Characteristic and non-characteristic X-ray emission from SF6 and SO2 molecules by electron impact

Cross sections for K-shell ionization of sulphur in collisions of electrons with kinetic energies of 3.5–14.0 keV with SF₆ and SO₂ gases have been measured. In addition, the impact energy dependence of the bremsstrahlung radiation emitted at different photon energies was investigated. The experimental results are compared with theoretical calculations in plane wave Born approximation and with the available semi-empirical models.     

Coherent excitation of H(n=2) in H+, H - He collisions

The coherent excitation of H(n=2) in H⁺, H - He collisions was investigated at incident energies of 5–25 keV. From a polarization analysis of the emitted Lyman-α radiation as a function of an external electric field, the partial cross sections for excitation to the H(2s) and the H(2p _m) magnetic substates and the real part of thes ?p ₀-coherence were extracted. For H⁺-He collisions, the measured partial cross sections are in fair agreement with previous two-electron calculations by Kimura and Lin; the agreement with one-electron calculations of Jain et al. is, particularly at the lower incident energies, less satisfactory. For both collision systems, an energy-dependent forward-backward asymmetry corresponding to a shift of the center-of-charge relative to the center-of-mass (dipole moment) was observed. In H⁺ - He collisions, the measured dipole moment was positive; it thus corresponds to an electron trailing behind the proton. The same analysis applied to the H - He system showed the electron riding in front of the proton.     

Kβ/Kα X-ray intensity ratios for elements in the range 16⩽Z⩽92 excited by 5.9, 59.5 and 123.6 keV photons

The K shell intensity ratios K_β/K_α for 59 elements in the atomic region 16⩽Z⩽92 have been measured at excitation energies of 5.9, 59.5 and 123.6 keV. K X-rays emitted by samples have been counted by a Si(Li) detector with resolution 160 eV at 5.9 keV. The measured values were compared with the theoretical values calculated using Scofield's tables based on the Hartree–Slater and Hartree–Fock theories and available experimental values. Reasonable agreement is typically obtained between present and theoretical values.     

Relativistic (e, 2e) collisions on atomic inner shells in symmetric geometry

We report here on an (e, 2e) experiment at relativistic electron energies (E ₀=300 keV and 500 keV) in coplanar symmetric geometry. Absolute triple differential cross section measurements forK-shell ionisation of gold, silver and copper are compared with a number of simple first order approximations. Appreciable discrepancies between theory and experiment are found, which reduce with decreasingZ and increasing primary energy. The theoretical calculations show that spin flip effects are important in symmetric geometry, in earlier works these had been neglected.     

Variation of K X-ray fluorescence cross-sections of Fe in F compounds at an energy interval of 7.6–14.4 keV

The photon-induced K X-ray fluorescence cross-sections (σ_Kα, σ_Kβ) for Fe in F compounds were investigated. Measurements were carried out at 10 different energies in the interval of 7.6–14.4 keV by using a secondary excitation method. K X-rays emitted by samples were counted by a Si(Li) detector with a resolution of 160 eV at 5.9 keV. Obtained values were compared with the theoretical values of pure elements.     

A possible contribution to the population of triplet states in electron impact measurements of total cross sections for triplet state excitation

It is proposed that low energy secondary electrons produced in medium energy electron impact experiments may play an important role in the excitation of triplet states even at low sample gas densities. A model calculation is carried out which shows that the population of the 4³S, S³P and 4³D triplet states in He from secondary electron excitation can be comparable to the population of these states by direct excitation at an incident electron energy of 1000 eV and sample gas pressures as low as 10^?3 torr. The model calculations show that the secondary electron mechanism becomes more important as the incident energy increases and that the produced populations are of a similar magnitude for the 3³P and 4³D states which in turn are about a factor of 4 larger than the population found for the 4³S state. The results indicate that the effect of secondary electron excitation in careful experimental measurements of electron impact total triplet state cross sections may have to be considered when incident electron excitation energies in the range of 1 keV or higher are used.     

Charge cloud orientation in excitation or deexcitation of Na* (3p) in Na-He collisions

The direct excitation and deexcitation probabilities of Na(3p) are calculated in an 8-state model using the Projected Valence Bond potential energies of NaHe, the impact parameter method is used in the 0.5 keV<E _CM<100 keV energy range. A 3-state model predicts propensity rules which are confirmed by the present more elaborate model in the energy range of the maximum of the 3s–3p transition, but a discrepancy attributed to ionisation of Na occurs at energies beyond the excitation probability maximum (E _CM>2 keV).     

M 3-alignment of gold by 0.16-5 MeV proton impact ionization

The alignment of theM ₃ subshell of gold following proton impact ionization was studied with projectile energies in the range from 160 keV up to 5MeV. The experiments were performed by measuring the polarization of emittedM X-rays be means of a 5 in. Johansson crystal spectrometer. A good agreement of the experimental data and the results of theoretical SCA calculations was found.     

Angular distributions in the double ionization of noble gases by electron impact

Electron-impact double ionization of noble gases is investigated theoretically for the case of high incident energies (5 keV). An ab initio calculation is made including partial correlation in the initial state as well as in the final state. The results of the calculations are compared with those of other theories and with the first available (e, 3e) experimental data on krypton 4p ⁶.     

Impact parameter dependence ofL-shell vacancy production in slow Kr-Kr and Xe-Xe collisions

The impact parameter dependence ofL-shell vacancy production in slow Kr-Kr and Xe-Xe collisions has been investigated both experimentally and theoretically. For collision energies between 711 keV and 2.49 MeV collisionally inducedL x-rays have been observed in coincidence with scattered ions. The experimental data are compared with theoretical predictions deduced from seven-state adiabatic 1 φ _u ?1 δ _u ?4 π _u ?3 π _u ?2 π _u -4 σ _u ?3 σ _u coupled-channel calculations. Contrary to similar calculations performed previously, no approximations regarding the dependence of molecular-orbital energies and coupling matrix elements on internuclear separation have been introduced in the present calculations. For large impact parameters, agreement between experiment and theory is found on an absolute scale. For smaller impact parameters, the experimental data tend to be consistently larger than the theoretical predictions. The relative deviations for Kr-Kr collisions with impact energy smaller than 2 MeV and for Xe-Xe collisions can be parametrised by means of universal functions depending only on the minimum internuclear distanceR ₀ reached in the collision. This finding is discussed in terms of possible contributions from direct molecular-orbital ionisation and of anR ₀-dependence of the heavy-ion induced fluorescence yield.     

Measurement of L subshell photoionisation cross sections of Th and U at 22.6, 25.8, 29.2 and 32.9 kev

Bremsstrahlung from an X-ray tube was used to excite secondary targets of Ag, Sn, I and Ba to get nearly monochromatic excitation energies of 22.6, 25.8, 29.2 and 32.9 keV, respectively. Th and U were used as targets. The L X-ray fluorescence cross sections of different lines from the targets have been measured. Of the several methods to obtain L subshell photoionisation cross sections from these fluorescence data, the merits and demerits of four common methods have been explained and the method with least uncertainty was suggested as the best one for such analysis. Following this method, with intensities of the resolved L_γ lines, three L subshell photoionisation cross sections have been obtained using six different sets of atomic parameters. The variation of these cross sections with different atomic parameters has been discussed. For σ₁, all the derived values are within 30% of one other while for σ₂ and σ₃, they are within 12%. Measured cross sections have been compared with the data of others and with the theoretical values of Scofield. Finally, the intensity ratios of different L lines have also been compared with available data and the theoretical values. Within experimental errors, our data are in good agreement with the data of others and with the theoretical predictions.     

Understanding on absorption and fluorescence electronic transitions of carbazole-based conducting polymers: TD-DFT approaches

The electronic excitation transitions of carbazole-based oligomers, (Cz-co-Cz) _N , (Cz-co-Fl) _N and (Cz-co-Th) _N (N = 2–4) were investigated using density functional theory (DFT) and time-dependent (TD) DFT methods. Our results show that the calculated ground state geometries favor a more aromatic, planer structure, while the electronically excited geometries favor a quinoidic type structure. Absorption and fluorescence energies have been obtained from TD-B3LYP/SVP calculations performed on the S₁ optimized geometries and are in excellent agreement with experimental data. The experimental fluorescence excitation energies for (Cz-co-Cz)₄, (Cz-co-Fl)₄ and (Cz-co-Th)₄ (2.76, 2.63, and 2.25 eV, respectively) correspond closely with the predicted S₁ transitions (2.84, 3.91 and 2.43 eV, respectively). We also report the predicted radiative lifetimes 0.52, 0.47, and 0.99 ns for (Cz-co-Cz) _N , (Cz-co-Fl) _N and (Cz-co-Th) _N , discuss the origin of the small stoke shift of the carbazole based oligomers and the magnitude of bathochromic shifts. We conclude by discussing the benefits of theoretical calculations, which can provide critical structural and electronic understanding of excitation–relaxation phenomena that can be exploited in design of novel optical materials.     

High-energy collisional activation studied via angle-resolved translational energy spectra of survivor ions

Angle-resolved translational energy spectroscopy has been applied to Cs₄I ₊ ³ ions that survived 8 keV collisions with a range of collision gas targets, including inert gases and deuterium. The experimental data comprise values of the translational energy loss ΔT_R as a function of the (laboratory-frame) scattering angle θ _R for each collision gas under conditions such that single-collision events dominated the scattering. The values of ΔT_R increase with θ _R, in accordance with very general expectations. However for any value of θ _R, the values of ΔT_R for helium and deuterium as targets were almost indistinguishable from one another but were at least five to six times larger than those for neon and all other collision gases. These data have been shown to be consistent with theoretical considerations based upon conservation of energy and linear momentum. Theoretical approaches include the simple “elasticlimit” model, which makes no mechanistic assumptions, and a particular “binary-model” theory, which excludes electronic excitation as a possibility. Both theories are consistent with the experimental data and interpret the surprisingly large values of ΔT_R for low-mass targets in terms of large recoil energies of the target required to ensure conservation of momentum. The most likely alternative candidate as sink for ΔT_R is internal excitation of the target, but this possibility was excluded in the present work by choosing ΔT_R values less than the lowest excitation energies of the inert gas targets. Moreover, such an interpretation cannot explain the similar results obtained using helium and deuterium, which were markedly different from those obtained for all other collision gases.     

Non-validity of Bragg's additivity law for rare-earth compounds

Mass attenuation coefficients have been measured for rare-earth compounds at averaged photon energies 5.947 and 8.118 keV. The measured values are compared with theoretical calculations. The disagreement between experiment and theory is larger than the experimental error when the incident photon energy is close to the L shell absorption edge. Further away from the edge the agreement between experiment and theory is within 3%. The breakdown of the mixture rule close to L absorption edges is discussed.     

Charge transfer inp-H and H2 collisions at low and high energies

The previous modified two-orthogonal state expansion method with the initial united-atom effect is extended to study the charge transfer for the resonant and nonresonant reactions at low and high energies and the theory is no longer restricted to low capture probabilities. The impact energy covers a wide range: 0.17 eV–200 keV and 0.1–200 keV forp-H and H₂ collisions, respectively. The present calculations show good agreement with experimental data at energies 0.17 eV–10 keV and 75–200 keV for thep-H reaction and 0.2–200 keV for thep-H₂ reaction.     

Electron scattering from tetrafluoroethylene

We report experimental results for electron scattering from tetrafluoroethylene, C2F4, obtained from measurements in two laboratories. An extensive set of differential, integral, and momentum transfer cross sections is provided for elastic scattering for incident electron energies from 1 to 100 eV and inelastic (vibrational excitation) scattering for incident electron energies at 3, 6, 7.5, 8, and 15 eV, and for scattering angles ranging from 10 degrees to 130 degrees. To highlight the role of intermediate negative ions (resonances) in the scattering process we have also measured excitation functions for elastic scattering and vibrational excitation of the ground electronic state of C2F4 for incident energies between 1.5 and 20 eV. Our results are compared with recent theoretical calculations and a limited number of other experimental results.     

Potential energy curves for ground and excited states of NaLi from ab initio calculations with effective core polarization potentials

Sixteen low-lying electronic states of NaLi are investigated by SCF/valence Cl calculations including core polarization effects by means of an effective potential. Spectroscopic constants are obtained with estimated uncertainties of ΔR_e ? 0.01 Å, Δω_e ? 0.6 cm^?1 and ΔD_e ? 80 cm^?1. From a comparison of experimental and theoretical G(υ) values, we suggest a ground-state dissociation energy of 7093 ± 5 cm^?1. Using our rovibrational energies and recently measured excitation lines, we are able to improve the T_e values and dissociation energies of five excited states to an accuracv of ±8 cm^?1.     

Excitation of He—41 D-substates by He+- and Ne+-projectiles investigated by level-crossing techniques

Collisions of He⁺- and Ne⁺-projectiles with He at impact energies between 90 keV and 800 keV were investigated. Relative excitation cross sections for magnetic sublevels of He—4¹ D were determined using level-crossing techniques. Absolute excitation cross sections σ _m of the Zeeman-sublevels are given using He—4¹ D cross sections from earlier measurements. The results show strong variations of the cross sections σ₀ and σ_±1 with a quasi-oscillatory behaviour. σ_±2 is much smaller than σ₀ and σ_±1.     

On the vibrational dependence of electron impact ionization of diatomic molecules

The dependence of the Na₂ electron impact ionization rate is measured as a function of vibrational excitation in a crossed molecule-electron beamm arrangement at collision energiesE _coll ≤ 3 eV above the ionization threshold. Specific vibrational distributions in theX ¹∑ _g ⁺ state with average vibrational energies of 0.17 eV, 0.276 eV, and 0.349 eV, are prepared via Franck-Condon pumping using a narrow-band cw laser. Enhancement of the ionization rate is observed only at impact energies near the ionization threshold where the ionization rate increases linearly as a function of vibrational excitation. Analysis of the experimental data is based on three model calculations. The first of these calculations equates vibrational energy with kinetic energy and agrees well with the experimental data. A second, more refined model allows for differences in state-to-state ionization rates and uses Franck-Condon factors to estimate transition probabilities, but leads to a less favorable agreement. The third one employs a semi-classical formulation of the Franck-Condon principle. It provides the best agreement with the experimental data. In contrast with an earlier study of electron impact ionization of diatomic molecules [20], we find no evidence of dynamical modification of the ionization rate, due to vibrational motion of the nuclei, at the present level of accuracy of our data and analysis.