I)2I)]. The calculated charge distribution indicates significant ion-pair character as in all the other HXY molecules observed thus far.1-4 Xenon carries a relatively large positive charge, and the SH group is negatively charged.

Spectroscopic Techniques using Synchrotron Radiation and Free-Electron and Conventional Lasers.

Considerable progress in the investigation of the electronic and vibrational properties of atoms, molecules, materials, surfaces and interfaces has been achieved by combining different photon sources of complementary characteristics. In this paper some experimental results obtained recently at LURE by using two synchronized sources, such as the IR free-electron laser (FEL) CLIO, the VUV storage ring FEL, synchrotron radiation and table lasers, are presented. Using CLIO synchronized with a YAG laser allows the investigation of the vibrational properties of adsorbed species by the non-linear optical technique of visible-IR sum (difference) frequency generation, as shown for the adsorption of hydrogen on platinum in the electrochemical environment. The second result reported here relates to the study of the intersubband stimulated emission in GaAs/GaAlAs quantum wells by pump-probe experiments using the two-colour configuration of CLIO. The combination of a mode-locked Ar(+) laser and synchrotron radiation has been used for investigations in a pump-probe arrangement of the ionization of Xe atoms via the resonant state Xe* 5p(5)5d [3/2](1). The final example is a time-resolved core-level spectroscopy study of photoexcited Si(111) 2 x 1 surfaces by using a combination of the naturally synchronized UV storage ring laser and synchrotron radiation.

A nano-stabilization technique for low repetition rate phase-sensitive optical experiments

A nano-stabilization technique is developed for the observation of Ramsey fringes in low repetition rate Rydberg electron wavepacket experiments. The stabilization system consists of a CCD array monitoring optical interference fringes of a reference CW laser beam passing through a Michelson interferometer, a PID feedback circuit generating an error voltage, and a piezo responding to the error voltage to compensate for geometric fluctuations in the interferometer of nm with a response time ms. The proficiency of the stabilized interferometer is demonstrated in an experiment to observe Ramsey fringes in autoionizing Rydberg states of Xe.

Production of Rg + ions in the resonance-enhanced multiphoton ionization spectroscopy of Rg · NO (Rg = Ar, Kr and Xe)

The observation of rare gas cations (Rg +) in the multiphoton ionization of Rg · NO is studied. Kr + and Xe + are produced when exciting via the à state of Rg · NO, addition to Rg · NO +. In contrast, only Xe + is seen when exciting via the C̃ state of Xe · NO, and only Kr · NO + is seen when exciting via the C̃ state of Kr · NO. Ar + is not seen when exciting via either the à or C̃ states of Ar · NO. The production of the different ions is studied as far of laser power by altering the laser intensity and focusing conditions. For excitation via the à state of Kr · NO, it is found that Kr + production is favoured by high laser intensities, with Kr · NO + being favoured by low intensities. Under tight focusing conditions for the one-colour experiments on Kr · NO, Kr + ions are also produced on NO A ← X resonances. Two-colour experimenrs are also performed on Kr · NO. The observation of Kr + on Kr · NO resonances is explained by intracomplex photoinduced charge transfer within the core of a Rydberg state, with collisional charge transfer between NO + and Kr · NO also playing a role. The production of Kr + on NO resonances is explained by photoinduced collisional charge transfer.

Role of Core Interactions and External Perturbations in the Autoionization of Xe

In a recent paper (Vrakking, M. J. J.; Lee, Y. T.; J. Chem. Phys. 1995, 102, 8833) we reported that autoionization rates of Xe Rydberg states converging on the P1/2 upper spin orbit state of Xe+ strongly depend on the presence of dc electric fields and surrounding ions. In the current paper, we present theoretical calculations that illustrate that the observed electric field dependence of the autoionization rates is influenced by the existence of short-range Rydberg electron-core interactions. In the presence of dc electric fields the autoionization rates decrease as a result of orbital angular momentum mixing. In the case of the ns′[1/2]1 series, mixing with the high-l manifold is mediated by short-range interactions with the rapidly autoionizing nd′[3/2]1 series. As a result, effective l-mixing takes place at substantially lower electric field strengths than would otherwise be the case. The experimental results of the dc electric field dependence of Xe pulsed field ionization (PFI) signals are compared with the theory, and predictions are made for sub-nanosecond timeresolved ionization experiments.

Double Auger Probabilities from Xe4dj, Kr3dj, and Ar2pjHole States

The yield ratios of triply charged ions to doubly charged ions produced from Xe + with 4 d 3/2 and 4 d 5/2 holes, from Kr + with 3 d 3/2 and 3 d 5/2 holes, and from Ar + with 2 p 3/2 and 2 p 1/2 holes have been measured using a coincidence technique between an energy-selected photoelectron and a charge-specified ion. The difference in the ratios of the double to the normal Auger transitions from the 2 p 3/2 and 2 p 1/2 hole states in Ar + and that in the ratios from the 3 d 5/2 and 3 d 3/2 hole states in Kr + are small. However the difference in those from the 4 d 5/2 and 4 d 3/2 hole states in Xe + is significant. This difference probably originates from the fact that the energy of the ground state of Xe 3+ is just below that of the 4 d 5/2 hole state of Xe + , i.e., the difference in the excess energy and the number of available states play a critical role in the transition.

Photorecombination to excited states of Xe in laser produced plasmas

In the present work, we report the frequency spectra of the continuum generated in a laser produced plasma of Xe, in the region 3000–5800 Å. From these studies it can be concluded that we have identified the photorecombination thresholds (radiative recombination, RR) to the excited levels of Xe I and Xe II and some autoionizing resonances nl−El′ transitions where with nl we label bound levels and with El′ auto-ionizing levels are indicated.

Angular distribution of Auger electrons in the decay of resonantly excited 4d3/2,5/2-1 6p states in Xe.

The angular distribution of Auger electrons originating from the decay of resonantly excited 4${\mathit{d}}^{\mathrm{\ensuremath{-}}1}$6p states in Xe has been studied with very high photon and electron energy resolution. The resolution enhancement via the Auger resonant Raman effect has enabled to resolve the \ensuremath{\beta} parameters for individual resonant Auger transitions. The results are compared to previous experiments and to theoretical predictions calculated by using the multiconfigurational Dirac-Fock approach. \textcopyright{} 1996 The American Physical Society.

Interfacial quantum well states of Xe and Kr adsorbed on Ag(111)

The energies and dispersions of the image states and quantum well electronic states in layers of Xe and Kr on a Ag(111) substrate were determined by angle-resolved two-photon photoemission (ARTPPE). For Xe, we measured binding energies of unoccupied electronic states for 1–9 layers and their parallel dispersion out to 4 layers. We measured the binding energies for a monolayer of Kr and dispersions for one and two layers. The n=2 and n=3 image states of the bare metal evolve into quantum well states of the layer (states of the Xe conduction band discretized by the boundary conditions of a 2-D slab) at higher Xe thicknesses, where the n=2,3 states exhibit both a perpendicular and parallel dispersion similar to that of the bulk Xe conduction band. The n=1 state appears to evolve with coverage as an image state screened by the Xe layer, with appreciable electron density in the vacuum. A continuum dielectric model (modified image state picture) reproduces the gross trends in the data, while an explicit quantum well analysis is used to extract the bulk Xe conduction band dispersion. A simple model which takes into account the band structures of the substrate and the overlayer, as well as the image potential, gives good agreement with the binding energy data. The combination of high energy and momentum resolution along both the surface parallel and surface normal yields very precise measurements of the bulk Xe conduction band as well as information about the behavior of conduction band electrons at interfaces.

Controlled lateral and perpendicular motion of atoms on metal surfaces.

We present the theoretical study of the controlled lateral and perpendicular motion of Xe on the Pt(111) surface. The lateral translation of Xe is manipulated by a tungsten tip of a scanning tunneling microscope. Using molecular statics and dynamics the energetics and different modes of atom translation are revealed. In the controlled and reversible transfer of Xe between two flat Pt(111) surfaces, effective charge on Xe, and the dipole moment of the Xe-Pt bond, are calculated as functions of the Xe-surface separation. The contributions of various mechanisms to the transfer rate of Xe are investigated by using the calculated quantum states of Xe under the applied bias voltage. These are tunneling and ballistic transfer, dipole excitation and excitation due to resonant tunneling of electrons, and electron wind force. We found that a single power law for the transfer rate does not exist in the whole range of applied pulse voltage. At high pulse voltage the transfer rate is dominated by the inelastic electron tunneling. At low pulse voltage the rate due to thermally assisted tunneling and ballistic transfer becomes important. \textcopyright{} 1996 The American Physical Society.

A simple optical emission method for measuring percent dissociations of feed gases in plasmas: Application to Cl2 in a high-density helical resonator plasma

Optical emission was used to determine absolute Cl2 number densities in a Cl2 high-density, helical resonator plasma that was used to etch Si. Emission from Cl2 at 306 nm was monitored, and rare-gas actinometry was used to correct for changes in the electron energy distribution. This emission band of Cl2 is ascribed in this study to a state at 8.4 eV, or possibly to one at 9.2 eV. A state of Xe at 9.8 eV, emitting at 823.2 nm, was chosen as a suitable match to the energy of the emitting state of Cl2. Absolute number density measurements were obtained by extending the measurement to extremely low powers where little dissociation occurs. This simple self-calibration method has advantages over other more complicated techniques such as laser induced fluorescence that rely on external calibrations. It is also much easier to carry out than mass spectrometry and is more sensitive than optical absorption. It is applicable to feed gases with optical emission from states with energies above about 8 eV. In this study it was found that a Cl2 helical resonator discharge is nearly completely dissociated (95%) at a power density of 0.4 W/cm3 in the glass tube source region. The percent dissociation drops to 85% below the source and above the Si substrate. Near the stainless-steel walls, fast Cl-atom recombination reduces the dissociation to ∼10% (determined by line-of-sight mass spectrometry). The high degree of dissociation in the source and near-downstream region is in agreement with recent model predictions for high-density discharges.

`Short' pulse MPI of xenon: the ionization channel

We present photoelectron spectra measured by `short' pulse ( ps) MPI of xenon at 526.5 nm excitation wavelength in the intensity range to . Ionization into and channels is observed and resonance structures are found in both ionization channels. They are induced by the AC Stark shift of bound Xe states within the light pulse which brings them into transient n-photon resonance with the Xe ground state. The resonant states are tentatively identified as d- and s-angular momentum states having AC Stark shifts which differ from the shift of the xenon ionization threshold. In the high-intensity limit, ionization into the channel is completely suppressed.

Ab initio calculations and high resolution infrared investigation on XeF4

Ab initio calculations employing effective core potentials and polarized triple-zeta basis sets have been carried out for XeF4 at the following levels: self-consistent-field (SCF) theory, Mo/ller–Plesset second-order perturbation theory (MP2), and coupled cluster theory with single and double excitations and a perturbational treatment of triple excitations (CCSD(T)). Pronounced correlation effects are found, especially for the Xe–F bond length and the vibrational frequencies. The theoretical predictions for the harmonic and anharmonic force fields and the associated spectroscopic constants have guided the analysis of the experimental data. Fourier transform infrared spectra of monoisotopic 136XeF4 have been recorded between 100–1170 cm−1 with an effective resolution of 0.002–0.004 cm−1, and a rotational analysis has been performed for the ν2 band. The high resolution results provide accurate molecular parameters for the ground state and the v2=1 excited state of XeF4 and allow a precise determination of the ground state Xe–F bond length, r0=193.487(3) pm. The agreement between the experimental and the correlated theoretical results is generally quite good.

A resonance enhanced multiphoton ionization study of the gerade excited states of Xe2 with a Xe 1S+Xe* 6s[3/2]1 dissociation limit

Low-lying gerade electronically excited states of the Xe dimer, viz., 1g and 0+g, with a Xe 1S0+Xe* 6s[3/2]1 dissociation limit have been investigated using two-photon resonance enhanced multiphoton ionization techniques. High resolution is achieved by using this method in combination with a supersonic expansion. Moreover, the mass-resolved ion detection used in the present experiments has led to the observation of both vibrationally well-resolved spectra and continuous features, allowing the bound and dissociative parts of the excited states to be investigated separately. All spectra have been measured under both linearly and circularly polarized excitation conditions, enabling an unambiguous assignment of the signals. On the basis of Franck–Condon calculations information on the potential energy curves of the bound and dissociative parts of the excited states has been obtained.

An ab initio study of the chemical bond and the 129Xe NMR chemical shifts in M+ −Xe compounds, M = Li, Na, K, Cu, Ag

Quantum chemical ab initio calculations have been performed for the interaction between ground state Xe atoms and the monopositive metal cations Li+, Na+, K+, Cu+, and Ag+. Potential energy curves have been calculated at the SCF and CEPA (coupled electron pair approach) levels. The mechanism of the bonding between Xe and the cation has been analyzed. For the alkali ions Li+, Na+, and K+ the bonds are rather weak (0.367, 0.210, 0.112 eV, respectively) and are caused mainly by inductive forces. In Cu+ −Xe and Ag+−Xe the binding energies are slightly larger (0.632 and 0.380 eV) and contain small contributions of genuine chemical effects and dispersion. 129Xe NMR chemical shifts have been calculated at the SCF level by means of the IGLO method. Only the paramagnetic part of the magnetic shielding constant σ of Xe is affected by the interaction with the cation, the diamagnetic part remains unchanged. For the alkali ions a small downfield shift is obtained which is attributed to the polarization of the Xe wave function by the charge of the cation. For the noble metal cations Cu+ and Ag+ an additional upfield shift is found which is caused by a mixing of the 54p, 4p, 3p orbitals at Xe with the 3d or 4d orbitals at the noble metals in the magnetic response function. This leads to an increased magnetic shielding. Comparison with 129Xe NMR data in X and Y zeolites is made.

Decay of the resonantly excited states of atomic Xe

The 4p −1 → 6s 1 resonant excitation energy and decay width of the Xe atom are calculated by an ab initio Green's function method. The dipolar relaxation energy shift of the 4p 3 2 −1 → 6s 1 resonantly excited state of the Xe atom is due mainly to the 4p 3 2 −16s 1 → 4d −26s 1 ϵf super-Coster-Kronig (sCK) spectator decay process. The calculated excitation energies are overestimated by 1.0 eV. For the 4p 1 2 −1 → 6s 1 resonant excitation, the quasi-particle picture of the resonant excitation breaks down completely because of the 4p 1 2 −16s 1 → 4d −26s 1 ϵf sCK spectator decay process.

Deactivation of two-photon excited Xe(5p56p,6p′,7p) and Kr(4p55p) in xenon and krypton

Lifetimes and bimolecular quenching rate constants have been determined for two-photon laser excited states of Xe*(5p56p,5p56p′,5p57p) and Kr*(4p55p) in krypton and xenon buffer gases. Collisional mixing between Kr*5p[5/2]2 and Kr*5p[5/2]3 in krypton is observed and analyzed using a coupled two-state model to obtain the rate of mixing. The measured rate constants for quenching of Xe*(6p′,7p) by krypton are 15%–20% smaller than those measured previously in xenon while bimolecular rates for the Kr*(5p) states are an order of magnitude larger in xenon than those in a krypton buffer. Measurements of state-to-state rate constants for deactivation and excitation transfer are also reported for these states in krypton and xenon buffer gases.

Ground and excited states of Xe+2 observed by high resolution threshold photoelectron spectroscopy of Xe2

Threshold photoelectron spectra of the xenon dimer have been observed with a resolution of 2 meV in the wavelength range 920–945 Å and 1022–1112 Å using the penetrating field technique and synchrotron radiation. Threshold photoelectron bands associated with transitions to the A2 Σ+1/2u, B2 Π3/2g, C2 Π3/2u, C2 Π1/2u, and D2 Σ+1/2g states of Xe+2 have been identified. Vibrational structure associated with the C2 Π1/2u state has been observed for the first time and a new value of the D2 Σ+1/2g state ionization potential is reported.

Electronic state distribution of Xe+* formed by excitation transfer from He(2 3S) to Xe+(2P3/2) at thermal energy

The formation process of Xe+* in the He afterglow reaction of Xe has been studied by observing Xe ii lines in the ultraviolet and visible regions. Sixty one Xe+* states in the 13.86–19.49 eV range were excited by the He(2 3S)+Xe+(5p5 2P03/2) excitation-transfer reaction. It was found that Xe+* was not formed by the He(2 3S)+Xe(6s 3P02) Penning type reaction and the He++Xe(6s 3P02) charge-transfer reaction. There were some unclassified Xe ii lines, which occupied 14% of the total production of Xe+*. Most of them were attributed to Xe+ transitions from unknown high energy Xe+* states in the 18–19.5 eV range. The electronic state distribution of individual Xe+* levels has been determined by taking account of radiative cascade for low lying electronic levels. The He(2 3S)+Xe+(2P03/2) reaction expressed no resonant character. The electronic state of Xe+* was distributed more widely than those of Ar+* and Kr+* in the He(2 3S)+Ar+(3p5 2P03/2), He(2 3S)+Kr+(4p5 2P01/2), and He(2 3S)+Kr+(4p5 2P03/2) reactions. The excitation mechanism of rare gas cations due to collisions between a rare gas metastable atom and a rare gas ion is discussed. The lack of the excitation processes of Kr+* and Xe+* by the He++Kr(3P02) and He++Xe(3P02) reactions was attributed to the absence of near-resonant Kr+(5p) and Xe+(6p) states whose excitation satisfies the selection rule of Δl=±1.

Intense high charge state ion sources (invited)

A review of Penning ionization gauge (PIG), duoplasmatron (DP), laser-plasma ion source (LPIS), Electron cyclotron resonance ion source (ECRIS), and electron beam ion source (EBIS) used for the production of intense and high charge state ion beams is given. The outputs of high charge states of different elements of the Periodic Table from ion sources of these types are compared. Bare nuclei up to Ar can be obtained from EBIS and ECR and Xe+54 from EBIS. Comparisons are made of compilations of the output (high average current of 107–1017 pps) for different charge states of Xe for PIG, DP, ECRIS, EBIS ion sources and Ta, Pb for different charge states of the LPIS. The present level and perspectives of development (a pulsed gas feed, stationary electric field) of the PIG ion source are presented. The beam performance and future development of these intense and high charge state ion sources are discussed from the point of view of the perspectives of the cyclotron programs at FLNR and JINR.