Threshold electron impact excitation of Cl2

A high resolution electron impact threshold spectroscopy technique was used to examine the excitation of Cl₂ in the 2–14 eV region. This study complements previous photon absorption and emission measurements, because it is capable of detecting transitions which are optically forbidden. In the region up to 7.5 eV, broad dissociative structures are correlated with optically active valence states, although relative intensities in the threshold spectrum differ considerably and indicate a substantial contribution from the optically forbidden transitions. At 7.46 eV a series of 5 equidistant sharp peaks is detected and interpreted as arising from the² π _g Feshbach resonance, which differs from the ground state positive ion Cl ₂ ⁺ by a pair of Rydberg electrons: (4sσ)². The decay channels responsible for the appearance of the resonance in a threshold spectrum are discussed and it is suggested that they include several valence states of the (2431) and (2341) configurations, whose potential energy curves cross the Cl ₂ ^? ,²π_g curve in the region of energy at which the resonance state is formed. At higher incident electron energies and up to ionisation, Rydberg states predominate, starting with (2430) 4s^3,1 π _g states detected for the first time. The absence of broad peaks above 8 eV and the irregular appearance of Rydberg bands is consistent with the strong Rydberg-valence configuration mixing proposed by Peyerimhoff and Buenker. Where our resolution permits comparison, good general agreement is found with recent synchrotron radiation absorption measurements of optically allowed transitions.     

The quadrupole moments of carbon dioxide and carbon disulphide

The electric quadrupole moments of carbon dioxide and carbon disulphide have been measured through the birefringence induced in gaseous samples over a range of temperature. The value for CO₂, Θ = (-14·98 ± 0·50) × 10^-40 C m², is consistent with earlier measurements, showing that the temperature-independent hyperpolarizability contribution to the birefringence is insignificant. For CS₂, Θ = (+12·0 ± 0·6) × 10^-40 C m². The positive sign reflects the increased importance of the π electron contribution to Θ in CS₂.     

Threshold peak structures in the vibrational excitation of HCl by low-energy electron impact

A new model which describes dissociative electron attachment and vibrational excitation for the HCl molecule is proposed. The model is based on ab initio scattering data calculated in the fixed nuclei approximation by Morgan et al. [J. Phys. B 23 (1990) 99]. The cross sections for vibrational excitation exhibit interesting oscillatory structures in the threshold peak region.     

Integral cross sections for electron impact excitations of argon and carbon dioxide

Electron-impact excitation integral cross sections play an important role in understanding the energy transfer processes in many applied physics. Practical applications require integral cross sections in a wide collision energy range from the excitation threshold to several keV. The recently developed BE-scaling method is able to meet the demands of integral cross sections for dipole-allowed transitions while the prerequisite relies on the accurate generalized oscillator strengths. Fast electron and x-ray scatterings are the conventional experimental techniques to approach the generalized oscillator strengths, and the joint study by both methods can provide credible cross-checks. The validated generalized oscillator strengths can then be used to extrapolate optical oscillator strengths by fitting the data with the Lassettre formula. The fitted curve also enables the integration of generalized oscillator strengths over the whole momentum transfer region to obtain the BE-scaled integral excitation cross sections. Here, experimental measurements by both fast electron and x-ray scattering of argon and carbon dioxide are reviewed. The integral cross sections for some low-lying states are derived from the cross-checked generalized oscillator strengths for the first time. The integral cross sections presented in this paper are openly available at https://doi.org/10.11922/sciencedb.01466.     

Longitudinal RF excitation of carbon dioxide lasers

Longitudinal RF excitation of CO₂ — Lasers at relatively low radio frequencies in the 100 to 200 kHz range permits an efficient AC to RF conversion in the power supply while maintaining all the advantages of RF excited lasers. Existing tube designs for DC excitations can be easily modified, and peripheral circuitry like laser stabilizers remain useable too. Experimental results for a prototype laser are given.     

Glow discharge lamps as electron sources for electron impact excitation

It has been found possible to utilize the plasma in the discharge lamp of a conventional ultraviolet photoelectron spectrometer as an electron source f     

Dissociative excitation of the fluoromethanes by electron impact

Emission spectra following electron impact on molecules of the homologous series of fluoromethanes CH_xF_4-x with x=0-4 have been investigated from the near infrared at 700 nm to the ultraviolet VUV-spectral region at 100 nm. Earlier experimental data for the visible and ultraviolet spectral region were revised and evaluated again on the basis of reliable new data for the dynamic viscosity of the molecules. The measurement of absolute and relative cross-sections were systematically extended into the VUV region from 100 nm to 200 nm. The examination of atomic lines as well as molecular band systems in the VUV gives further insight into the dissociation mechanism and shows that many excited levels even of atomic and molecular species cannot be populated directly, but only by transitions from higher-lying energy levels. Simple steric effects can be distinguished from more complex transition phenomena. Received: 25 February 1998 / Revised: 29 May 1998 and 18 June 1998 / Accepted: 23 June 1998     

Ionization of nitrogen,oxygen, water,and carbon dioxide molecules by near-threshold electron impact

An experimental technique for measuring the cross sections of direct and dissociative ionization of N₂, O₂, H₂O, and CO₂ molecules by electron impact in the near-threshold energy range is described. The setup used in the experiments allows mass separation of ions with a monopole mass spectrometer. It is shown that such a setup can be used to advantage in separation experiments. For incident electron energies between 7 and 35 eV, the energy dependences of the cross sections of generation of parent ions and ion fragments due to parent molecule dissociation are obtained.     

Two-electron excitation in helium-like ions by electron impact

Calculation of cross-sections for the two-electron excitation in helium-like ions by electron impact employing Coulomb-Born-Oppenheimer (CBO) approximation is presented. Analytical expressions for the differential and total scattering cross-sections without using partial wave expansion of the wavefunction reported earlier have been used. The total and differential scattering cross-sections for each of the excitations 1s ^{2 1} S* → 2s ^{2 1} S ^e , 2s2p ^1.3 P ⁰, 2p ^{2 1} S ^e ,³ P ^e,¹ D ^e in Be²⁺ and B³⁺ are computed. Results for Li⁺ reported earlier are also included for comparison.     

Ejected electron spectrum of sodium autoionizing levels obtained by electron impact excitation 'at 500 eV incident energy

The ejected electron spectrum of sodium vapour has been observed at 90° to the direction af an incident electron beam with kinetic energy 500 eV. Comparisons are made with the ultraviolet absorption data.     

Impact parameter calculation of electron impact excitation of hydrogen

The convenience of a diabatic or an adiabatic interpolation of frequencies in variational transition state calculations involving interpolation methodologies is discussed. The gas phase proton transfer between butanone and a hydroxide anion has been used to illustrate the theoretical discussion. For example, it has been shown that if vibrational normal mode crossings exist the simple adiabatic interpolation can produce incorrect entropy contributions and, as a consequence, a spurious displacement of the generalized transition state.     

K-shell excitation of HCN by electron energy loss spectroscopy

Small angle inelastic scattering of fast electrons has been used to study carbon and nitrogen K-shell excitation and ionization of HCN. The K→π* transitions in HCN have been investigated with high resolution.     

Resonance enhanced electron impact excitation for P-like Cu XV

Employing both the Dirac R-matrix and the relativistic distorted wave with independent process and isolated resonance approaches, we report resonance enhanced electron impact excitation data(specifically, effective collision strengths)among the lowest 41 levels from the n = 3 configurations of Cu XV. The results show that the latter approach can obtain resonance contributions reasonably well for most excitations of Cu XV, though a comparison between the two approaches shows that the close-coupling effects are truly significant for rather weak excitations, especially for two-electron excitations from the 3s3p4 to 3s23p23d configuration. Resonance contributions are significant(more than two orders of magnitude) for many excitations and dramatically influence the line intensity ratios associated with density diagnostics.     

Vibrational excitation in the nitrogen molecule by electron impact

The European Physical Journal D - We report an experimental study on the effect of detuning on velocity-induced population oscillation (VIPO) for a Doppler mismatched double resonance. The double...     

Elemental analysis by high‐energy electron excitation

The possibility of using high‐energy β‐particles (10²–10³ keV) to induce the emission of characteristic x‐rays from pure chemical elements, with important improvements with respect to conventional excitation methods, has been recently reviewed. An excitation procedure named BIXE (β‐induced x‐ray emission) is used for implementing a spectrometric technique along the lines developed for EPMA (electron probe microanalysis). We have found that by using BIXE it is possible to determine binary sample compositions of elements present at concentrations higher than 1%, by comparisons with reference samples to obtain calibration curves. Experience with EPMA shows that when ternary and higher order samples are analyzed, the use of reference samples is not enough and it is necessary to perform theoretical corrections to the relationship between the line intensity and the corresponding concentrations, as a suitable complement to the analytical procedure. Semi‐quantitative results are thus obtained with corrections applied through the ZAF method usually used in EPMA. In this work we concentrated in finding whether calibration curves as used in EPMA (where the electron beam is monochromatic) can be used in BIXE, where the electron beam is polychromatic (the β spectrum). We expect that BIXE can be developed as a spectrometric technique whose main advantages are that it is a low‐cost technique suitable for in situ studies and that the experimental arrangement and data acquisition and its evaluation are comparatively simple. Copyright © 2004 John Wiley & Sons, Ltd.     

Near-threshold electron impact excitation of ultraviolet-emitting levels of helium atoms

The excitation by electron impact of levels of helium atoms that decay by emission of ultraviolet light has been studied as a function of incident energy up to approximately the first ionization energy. The width (FWHM) of the energy spread of the incident beam was less than 30 meV. A crossed-beam interaction region was used and UV photons emitted approximately normally to the electron and gas beam directions were detected by a channel electron multiplier. Precautions were taken to avoid detection of scattered electrons and metastable atoms by the photon detector. The onset position of the lowest excited level was used in calibrating the incident energy scale. The energies of resonance structures in the cross-sections could therefore be obtained and are compared with the results of experiments detecting other products of the interaction.