States Of Xe Research Articles

A linear magnetic field spectrometer for electron energy-loss measurements

A new type of electron energy-loss spectrometer has been developed, in which the electrons are confined by a unidirectional magnetic field that extends over a length of 2 m. Magnetic-field-immersed Wien filters are used for energy selection and analysis. The new spectrometer is characterized by a high collection efficiency and hence it is especially suitable for the study of processes that have low cross sections, such as excitation of autoionizing and inner-shell states. Electron energy-loss spectra of inner-shell states of Xe have been obtained with the new spectrometer.

A comparison of Kr( 3P 1) and Kr( 3P 2) quenching reactions

The relative total quenching rate constants for Kr( 3P 1) by Xe and several molecules at 300 K were measured using the sensitization technique. Electronically excited products resulting from the quenching of Kr( 3P 1) were characterized by emission spectra observed in the wide spectral range from the vacuum UV to the near IR. The results are compared with those of Kr( 3P 2). Collisions of Kr( 3P 1) and ground state Xe result in the formation of Xe( 3P 1) and all six Xe(6p) states. For the reaction of Kr( 3P 1) with CO, the endoergic product level CO(d, υ′ =22) is dominant over the exoergic product level CO(d, υ′ =21, 20).

Calculations of inelastic collisions of excited states of Xe with He and Ar

Coupled channel calculations of thermal collisions of Xe(5p5nl) (nl=6s,6p,5d) with He and Ar have been performed using potential curves and coupling matrix elements that we previously calculated. Coupling among all of the closely spaced 6s′, 6p, and 5d fine structure levels of Xe is fully included. These excited levels lie in the range 76 000–84 000 cm−1 above the Xe ground state, and the spacing of adjacent levels ranges from 84 to 1460 cm−1. Rate constants for level-to-level transitions are calculated at T=300 and T=800 and compared with recent experiments; agreement is generally good. The rate constants are very sensitive to the specific initial and final levels and to the collision partner. This sensitivity can be explained by the pattern of crossings and avoided crossings of potential curves corresponding to individual fine structure levels. The calculations thereby illustrate the importance of spin–orbit effects in heavy atom systems and provide insight into a large amount of data.

Interference between nearly resonant three-photon excitation and third-harmonic generation probed by the cancellation of four-photon resonances.

The destructive interference between the three-photon and the third-harmonic one-photon absorption in the vicinity of the 6s[3/2${]}_{1}$ state of Xe has been studied in the resonantly enhanced multiphoton ionization via nf states, through the cancellation of the nf, J=2 resonances of the spectrum, accessible by four-photon absorption. The gas-pressure, i.e., phase-matching, dependence of the three-photon detuning range from the 6s[3/2${]}_{1}$ state for which cancellation occurs has been investigated in one- and two-color experiments, including positive detuning from this state in positively dispersive regions. For obtaining cancellation, the theoretically predicted requirement of optically thick medium at the third-harmonic frequency is corroborated by present observations for negative detunings. The different cancellation behavior of the nf[3/2${]}_{2}$ and nf[5/2${]}_{2}$ states observed indicates the dependence of the effect on the atomic parameters of the coupled four-photon resonance. The experimental results are found to be in partial agreement with existing theoretical models.

High charge states of Xer+ recoil ions (r = 1 - 15) produced by multicapture processes in low energy Xe27+ collisions on Xe

Abstract Cross sections of multicapture processes for Xe27+ ions colliding with Xe at an energy of 4 keV/u are reported. Projectiles and recoil ions were detected in coincidence, allowing the determination of recoil ion charge states from time of flight measurements. For processes where the projectile stabilizes five electrons, we measured that the target exchanges up to 15 electrons. Cross sections are compared with the theoretical predictions based on the Extended Classical Barrier Model (E.C.B.M.) [10].

Single-photon ionization study of the vibrational structure in the ground state of Xe+2

The vibrationally resolved spectra of the ground state A 2 Sigma +u of Xe+2 were observed at wavelengths from 1023 AA to 1116 AA by using the method of single-photon ionization with synchrotron radiation as a light source. A long progression beginning at v=0 was clearly observed in the threshold electron spectra for the first time. The spectroscopic parameters for the ground state of Xe+2 (average mass=131.3, 131.3) calculated from the experimental data are: omega e=122.9 (+or-0.1) cm-1 omega e chi e=0.481 (+or-0.001) cm-1, the dissociation energy D0=7849 (+or-9) cm-1, the zero point energy 61.3 (+or-0.1) cm-1, Te=90 119 (+or-8) cm-1, and the adiabatic ionization potential 90 170 (+or-8) cm-1. Autoionizing Rydberg series were found and assigned to converge to the C 2 Pi 32u/ and B2 Pi 12g/ states of Xe+2.

The total photoionization cross section of barium including 7snp, 6dnp and 4fnd double excitations

The total photoionization cross section of transitions from the ground state (Xe)6s2 of neutral barium atoms to 5dnp, 5dnf, 6pns, 7snp, 6dnp and 4fnd (J=1, odd parity) doubly excited autoionizing states in the energy range between 42000 cm-1 and 89000 cm-1 has been determined. The experiment used a linear thermionic diode detector and monochromatized synchrotron radiation from the DORIS II storage ring with a bandwidth of 0.3 AA. The diode detector allowed for a significant improvement of the signal-to-noise ratio over previous measurements and is described in some detail. The photoionization cross section of the 5d and 6p channels is compared with recent calculations. For the Rydberg series converging to the 7s1/2, 6d3/2 and 6d3/2 states of Ba+ a tentative assignment for the doubly excited states in terms of the independent particle model is proposed.

Local effects in the interaction of potassium with Rh(111)

As a model system for alkali promotor effects on catalysts we studied the potassium interaction with Rh(111) by UPS, AES, LEED, TDS and work-function (WF) measurements. For the first time also rare gas probe atoms other than Xe atoms have been applied to determine the local surface potential around the adsorbed potassium atoms. The photoemission of adsorbed Ar, Kr and Xe has been measured at 45 K on Rh(111) covered with about 3 at% potassium and on clean Rh(111). On the clean surface all three physisorbed gases show a single adsorption state below 1 ML, with the position of the Ar3p12, Kr4p12andXe5p12 photoemission peak at 8.7, 7.6 and 6.7 eV, respectively. For the potassium-covered surface all three rare gas atoms exist in three different adsorption states, being associated with rare gas atoms ontop of K, next to K and on bare Rh patches. The respective p12 signal from the Rh sites (located at lower binding energy (BE)) is shifted by 0.5 eV to higher BE with respect to clean Rh(111) independent of the nature of the probe rare gas atom. In turn, the state attributed to adsorption sites next to K atoms shows a probe-atom dependent shift by 0.74 (Ar), 0.63 (Kr) and 0.53 eV (Xe) to higher BE with respect to the Xe state on Rh sites.

An attempt to observe Mott scattering optically

The authors report an attempt to make an optical observation of Mott scattering, involving the first studies of inelastic collisions between polarized electrons and a heavy noble gas. Polarization fractions of light emitted by the 5p 56p(5/2)3(3D3) state of Xe following impact excitation in an axial collision geometry were measured as a function of the incident electron energy, and compared with distorted-wave Born calculations. The theoretical and experimental results agree qualitatively in the energy range where cascade contributions to the photon signal are small. The authors failed to measure non-zero values of the linear polarization fraction eta 1 which would have constituted unambiguous evidence for Mott scattering and/or the importance of higher-order excitation processes in this system.

An electron spectrometer to record gas phase photoelectron-photoelectron coincidence spectra following double photoionization

An electron spectrometer employed to record gas phase photoelectron-photoelectron coincidence (PEPECO) spectra following valence shell single-photon double ionisation is described. The spectrometer is based on two opposed hemispherical electron energy analysers, and employs a helium discharge lamp as a source of ionizing radiation. To achieve a practical coincidence count rate each analyser is designed to gather electrons from a large solid angle. The successful operation of the experiment is illustrated by new PEPECO results recorded following double photoionization of xenon at 40.8 eV. These spectra record the excess energy distribution between the pair of ejected photoelectrons following population of the 5p-2 (3P and 1D) states of Xe2+, and confirm that the double photoionization mechanism is predominantly indirect.

Vacuum ultraviolet laser spectroscopy of Xe 2 between 116.9 and 123.6 nm

Vibrationally resolved fluorescence excitation spectra of jet-cooled Xe 2 have been recorded between 116.9 and 123.6 nm with tunable coherent radiation generated by four-wave mixing in Kr. Three molecular systems were observed. One is a new band system in the vicinity of the 119.2-nm Xe resonance line. The other two are system X and XI which have been identified by absorption spectroscopy [M. C. Castex, Chem. Phys. 5, 448–455 (1974)]. The analyses of the latter two band systems provided accurate spectroscopic constants of the upper states for the first time. The new band and X band systems were shown to be predissociated to the ground ( 1 S 0) and the excited state (5 p 56 p) Xe atoms.

Interatomic potentials for excited states of XeHe and XeAr

Interatomic potentials for the interaction of low-lying excited states of Xe(5p5nl) (nl=6s,6p,5d) with He and Ar have been calculated. A novel method has been developed that refines and extends the results of ab initio electronic structure calculations by incorporating available spectroscopic information using a model Hamiltonian. The ab initio calculations treat the heavy-atom cores using relativistic effective potentials and include spin–orbit effects at the configuration-interaction level. The model Hamiltonian depends on a small number of physically sensible parameters, some of which are extracted from the ab initio calculations, and others of which are determined more accurately from spectroscopy. The model Hamiltonian is then used to recalculate the potential curves and coupling matrix elements. The results obtained have significant implications for two classes of recent experiments. Recent measurements of rate constants for state-to-state transitions of Xe* induced by collisions with He or Ar have revealed that the magnitude of the rate depends strongly on the specific transition and collision partner. The present adiabatic potential curves exhibit several regions of strong coupling, which can be associated with the crossings of ‘‘diabatic states,’’ and which can be correlated with those transition rates found to be large. Other experiments have measured multiphoton ionization spectra of XeAr, thereby providing information on the vibrational levels of the excited-state potentials that can be compared with the present calculations.

Vibrational spectroscopy of Xe+2 via pulsed field ionization

Delayed, pulsed field ionization of extremely high-n Rydberg states has been used in combination with a high-resolution vacuum ultraviolet radiation source to measure the vibrationally resolved spectra of the first three bound states of Xe+2. For the ground state of the ion (A 2Σ+u), a long progression beginning at v+=56 was observed which included isotopically resolved energy levels. The spectroscopic parameters obtained from a fit of the data to anharmonic oscillator energy levels are reported. The resulting Xe2 ionization potential is somewhat higher than previously reported. Several vibrations of the first excited state (B 2Π(3/2)g) were observed near 97 000 cm−1. An absolute vibrational assignment was not possible due to an unusual alternation of intensities. Nevertheless, a lower bound on the binding energy and an estimated vibrational frequency are reported. For the C 2Π(3/2)u state, the v+=0–2 vibrational levels were observed with an intensity distribution consistent with the weakly bound nature of this state. The derived binding energy and vibrational frequency for the C 2Π(3/2)u state are reported.

Cancellation effects in four-photon-resonant five-photon ionization through the nf J=2 states of Xe.

Partial cancellation of the ionization signal in the four-photon-resonant five-photon ionization via J=2 4f and 5f resonances in Xe has been observed at increased gas pressures. Experiments with one and two counterpropagating laser beams provide strong evidence that cancellation is due to destructive interference at the three-photon step between the coherent three- and one-photon excitation channels, the latter involving one vacuum ultraviolet photon of the third-harmonic generation that occurs above the 6s[3/2${]}_{1}^{\mathit{o}}$ state. The well-known destructive interference for the ``on-resonance'' case is now shown to occur also for large detuning from the resonance (up to 2000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) probed by four-photon-resonant five-photon ionization. This type of cancellation may provide a new insight in interference effects involved in multiphoton-excitation processes.

Photoelectron angular distributions in 2+1 REMPI through nf (3/2)2e and nf (5/2)2e states of Xe

Angular distributions of photoelectrons ejected through 2+1 REMPI via nf (3/2)2e and nf (5/2)2e (n=4, 5 and 6), Jc=1/2 and 3/2 states of Xe have been measured. Ionization ratios between the two channels leading to the two fine-structure levels of the ground state of the Xe+ ion have been extracted. The experimental results have been compared with the results of the QDT calculations of Tang and Lambropoulos (Ref. unobtainable).

Emission spectra and potentials of the 0+u and 1u states of Xe2

An extensive series of time-resolved measurements of Xe*2 second continuum emission spectra has been completed to reveal the spectra of the relaxed individual emitting states. Optical pumping of high-pressure Xe by a short-pulse F2 laser at 157.6 nm was used together with a gated optical multichannel analyzer detector or monochromator and boxcar integrator to time resolve the emission bands of the 0+u and 1u states. The bands are broader than expected and have center wavelengths close together. Curve fits to the data yield the parameters for the fully relaxed states: 1u:λc =171.4±0.6 nm, Δλ=12.2±0.2 nm and 0+u: λc =170.4±0.6 nm, Δλ=13.1±0.3. Temperature-dependent measurements from 246 to 413 K show a rapidly broadening band width with temperature. The results provide the first definitive measurement of the individual excimer emissions from the Xe2(0+u) and Xe2(1u) states. From these measurements and the known ground-state potential, we have derived 0+u and 1u potentials that reproduce the emission bands and temperature dependence very well.

Decay of the 4d hole states of Xe studied by photoelectron-photoelectron coincidence spectroscopy.

The formation of ${\mathrm{Xe}}^{2+}$ by decay of 4d hole states in xenon has been investigated by observing both the ${\mathit{N}}_{4,5}$OO normal Auger spectra and the resonantly excited spectra using synchrotron radiation and a new form of electron\char21{}electron coincidence spectroscopy involving a magnetic-bottle time-of-flight analyzer. Direct double Auger processes from the resonance populate all accessible states of ${\mathrm{Xe}}^{2+}$ while indirect processes via superexcited ${\mathrm{Xe}}^{+}$ levels populate ground and lower excited states of ${\mathrm{Xe}}^{2+}$ preferentially.

Radiative lifetimes and collisional deactivation of two-photon excited xenon in argon and xenon

Radiative lifetimes and bimolecular rates have been determined for two-photon laser excited states of Xe* (5p56p, 5p56p′, 5p57p) in argon and xenon buffer gases. The collisional deactivation rates are found to be very large for the Xe 6p′ and 7p states [∼(2–5)×10−10 cm3/s] while the rates for the Xe 6p states are comparatively smaller [∼(0.06–1.2)×10−10 cm3/s]. In general, the quenching rates in argon are about a factor of 2 smaller than the xenon quenching rates for the same excited state with the notable exception of Xe 6p[1/2]0. For Xe 6p[1/2]0, a multicomponent decay has been observed in argon buffer gases. The second component is attributed to collisional coupling to Xe 5d[1/2]1 which lies 132.3 cm−1 below Xe 6p[1/2]0. Quench rates determined from the collisionally induced VUV fluorescence from Xe 5d[1/2]1 at 125 nm are in excellent agreement with this assignment. Furthermore, these experiments have unambiguously identified the product channel involved in the curve crossings observed in studies of optical collisions [N. Böwering, T. D. Raymond, and J. W. Keto, Phys. Rev. Lett. 52, 1880 (1984)].

Collisional relaxation of the 6s[3/2]10and 6s’[1/2]10mJlevels of xenon using resonantly enhanced multiphoton ionization

Collisional relaxation of the 6{ital s}(3/2){sub 1}{sup 0} and 6{ital s}{prime}(1/2){sub 1}{sup 0} {ital m{sub J}} levels of Xe has been investigated both experimentally and theoretically. Collisional disalignment and dephasing of the {ital m}{sub {ital J}} levels have been studied by measuring the pressure and laser intensity dependence of the ratio of the ionization from selectively excited {ital m}{sub {ital J}}=0 and {plus minus}1 sublevels in a three-photon resonant-ionization experiment via the 6{ital s} and 6{ital s}{prime} states of Xe. The selective ionization is achieved by a novel use of two counterpropagating laser beams having parallel and orthogonal polarization, respectively. The pressure dependence of the ionization ratio has been calculated by solving a system of rate equations derived from a full set of density-matrix equations pertaining to the particular experimental conditions. Collisional-relaxation cross sections of the investigated levels have been estimated by fitting the calculated results to the experimental data. The ionization ratio is found to be sensitive to both disalignment and dephasing of the {ital m}{sub {ital J}} levels.

Two-photon spectroscopy of autoionizing states of Xe 2 near threshold

The two-photon ionization spectrum of Xe 2 in the region of the first ionization threshold is presented. Vibronic bands corresponding to at least four different autoionizing electronic states of Xe 2 are observed for the first time and are tentatively assigned. The observed appearance potential is significantly higher (by 415 cm −1) than the earlier single-photon ionization result (C.Y. Ng, D.J. Trevor, B.H. Mahan and Y.T. Lee, J. Chem. Phys. 65 (1976) 4327).