The electronic spectrum of linear HC9H

Large-scale ab initio multi-reference configuration interaction (MRD-CI) calculations are carried out to determine the vertical electronic spectrum of HC₉H. The calculated energy for the first dipole-allowed ³Σ_u⁻←X ³Σ_g⁻ transition of 2.38 eV is in reasonable agreement with recent measurements of 582 nm (2.13 eV) in neon matrix. The oscillator strength is computed with f=0.007. In addition a second very strong (f=4.7) ³Σ_u⁻←X ³Σ_g⁻ transition is predicted by the calculations to be located near 6 eV.     

Photoionization cross sections involving an explicitly correlated initial state: Combination of multiconfigurational linear response and the stieltjes—tchebycheff moment theory

We have performed outer valence photoionization cross section calculations for N₂ and O₂. To do this we have combined several linear response techniques, in particular time-dependent Hartree—Fock (TDHF), multiconfigurational time-dependent Hartree—Fock (MC TDHF), and a modification of MC TDHF (MMC TDHF) with Stieltjes—Tchebycheff moment theory (STMT). To our knowledge, these MC TDHF and MMC TDHF calculations are the first which combine explicitly correlated Green function approaches with STMT. Since, in addition, these calculations are fully coupled, we expect the MC TDHF and in particular the MMC TDHF—STMT results to be highly reliable. For both N₂ and O₂ our MC TDHF—STMT and MMC TDHF—STMT results are in overall agreement with previous static exchange STMT results; however, there are a few significant differences and differences in detail in the partial and total photoionization cross sections. In particular, for example, for N₂ we note that the MMC TDHF—STMT does not give a “hump” resonance in the cross section for the (1π_u⁻¹)A²Π_u ionic state. In O₂ we note that the (3σ_g⁻¹) cross section obtained using MMC TDHF—STMT is substantially lower than the static exchange results.     

Low energy (e,2e) measurements of CH4 and neon in the perpendicular plane

Low energy experimental and theoretical triple differential cross sections for the highest occupied molecular orbital of methane (1t(2)) and for the 2p atomic orbital of neon are presented and compared. These targets are iso-electronic, each containing 10 electrons and the chosen orbital within each target has p-electron character. Observation of the differences and similarities of the cross sections for these two species hence gives insight into the different scattering mechanisms occurring for atomic and molecular targets. The experiments used perpendicular, symmetric kinematics with outgoing electron energies between 1.5 eV and 30 eV for CH(4) and 2.5 eV and 25 eV for neon. The experimental data from these targets are compared with theoretical predictions using a distorted-wave Born approximation. Reasonably good agreement is seen between the experiment and theory for neon while mixed results are observed for CH(4). This is most likely due to approximations of the target orientation made within the model.     

Electron and positron atomic elastic scattering cross sections

A method was developed to calculate the total and differential elastic-scattering cross sections for incident electrons and positrons in the energy range from $\text{0.01}\text{eV}$ to $\text{1}\text{MeV}$ for atoms of Z=1–100. For electrons, hydrogen, helium, nitrogen, oxygen, krypton, and xenon, and for positrons, helium, neon, and argon atoms were considered for comparison with experimental data.First, the variationally optimized atomic static potentials were calculated for each atom by solving the Dirac equations for bound electron states. Second, the Dirac equations for a free electron or positron are solved for an atom using the previously calculated static potential accomplished (in the case of electrons) by “adjusted” Hara's exchange potential for a free-state particle. Additional to the exchange effects, the charge cloud polarization effects are considered applying the correlation-polarization potential of O'Connell and Lane (with correction of Padial and Norcross) for incident electrons, and of Jain for incident positrons.The total, cutoff and differential elastic-scattering cross sections are calculated for incident electrons and positrons with the help of the relativistic partial wave analysis. The solid state effects for scattering in solids are described by means of a muffin-tin model, i.e. the potentials of neighboring atoms are superpositioned in such a way that the resulting potential and its derivative are zero in the middle distance between the atoms. The potential of isolated atom is calculated up to the radius at which the long-range polarization potential becomes a value of −10⁻⁸.     

Velocity dependence of the total cross section for helium-argon,neon-krypton and neon-xenon scattering

Velocity dependences of the total collision cross sections for scattering of helium by argon, neon by krypton and neon by xenon were measured using a time of flight method of velocity selection. For neon scattering glory oscillations were observed from which values of ?r_m (the product of the well depth and the internuclear separation at the minimum) were determined for various Lennard-Jones potentials. Comparison is made with the results from diffusion experiments and differential scattering measurements.     

Chemi-ionization in alkali-methylhalogen collisions

Chemi-ionization has been observed in collisions between the alkali atoms K, Na and Li and the methylhalogens CH₃] and CH₃Br. Total cross sections were measured as a function of the translational energy and the internal energy of methylhalogens. The molecular electron affinities were determined from the thresholds of these reactions. The thresholds indicate that these negative ions are formed by vertical or near-vertical transitions, in contrast to most experiments on chemi-ionization, which yield the adiabatic electron affinities. In the threshold region, the cross sections are strongly dependent upon the internal energy of the molecules. At very high temperature the methyliodide dissociates, and so chemi-ionization was observed with iodine atoms.     

K-shell ionization induced by 30 MeV/u argon and neon beams

We have studiedK-shell ionization induced by 30 MeV/u Ne and Ar projectiles on target atoms with atomic numbers ranging from 27 to 90. X-ray production cross sections and energy shifts were measured with Si(Li) detectors. In most cases satisfactory agreement between measurements and theoretical direct ionization cross sections is obtained when the contribution of electron capture is included. The influence of multiple ionization on the fluorescence yield ω _K is discussed.     

Vibrational relaxation and collision-induced dissociation of xenon fluoride by neon

Rate coefficients were calculated for vibrational relaxation and collision-induced dissociation of ground state xenon fluoride in neon at temperatures between 300 and 1000 K for each of nine vibrational levels. These coefficients were calculated using a pairwise additive potential energy surface, which consists of a Morse function for the XeF interaction and Lennard–Jones functions for the NeXe and NeF interactions. Rate coefficients are provided for both temperature and v- dependences. The vibrational relaxation and dissociation processes occur by multiquanta transitions. Dissociation can take place from all v-levels provided that the internal energy of the XeF molecule is close to the rotationless dissociation limit. The order of increase effectiveness of the various forms of energy in promoting dissociation in XeF was found to be translation–rotation-vibration. At room temperature, neon atoms were found to be more efficient than helium atoms in the dissociation processes; helium atoms were found to be more efficient than neon atoms in the vibrational relaxation of XeF. Strong vibration–rotation coupling in both vibrational relaxation and in the dissociation processes is demonstrated.     

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.     

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.     

Semiempirical method for extracting electron molecule cross sections from experimental data: CF4 as an example

A semiempirical method is described for calculating electron-molecule interaction potential, electron density, and elastic and vibrational inelastic cross sections using experimental data.CF ₄ was considered as un example. Experimental data from IR and Raman spectra, molecular property calculations (dipole matrix elements, polarizability, distance between atoms in molecule), beam measurements of total, momentum transfer, and differential cross sections, and swarm data (diffusion and drift veolcity) are used for extracting the interaction potential, the molecule electron density, avid the differential cross sections. Electron energy distribution functions, drift velocity, and characteristic energy are calculated with the obtained differential e-CF ₄ cross sections lierAr/CF ₄ mixtures.     

Total reaction probabilities for translationally hot (3.7 eV)D atoms reacting with H2 and their astrophysical implications

Translationally hot (3.7 eV center-of-mass collision energy) D atoms can be produced by the dissociative recombinationsof DCO⁺ ions with electrons in interstellar clouds. The reactions of hot D atoms with H₂ molecules is an important factor in determining the D to H abundance ratio in the universe from observations of deuterated species in dense interstellar clouds. A simulation of this reaction has been carried out using exact quasiclassical reaction cross sections, together with a series of approximate total collision cross sections. It was found that the total probability of reaction, P^R, at 3.7 eV was approximately 0.4. A reaction probability of this magnitude suggeststhat the loss of D atoms by the title reaction be considered in astrochemical modeling of deuterated molecules.     

Dynamics of the reaction H2+(He,H)HeH+. Influence of various forms of reactant energy on the total and differential cross section

Results of quasiclassical trajectory calculations of reactive processes between He atoms and H₂⁺ (υ, J) molecular ions in the collision energy interval 0.5–5.0 eV (c.m.) for a large number of selected υ, J combinations are analyzed with respect to the influence of the initial translational, vibrational, and rotational energy on the total and differential reaction cross sections. Vibrational energy is more effective in promoting the reaction than translational energy. Small rotational excitation has a negligible effect, whereas high rotational excitation has a similar influence on the reaction cross sections as the vibrational excitation of the same magnitude.     

On trends in total cross sections for electron (positron) scattering on atoms and molecules at intermediate energies

An examination of total electron and positron scattering cross sections in a large variety of atoms and molecules reveals a trend related to ground state target dipole polarizability. Some correlations of total cross section with diamagnetic susceptibility and the number of target electrons have also been noted. Experimental positron and electron cross sections between 100 and 500 eV can be reasonably well described by a simple formula based on polarizability and energy correlation.     

Potential energy curves for the (ArH)+ and (NeH)+ systems from the interplay of theory and experiments

The ground state potential energy curves for protons interacting with Ar and Ne atoms are determined by the analysis of new, highly accurate measurements of the elastic differential cross sections at a laboratory collision energy of 14.8 eV. Accompanying theoretical results from SCF-CI calculations are used as starting points to generate analytic potentials that are able to fit all available experimental cross sections for both systems. The final results provide the full shape of the potential curves and give the best existing fit to the measured cross sections for elastic scattering at several energies from 2eV to 30eV.     

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.     

Quasiresonance charge exchange of hydrogen isotopes mesic atoms in excited states

Processes of charge exchange of hydrogen isotopes mesic atoms in excited states at low collision energies 10^?2?E?1 eV are studied. The cross sections calculated depend on energy like ～E ^?1 and are of an order of the atomic cross sections (～10^?16 cm²). It is shown that the high rates (～10¹² s^?1) of charge exchange and thermalization of mesic atoms in excited states at the liquid hydrogen density are comparable with the rates of cascade transitions in mesic atoms.     

Differential cross sections and angular correlation functions in electron-hydrogen scattering for energies betweenn=3 andn=4 thresholds

Differential cross sections and electron-photon angular correlation functions for electron impact excitation of hydrogen atoms in the energy range between then=3 andn=4 thresholds were calculated using the algebraic variational method. The cross sections and correlation functions show complicated and irregular structures associated with overlapping resonances in different partial waves. The availability of these quantities may help interested experimentalists to determine resonance positions, and also may furnish an important test of the adequacy of the computations.     

A method for measuring cross sections for electron-impact excitation out of metastable levels of atoms

A method for measuring cross sections for electron-impact excitation out of the metastable levels of the He atom is described. A hollow cathode discharge is used to produce an atomic beam consisting of ground-level He atoms and the He(2¹S) and He(2³S) metastable atoms. An electron beam of energy below 20 eV crosses the atomic beam exciting the metastable atoms to higher levels, and the intensity of the radiation emitted by atoms in these higher levels is utilized to determine the cross sections. Because of the very low concentration of metastable atoms in the atomic beam, the emission signal is extremely weak. A number of special techniques have been developed to detect these very low-level signals. Absolute calibration of the cross section is accomplished by referencing the emission signal that resulted from electron excitation out of the metastable level to the emission signal that resulted from the 2³S3³P or 2¹S3¹P laser optical absorption.