Experimental study of electron impact excitation of multiply charged ions

We report on measurements of angular differential cross sections for the excitation of multiply charged ions by electron impact. An ion beam is crossed by an electron beam; electrons which are inelastically scattered at different angles are identified by their energy loss due to the excitation process. Absolute excitation cross sections are obtained by comparing the signals of the elastic and the inelastic electron-ion scattering. Results obtained for the 3s→3p excitation of Ar⁷⁺ are discussed.     

The photoionization of atomic Eu in the vicinity of its giant resonance

It is demonstrated that the partial photoionization cross sections of outer subshells of atomic Eu in the giant resonance region are determined by the action of the 4d-electron excitations. The cross section for photoionization of the semifilled 4f ⁷ subshell is also entirely dominated by the interaction with 4d ¹⁰ electrons.     

Deactivation rate constants and cross sections for Ar3P2 metastables in collision reactions with several molecules

The rate constants for the deactivation of Ar³P₂ metastables by 26 different molecules were measured and the corresponding cross sections calculated. The measurements were done in a “flowing” afterglow system; the Ar³P₂ being produced by a low power microwave discharge. The concentration of Ar³P₂ is measured by light absorption at 8115 Å.     

Collisions at thermal energy between metastable hydrogen atoms and hydrogen molecules: total and differential cross sections

A metastable hydrogen (deuterium) atom source in which groundstate atoms produced by a RF discharge dissociator are bombarded by electrons, provides a relatively large amount of slow metastable atoms (velocity 3–5 km/s). Total integral cross sections for H*(D*)(2s) + H₂(X ¹Σ _g ⁺ ,v=0) collisions have been measured in a wide range of relative velocity (2,5–30 km/s), by using the attenuation method. A significant improvement of accuracy is obtained, with respect to previous measurements, at low relative velocities. Total cross sections for H* and D*, as functions of the relative velocity, are different, especially in the low velocity range. H* + H₂ total differential cross sections have also been measured, with an angular spread of 3.6°, for two different collision energy distributions, centered respectively at 100 meV and 390 meV. A first attempt of theoretical analysis of the cross sections, by means of an optical potential, is presented.     

Cross sections and spin polarizations of electrons elastically scattered from oriented molecules (CH3I)

Elastic differential cross sections and spin polarizations for electrons elastically scattered from CH₃I are calculated using the independent atom model. Three molecular orientations with respect to the incident electron wavevector are considered — first, the molecule is oriented randomly, second, the electron wave front and molecular bond are parallel, and third, the wavefront and the bond axis are perpendicular. It will be seen to what extent orientational averaging weakens features of the cross section and spin polarization. The calculations show that cross section and spin polarization measurements are a possible tool for determining the degree of molecular orientation. There is no degeneracy betweenI–C andC–I in cross section and spin polarization measurements. The results presented here for 200 eV and 600 eV electrons scattered by CH₃I should be considered as a case study and it should be possible to find molecules and electron energies for which even more dramatic differences between the various orientations between the molecules and the electrons can be expected.     

Electron scattering differential cross sections for C2H2, C2H4, and C2H6 by gas phase electron diffraction - binding effects

Experimental differential cross sections for 40 keV electrons scattered by C₂H₂, C₂H₄ and C₂H₆ molecules were measured using the gas electron diffraction method in the range of the scattering variable s from s = 1 A^?1 to s = 30 A^?1. The differential cross sections for neon were also measured and compared with calculated differential cross sections to calibrate the diffractograph. Experimental differential cross sections show significant deviations with respect to theoretical differential cross sections calculated from the Debye-Ehrenfest model, mainly in the range of small scattering angles. The observed differences are connected to chemical binding effects. From the experimental data, an estimation of the binding energy was carried out. The deduced values: ?0.58 ± 0.20 au for C₂H₂, ?0.94 ± 0.30 au for C₂H₄ and ?1.23 ± 0.40 au for C₂H₆ are in agreement with those obtained by thermochemical methods.     

A study of the negative ion states of selected cyclodienes by electron transmission spectroscopy

Electron transmission spectroscopy is employed to locate sharp variations in the total cross sections for electrons scattered from several cyclic compounds containing two carboncarbon bonds. For each molecule, structure is observed which we associate with the temporary occupation of the two low-lying, normally unfilled, π^* orbitals by impacting electrons. Electron affinities are reported for 1,5-cyclooctadiene, 1,4-and1,3-cyclohexadiene, norbornadiene and also cyclohexene, propene, and cis-butene.     

Threshold photoelectrons coincidence spectroscopy of the rare gases Ne,Ar, Kr and Xe

Threshold single photon double ionisation of the rare gases neon, argon, krypton and xenon has been studied in an electron-electron coincidence experiment. A new technique has been used where only near-zero energy electrons are detected and these are measured in coincidence. The spectrometer used here employs the penetrating field technique which provides very high detection efficiency, sensitivity and energy resolution. Relative partial double ionisation cross sections have been measured at threshold for then p⁴(³P,¹D and¹S) states of Ne⁺⁺, Ar⁺⁺, Kr⁺⁺ and Xe⁺⁺ and then sn p⁵ (³P and¹P) states of Ne⁺⁺, Ar⁺⁺ and Kr⁺⁺. The observed relative cross sections are in general agreement with a propensity rule for excitation of these states, except for the case of neon.     

Orientation and alignment in charge exchange processes involving sodium atoms studied by semi-classical molecular theory

The semi-classical molecular model is applied to study the charge exchange processes in the H⁺-Na 3p and Li⁺-Na 3p systems in the keV energy range. The dependence of the charge exchange on the orientation and the alignment of the initial or final state is obtained for the transition probabilities and for the differential cross sections. The 14 state present model for the H⁺-Na system is in good agreement with the experimental differential cross sections. The alignment is explained by orientation occurring in the transfer region. The cross sections predicted with a 28 state model for Li⁺-Na exhibit similar behaviour as for H⁺-Na.     

Alignment of atomic autoionizing states created by slow Li++Ne collisions

Ejected-electron spectra have been measured in collisions of Ne-atoms with Li⁺-ions, whose impact energy has been ranged from 0.4 to 7.0 keV. The ion impact-energy dependence has been investigated on the angular-differential cross-section of the ejected electrons from the aligned autoionizing state Ne** 2p ⁴3s ^{2 1} D. In the highest energies studied, cross section maxima have been found at both forward and backward angles with respect to the Li⁺-ion beam direction, whereas the cross sections are maximized at around right angles below intermediate energies. The data were analyzed semi-quantitatively, and the alignment of the autoionizing state was recognized to be characterized by a representative scattering angle of the Li⁺-ions.     

Triple differential cross sections in the vicinity of the (2s 2)1 S state of helium

Triple differential cross sections have been measured in the vicinity of the (2s ²)¹ S autoionising state of helium, following impact by 200 eV electrons. The scattered electron detector was set at an angle of ?12° (anti-clockwise) and the forward and backward ejected electron angular ranges scanned. The direct ionisation cross section at an ejected electron energy of 33.5 eV has been obtained and the results for the resonant ionisation of the¹ S state are presented in the Shore/Balashov parametrisation. These measurements are compared with previous experimental data and emphasise the need for new detailed theoretical calculations on the autoionisation process.     

Total cross-sections for electron scattering from iron at 10–5000 eV using a model optical potential

We report calculations on the total (elastic plus inelastic) electron-scattering cross sections in the energy range 10–5000 eV. A model complex optical potential, composed of static, exchange, polarisation and absorption terms, is employed to describe the collision system at each electron energy. The Iron atom is described by Dirac-Hartree-Fock-Slater self-consistent charge density. The complex phase shifts are computed in a variable phase approach. The absorption cross sections are compared with the experimental results. The experimental absorption cross sections are obtained by adding the experimental ionisation cross sections and available experimental excitation cross sections for electron impact of the allowed transitions a⁵ D → (x,y,z)⁵ D ⁰, (w,y,z)⁵ P ⁰. We have good qualitative agreement between our results and the experimental results available below 200 eV. The Born-Bethe parameters are also calculated. Elastic differential cross-sections with and without absorption are also reported at a few selected energies.     

The effect of collisions on the intensity and polarization of laser-induced D1π → X1Σ+ + fluorescence from NaK

Laser-induced fluorescence was applied for determining total deactivation cross sections of the level (17,94) D¹Π of NaK molecules in collisions with He, Ne, Ar, Kr, Xe, as well as with K atoms. Relaxation cross sections of population and alignment were found to coincide within experimental error. The kinetics of the transient process was used for determining the cross section of thermalization of the optically depopulated level of the electronic ground state (5,67) X¹Σ⁺ in NaK + Ar collisions.     

Formation of negative ions by interaction of electrons with heterocyclic compounds

Curves are derived for the effective yield of negative ions from the interaction of electrons with thiophene, 2-methylthiophene, 2-propylthiophene, 3-methylthiophene, 3-propylthiophene, furan, and selenophene. It is found that there are two quasi-stationary states of the molecular negative ions having lifetimes of about 10^–14 and 4 · 10^–15 sec. The cross sections for formation of these states are estimated, and also the cross sections for resonant elastic scattering of electrons. The states are shown to be related to excited states of the molecule. It is found that alkyl substitution has the following effects on resonant electron capture: a) reduces the probability of ring breakage and b) reduces the energy levels of the quasi-stationary state. Some aspects of dissociative ionization are discussed.     

Photoabsorption of Fe,Co, and Ni atoms embedded in Al-clusters near 3p-ionization threshold of impurity

Photoionization cross sections of Fe, Co, and Ni atoms inside Al jellium-clusters (containing up to 90 atoms) in the energy range near the 3p ionization threshold of the impurity atom are calculated using the time-dependent local spin-density approximation. The spectra are dominated by resonances and depend markedly on the cluster size. It is obtained that the resonances have an autoionization character and are caused by the interference between discrete electronic transitions from the 3p shell of the impurity atom to unfilled d shells of the jellium-cluster and ionization excitations ofd electrons. For comparison, within the same approach, photoionization cross sections of atomic Fe, Co, and Ni are computed.     

Collisional deactivation Of Ca(4p3PJ) by barium atoms

Collisional deactivation of Ca(4p³P_J by barium atoms proceeds with a thermal cross section of 0.26±0.02 nm² at 850 K. No evidence for the corresponding deactivation of electronically excited Ca(4p³P_J) by Ca(4s¹S_o) was obtained. The optical lifetime of Ca(4p³P_J) was measured to be 0.33 ± 0.03 ms, in good agreement with previous studies.     

Concentration-dependent absorption and emission behaviour of a pyrimidonecarbocyanine dye in hexafluoroisopropanol

Absorption spectra, refractive index dispersions, fluorescence quantum distributions and fluorescence quantum efficiencies of a pyrimidonecarbocyanine dye in hexafluoroisopropanol are measured for various concentrations in the region between 10⁻⁵ and 0.3 mol/dm³. The fluorescence quantum distributions and quantum efficiencies are separated into monomeric and dimeric (closely spaced pair) contributions. The fluorescence emission at high concentrations C( > 0.1 mol/dm³) is identified as closely spaced pair fluorescence. The absorption cross sections and the stimulated emission cross sections for the monomers and closely spaced pairs are resolved.     

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.     

Collisional Deactivation of K(7s2S) and K(5d2D) by No

Radiative lifetimes and total deactivation cross sections of K(7²S) and K(5²D) by collision with NO are studied. The K atomic vapor in either the 7²S or the 5²D state was prepared by two- photon absorption using a dye laser. The decay signal of the time-resolved fluorescence from the 7²S – 4²P_1/2 or 5²D – 4²P_3/2 transition was then monitored. Based on a Stern-Volmer analysis, the radiative lifetimes are 155 ±8 ns and 561 ± 18 ns for the K(7²S) and K(5²D) states, respectively. The total deactivation cross sections are 88 ±1Ų and 70 ±2Ų for the K(7²S)-NO and K(5₂D)-NO collisions, respectively. In the absence of NO collisions the radiative lifetimes obtained in this work show excellent agreement with those previously reported. The quenching cross sections for NO have been measured for the first time, and have values in a reasonable range, when compared with Na-N₂ collisions.     

State-to-state total cross sections for ion-pair formation

Relative total cross sections are reported for the production of electrons and anions in collisions between an alkali atom (Na, K, Cs) and the molecules O₂ and NO. The formation of electrons is due to autoionization of vibrationally excited O₂^? (gu′ ? 4) respectively NO^? (υ′ ? 1) molecules. The experiments have been performed with fast alkali atom beams in the laboratory energy range of 30–300 eV.