Positron scattering from vinyl acetate

Elastic scattering of electrons from alanine

A complex optical potential (OP) is employed to study the critical minima (CM) and spin polarization in the elastic e−–Ba and e+–Ba scattering for collision energies in the range Ei = 1–1500 eV and Ei = 1–100 eV respectively in terms of the Dirac partial wave method (OPMD) and the Schrodinger partial wave method (OPMS). The energy dependence of the integral elastic cross sections (IECSs), momentum transfer cross sections (MTCSs), viscosity cross sections (VCSs), inelastic cross sections (INCSs) and total cross sections (TCSs) is also calculated and discussed for both the projectiles in the same energy range. The OPMD optical potential is composed of the static, exchange, polarization and absorption components. On the other hand, the OPMS comprises the static, local exchange, polarization, spin–orbit, and absorption potentials. The number of CM in the DCS distribution of e−–Ba scattering has been found to be 11 using the OPMD method, while the OPMS calculation produces 12 CM points. However, the OPMS calculation with the static and spin–orbit potentials only produces 9 CM. The critical energies and the angular positions of these CM are revealed and discussed. There are 22 and 24 maximum spin polarization points found in the e−–Ba scattering using OPMD and OPMS respectively. For the case of e+–Ba scattering, we have found 2 CM with OPMD calculations of which the angular positions along with the critical energies are discussed. There are 4 maximum polarization points noted with OPMD calculations which fail to attain the condition for sharpness of S(θ) distribution. To the best of our knowledge, the present is the first work to determine the CM in the DCS distribution of e±–Ba scattering. The Sherman function and the DCSs for some energy points are also calculated for the first time.

An apparatus is described for measuring the inelastic differential cross section for vibrational excitation in collisions of diatomic molecules with monoenergetic ions at laboratory energies between 10 and 50 eV. The method consists of measuring the time of flight of single ions with a time amplitude converter and displaying the results on a 100 channel pulse height analyzer. From the shift in the time of flight relative to that expected for elastic scattering the final state of the molecule excited in a single collision is identified. By studying only central collisions with almost zero impact parameter rotational excitation is strongly suppressed. Measured times of flight after collisions of monoenergetic Li+ ions with H2 show that with increasing energy the most probable vibrational quantum jump increases from 0→1 to 0→2,0→3 etc. Contrary to the usual assumption of a small steric factor for vibrational excitation the results show that the inelastic cross section is larger than the elastic cross section. Using reported potential parameters the energy dependence of the most probable excited state is compared with the calculations of Secrest and Johnson for a one-dimensional collinear collision. The satisfactory agreement suggests that the steric factor is close to 1. From measurements at different scattering angles at 10 eV the integral inelastic cross section is found to be about 0.2 A2 corresponding to a differential cross section of 0.4 A2/sr. Measured values of integral and differential total cross sections for Li+-He andLi+-H2 are reported and compared with theory. Direct dissociation of D2 by Li+ in the energy range from 25 to 55 eV was not observed, yielding an upper limit for the cross section of 4 · 10−4 A2/sr.

We employed the Schwinger multichannel method to compute elastic integral, differential, and momentum transfer cross sections for low-energy electron collisions with Si$Y$${}_{4}$ ($Y$ $=$ Cl, Br, I) molecules. The calculations were carried out in the static-exchange and static-exchange plus polarization approximations for energies up to 10 eV. The elastic integral cross section for SiCl${}_{4}$ and SiBr${}_{4}$, computed in the static-exchange plus polarization approximation, shows two shape resonances belonging to the ${T}_{2}$ and $E$ symmetries of the ${T}_{d}$ group, and for SiI${}_{4}$ shows one shape resonance belonging to the $E$ symmetry of the ${T}_{d}$ group. The present results agree well in shape with experimental total cross sections. The positions of the resonances observed in the calculated integral cross sections are also in agreement with the experimental positions. We have found the presence of a virtual state for SiCl${}_{4}$ and a Ramsauer-Townsend minimum for SiI${}_{4}$ at 0.5 eV. The present results show that the proper inclusion of polarization effects is crucial in order to correctly describe the resonance spectra of these molecules and also to identify a Ramsauer-Townsend minimum for SiI${}_{4}$ and a virtual state for SiCl${}_{4}$.

We present results of measurements and calculations of elastic electron scattering from pyrimidine in the energy range 3–50 eV. Absolute differential and integral elastic cross sections have been measured using a crossed electron-molecule beam spectrometer and the relative flow technique. The measured cross sections are compared with results of calculations using the well-known Schwinger variational technique and an independent-atom model. Agreement between the measured differential cross sections and the results of the Schwinger calculations is good at lower energies but less satisfactory at higher energies where inelastic channels that should be open are kept closed in the calculations.

The PH radical is an interesting case to study because of its ${\ensuremath{\pi}}^{2}$ electronic configuration, which leads to the three low-lying electronic states, $X$ ${}^{3}{\ensuremath{\Sigma}}^{\ensuremath{-}}$, $a$ ${}^{1}\ensuremath{\Delta}$, and $b$ ${}^{1}{\ensuremath{\Sigma}}^{+}$. The singlet states possess long lifetimes, because direct combination with the ground state is not allowed. This is similar to the ${\mathrm{O}}_{2}$ case. The $\mathit{R}$-matrix method is employed to calculate elastic differential and integral cross sections, momentum-transfer cross sections, and inelastic cross sections. The energy range is 0--10 eV. The Hartree-Fock ground-state configuration of PH is $1{\ensuremath{\sigma}}^{2}2{\ensuremath{\sigma}}^{2}3{\ensuremath{\sigma}}^{2}1{\ensuremath{\pi}}^{4}4{\ensuremath{\sigma}}^{2}5{\ensuremath{\sigma}}^{2}2{\ensuremath{\pi}}^{2}$. We have included thirty-seven target states in the trial wave function of the scattering system in our best model. Each target state is represented by a configuration interaction (CI) wave function. In our CI model, we freeze ten electrons in orbitals $1\ensuremath{\sigma}$, $2\ensuremath{\sigma}$, $3\ensuremath{\sigma},$ and $1\ensuremath{\pi}$; the remaining six electrons are free to move among the seven orbitals $4\ensuremath{\sigma}$, $5\ensuremath{\sigma}$, $6\ensuremath{\sigma}$, $7\ensuremath{\sigma}$, $8\ensuremath{\sigma}$, $2\ensuremath{\pi},$ and $3\ensuremath{\pi}$. Our CI model yields a dipole moment of 0.67 D at the equilibrium PH bond length, ${R}_{e}$, of 2.687 ${a}_{0}$; this dipole moment compares favorably with the experimental value 0.70 D. We have carried our scattering calculations in one state with the CI wave function and thirty-seven-state model. We have detected a stable anionic bound state ${}^{2}{B}_{1}$ of PH at various bond lengths of PH molecule. The vertical electron affinity value is 0.97 eV, which is in excellent agreement with experimental value of 1.00 eV. We have included up to $g$ partial waves in the scattering calculations. The contribution of higher partial waves is accounted for by using a Born-closure procedure.

We report results from an ab initio calculation of low-energy electron scattering by CS2 molecules using the Schwinger multichannel method with pseudopotentials. We calculated elastic integral, differential and momentum transfer cross sections in an energy range from 5 eV up to 50 eV and compared our results with available theoretical results and experimental data. Through the symmetry decomposition of our integral cross section and eigenphase sum analysis, we found cross section peaks that may be interpreted as broad shape resonances in the cases of the Sg, Pg, Pu, and Du symmetries. Among these possible resonances, the Pg, Pu, and Du are being reported for the first time.

New evaluations of the neutron cross sections for 14N and 16O have been made for ENDF/B-VI. The evaluations are based at low energies on R-matrix analyses of reactions in the 15N and 17O systems, and at higher energies on GNASH calculations and experimental data evaluations, including covariance analyses. The 15N system R-matrix analysis includes data from reactions among the channels n+14N, p+14C, and α+11B at energies corresponding to excitations in 15N below Ex=13 MeV. The resonance structure of all cross sections in this energy range is fairly well reproduced. New data indicate a different J-value for the first resonance, however. Sub-threshold S-wave levels required to explain the large n+14N total and elastic cross sections near zero energy give scattering lengths that differ significantly from the previous values. The R-matrix analysis of the 17O system includes many new measurements of the n+16O total cross section, done primarily at Oak Ridge and at Karlsruhe. The resonance structure of all the cross sections [total, (n,n), (n,α), and (α,α)] is well represented by the fit in the region below En = 6.5 MeV. The new total cross section information gives different positions for some of the resonances and implies a different normalization for the (n,α) cross sections than that obtained in the ENDF/B-IV analysis. The evaluations at energies above the ranges of the R-matrix analyses incorporate results from a number of experiments performed since the previous ENDF/B evaluations. Especially important are new measurements of the total cross sections and differential elastic, inelastic, and gamma-ray production cross sections.

Potential anisotropy effects on inelastic differential cross sections

In the present work we report cross sections for electron collisions with the isomers propene (C3H6) and cyclopropane (c-C3H6). Electron-scattering differential cross sections (DCS) are reported for measurements carried out for energies 1.5-100 eV and the angular range of 20 degrees-120 degrees. Elastic integral cross sections (ECS), DCS, and momentum-transfer cross sections (MTCS) are reported for calculations carried out using the Schwinger multichannel method with pseudopotentials for the energy range of 2.0-40 eV and angular range of 0 degrees-180 degrees. The resemblance of the pi* shape resonance in the cross sections, observed at 1.5-2.0 eV for propene, to those in C2H4 and C2F4 clearly points to the effect of the double bond in the molecular structures for these molecules. Below 60 eV, we observed clear differences in peak positions and magnitudes between the DCS, ECS, and MTCS for C3H6 and c-C3H6, which we view as the isomer effect.

We present calculated cross sections for elastic and inelastic collisions of low-energy electrons with octafluorocyclobutane, c-C4F8. The integral elastic cross section displays a rich resonance structure, which we analyze in terms of temporary trapping in virtual valence orbitals. The differential elastic cross sections compare well with recent measurements at energies where the approximations used in the calculations are expected to be valid. Integral and differential cross sections for electron-impact excitation of the lowest singlet and triplet excited states were obtained. We relate the small magnitude of the inelastic integral cross sections and the unusual form of the inelastic differential cross sections to the symmetries of the electronic states involved in the transition.

Elastic differential, integral, and momentum-transfer cross sections are reported for electron scattering by ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ at impact energies ranging from 1 to 50 eV. The Schwinger iterative variational method in the fixed-nuclei, static-exchange plus correlation-polarization approximation is used to calculate the scattering amplitudes. Integrated cross sections are also calculated below 1 eV, showing the existence of a Ramsauer-Townsend minimum in this region. Our calculated cross sections are compared with recent experimental data and other theoretical results.

We report results for positron scattering by ground-state atomic hydrogen in the energy range 0 to 80 eV. The calculations have been performed in an 18-state Ps(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d)+H(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d) approximation where the pseudostates (denoted by a bar) have been taken from Fon et al. (1981). Cross sections are presented for elastic scattering, positronium formation, total scattering and ionization. The elastic scattering results are in good agreement with accurate variational numbers at low energies and with other sophisticated, but very different, theoretical approximations at higher energies. We estimate that the elastic cross section is now known to better than 10%. The cross section for positronium formation is dominated by capture into the 1s state and is in fairly good agreement with the measurements of Weber et al. (1994). The results for the total cross section are generally consistent with the upper and lower bounds of Zhou et al. (1994) but are a little larger than other theoretical estimates at the higher energies. There is also a general theoretical problem concerning the degree to which the total cross sections for electrons and positrons merge at energies above 31 eV. The calculated ionization cross section is in agreement with the measurements of Jones et al. (1993).

We present results of measurements and calculations of elastic electron scattering from 1,4-dioxane in the energy range of 0-1000 eV. Absolute differential and integral elastic cross sections have been measured using a crossed electron-molecule beam spectrometer and the relative flow technique, at four energies in the 10-30 eV range and for scattered electrons in the angular range 20°-129°. The measured cross sections are compared with results from R-matrix computations, at the static exchange plus polarization level, calculated at energies between 0-20 eV, and with calculations employing the independent atom model with the screening corrected additivity rule (IAM-SCAR). Those latter computations were conducted at energies between 1 and 1000 eV. Agreement between the measured and R-matrix cross sections was typically found to be good at all common energies, whereas agreement with IAM-SCAR was satisfactory only at 30 eV. To the best of our knowledge, the present results are the first absolute data to be published in the literature for this scattering system.

Recent work is reviewed on the theory of angular momentum decoupling approximations in inelastic molecular scattering theory. It is shown that the failure of these approximations is more extensive than previously believed, particularly for completely state-selected m transitions. A systematic study of a number of centrifugal sudden approximations is made and discussed. A new method is presented for improving these approximations, which recouples the approximate solutions. Completely state-selected differential and integral cross sections are computed under the centrifugal decoupling approximation and its correction for Ne+HD scattering at a total energy of 31.5 meV. Five choices of the partial wave decoupling parameter (including the three known ones) are studied. The numerical results show that with the partial wave parameter chosen to be the arithmetic mean of the initial and final l-quantum numbers, the coupled states approximation gives excellent results for orientation averaged transitions, but fails dramatically for state-selected m transitions. The corrected centrifugal decoupling approximation gives significantly more accurate inelastic differential and integral state-selected cross sections than its uncorrected counterpart. The extension of this correction technique to the energy sudden and the infinite order sudden approximations is discussed.