星野 正光

We report absolute differential cross sections (DCSs) for elastic electron scattering from OCS (carbonyl sulphide) and CS2 (carbon disulphide) in the impact energy range of 1.2-200 eV and for scattering angles from 10 degrees to 150 degrees. Above 10 eV, the angular distributions are found to agree quite well with our present calculations using two semi-phenomenological theoretical approaches. One employs the independent-atom model with the screening-corrected additivity rule (IAM-SCAR), while the other uses the continuum-multiple-scattering method in conjunction with a parameter-free exchange-polarization approximation. Since OCS is a polar molecule, further dipole-induced rotational excitation cross sections have been calculated in the framework of the first Born approximation and incoherently added to the IAM-SCAR results. In comparison with the calculated DCS for the S atom, atomic-like behavior for the angular distributions in both the OCS and CS2 scattering systems is observed. Integrated elastic cross sections are obtained by extrapolating the experimental measurements, with the aid of the theoretical calculations, for those scattering angles below 10 degrees and above 150 degrees. These values are then compared with the available total cross sections. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4788666]

We report on the first measurements of the electron impact electronic excitation cross sections for C4F6 isomers, hexafluoro-1,3-butadiene (1,3-C4F6), hexafluorocyclobutene (c-C4F6), and hexafluoro-2-butyne (2-C4F6), measured at 100 eV, 3 degrees scattering angle, while sweeping the energy loss over the range 2.0-15.0 eV. Under these experimental conditions, optically allowed transitions are favored. The electronic state spectroscopy has been investigated and the assignments supported by quantum chemical calculations. The n = 3 members of the Rydberg series have been assigned converging to the lowest ionization energy limits of the C4F6 isomers and classified according to the magnitude of the quantum defects (delta).

We report on the first measurements of the electron impact electronic excitation cross sections for C 4F 6 isomers, hexafluoro-1,3-butadiene (1,3-C 4F 6), hexafluorocyclobutene (c-C 4F 6), and hexafluoro-2-butyne (2-C 4F 6), measured at 100 eV, 3° scattering angle, while sweeping the energy loss over the range 2.0-15.0 eV. Under these experimental conditions, optically allowed transitions are favored. The electronic state spectroscopy has been investigated and the assignments supported by quantum chemical calculations. The n = 3 members of the Rydberg series have been assigned converging to the lowest ionization energy limits of the C 4F 6 isomers and classified according to the magnitude of the quantum defects (δ). © 2012 American Chemical Society.

We report absolute differential cross sections for elastic electron scattering from SiF4. The incident electron energy range is 1.5-200 eV, while the scattered electron angular range was from 15 degrees to 150 degrees. The absolute scale of the differential cross sections was set using the relative flow technique with helium as the reference species. Corresponding integral elastic and momentum transfer data have been derived from our differential measurements. As a part of this study, independent atom model calculations were also performed. Those computations were found to be in generally quite good accord with the experimental results at both the differential and integral levels. In addition, excitation function measurements at a scattering angle of 60 degrees and in the energy range 1-14 eV for the nu(3) (d-stretch) and nu(4) (d-deformation) fundamental vibrational modes are reported. Finally, differential cross sections for all four fundamental modes (nu(1), nu(2), nu(3), nu(4)) at the single electron energy of 7 eV are given. These vibrational excitation measurements, when coupled with the elastic angular distributions in the 5-8 eV energy range, demonstrate the presence of a t(2)-symmetry shape resonance.

A new experimental technique for the measurements of the total cross section for electron scattering from atoms and molecules at very low energy is described. Momentum transfer cross sections for scattering from Ar, Kr and Xe at very low energies were carefully derived using the modified effective range theory from the recently measured total cross sections, which were obtained with a new experimental technique utilizing the threshold photoelectron source. A significant discrepancy between the momentum cross sections derived from the present analysis and those determined in the previous electron swarm studies was found at energies below 100 meV. The findings emphasize the need of further high precision experiments in the very low energy region as well as re-analysis of the previous swarm data.

A new experimental technique for the measurements of the total cross section for electron scattering from atoms and molecules at very low energy is described. Momentum transfer cross sections for scattering from Ar, Kr and Xe at very low energies were carefully derived using the modified effective range theory from the recently measured total cross sections, which were obtained with a new experimental technique utilizing the threshold photoelectron source. A significant discrepancy between the momentum cross sections derived from the present analysis and those determined in the previous electron swarm studies was found at energies below 100 meV. The findings emphasize the need of further high precision experiments in the very low energy region as well as re-analysis of the previous swarm data. © The Author(s) 2012.

We report absolute differential cross sections (DCSs) for elastic electron scattering from GeF4. The incident electron energy range was 3-200 eV, while the scattered electron angular range was typically 15 degrees-150 degrees. In addition, corresponding independent atom model (IAM) calculations, within the screened additivity rule (SCAR) formulation, were also performed. Those results, particularly for electron energies above about 10 eV, were found to be in good quantitative agreement with the present experimental data. Furthermore, we compare our GeF4 elastic DCSs to similar data for scattering from CF4 and SiF4. All these three species possess T-d symmetry, and at each specific energy considered above about 50 eV their DCSs are observed to be almost identical. These indistinguishable features suggest that high-energy elastic scattering from these targets is virtually dominated by the atomic-F species of the molecules. Finally, estimates for the measured GeF4 elastic integral cross sections are derived and compared to our IAM-SCAR computations and with independent total cross section values. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3699040]

We report the first evidence for double-slit interferences in a polyatomic molecule, which we have observed in the experimental carbon 1s photoelectron spectra of acetylene (or ethyne). The spectra have been measured over the photon energy range of 310-930 eV and show prominent oscillations in the intensity ratios sigma(g)(upsilon)/sigma(u)(upsilon) for the vibrational quantum numbers upsilon = 0, 1 and for the ratios sigma(s)(upsilon = 1)/sigma(s)(upsilon = 0) for the symmetry s = g, u. The experimental findings are in very good agreement with ab initio density functional theory (DFT) calculations and are compatible with the Cohen-Fano mechanism of coherent emission from two equivalent atomic centers. This interpretation is supported by the qualitative predictions of a simple model in which the effect of nuclear recoil is taken into account to the lowest order. Our results confirm the delocalized character of the core hole created in the primary photoionization event and demonstrate that intramolecular core-hole coherence can survive the decoherent influence associated with the asymmetric nuclear degrees of freedom which are characteristic of polyatomic molecules.

Magnetic-field-free measurements of the absolute total cross sections (TCS) for positron-neon scattering have been performed. In the energy range of 9-15 eV, the present results are larger than those of the other groups except for the Toronto group.

We review earlier cross section data sets for electron-collisions with H-2, O-2, CO, CO2, H2O and N2O, updated here by experimental results for their electronic states. Based on our recent measurements of differential cross sections for the electronic states of those molecules, integral cross sections (ICSs) are derived by applying a generalized oscillator strength analysis and then assessed against theory (BEf-scaling [Y.-K. Kim, J. Chem. Phys. 126, 064305 (2007)]). As they now represent benchmark electronic state cross sections, those ICSs for the above molecules are added into the original cross section sets taken taken from the data reviews for H-2, O-2, CO2 and H2O (the Itikawa group), and for CO and N2O (the Zecca group).

Magnetic field free measurements of the total cross sections for positron neon and positron argon scattering have been performed using an electrostatic high brightness slow positron beam apparatus.

We have measured electronic excitation differential cross sections for C4F6 molecules isomers by electron impact. In the case of hexafluoro-1,3-butadiene we observed an optical forbidden transition at around 5 eV. The spectra of the three C4F6 isomers show the most intense band clearly shifted to lower energies when going from 2-C4F6, to c-C4F6 and to 1,3-C4F6.

In this study we have measured angular distributions of differential cross sections (DCS) for vibrational excitation and superelastic scattering from vibrationally excited N2O. The results are analyzed and interpreted using the angular correlation theory by Read.

We report absolute differential cross sections (DCSs) and integral cross sections (ICSs) for elastic electron scattering with CCl4 molecules in the energy range of 1.5-100eV. In addition, the experimental DCSs are compared with the results from theoretical calculations based on the Schwinger multichannel method with norm-conserving pseudopotentials.

Absolute total cross sections for electron scattering from Ar and Xe at electron energies ranging from 7 meV to 20 eV were obtained with the experimental technique employing the threshold-photoelectron source. The measured total cross sections are in good agreement with those obtained by other groups down to 100 meV, above which several experimental works have been reported. Scattering lengths for electron scattering from Ar, Kr, and Xe were determined from the present total cross sections and our recent results for Kr using the modified effective range theory. The values of the scattering length obtained in the present analysis differ from the values determined from the previous swarm experiments and beam experiments. The resonant structures in the total cross sections due to Feshbach resonances of Ar, Kr, and Xe with an improved energy resolution were also measured. Analyses of the resonant structure were carried out based on the spin-dependent resonant scattering theory in order to determine the values of the natural width of Feshbach resonances of Ar, Kr, and Xe precisely.

We report absolute elastic differential, integral and momentum transfer cross sections for electron interactions with CCl4. The incident electron energy range is 1.5-100 eV, and the scattered electron angular range for the differential measurements varies from 15 degrees-130 degrees. The absolute scale of the differential cross section was set using the relative flow technique with helium as the reference species. Comparison with previous total cross sections shows good agreement. Atomic-like behaviour in this scattering system is shown here for the first time, and is further investigated by comparing the CCl4 elastic cross sections to recent results on the halomethanes and atomic chlorine at higher impact energies [H. Kato, T. Asahina, H. Masui, M. Hoshino, H. Tanaka, H. Cho, O. Ingolfsson, F. Blanco, G. Garcia, S. J. Buckman, and M. J. Brunger, J. Chem. Phys. 132, 074309 (2010)]. (C) 2011 American Institute of Physics. [doi:10.1063/1.3669429]

High-resolution N 1s and O 1s photoelectron spectra (PES) of NO are presented together with spectra of the subsequent Auger decay. The PES are analyzed by taking spin-orbit splitting of the 2Π ground state into account providing detailed information on equilibrium distances, vibrational energies, and lifetime widths of the core-ionized states. In the Auger electron spectra (AES) transitions to five metastable dicationic final states are observed, with two of them previously unobserved. A Franck-Condon analysis of the vibrational progressions belonging to these transitions provides detailed information on the potential-energy curves of the dicationic final states as well as on the relative Auger rates. The present calculations of the potential-energy curves of NO2+ agree well with the experimental results and allow an assignment of the two hitherto unresolved Auger transitions to excited states of NO2+, C2Σ+and c4Π. © 2011 the Owner Societies.

Magnetic-field-free measurements of absolute total cross sections for positron scattering from helium have been performed using an electrostatic high-brightness slow positron beam apparatus. In the energy region above 20 eV, the present results are in good agreement with those of other studies. On the other hand, in the energy range of 4-20 eV, our values are higher than most of the other measurements except one study.

In this study, angle-resolved energetic-ion yield spectra were measured in the N 1s excitation region of N2O. A Franck-Condon analysis based on ab initio two-dimensional potential energy surfaces of the core-excited Rydberg states, which were calculated by the symmetry-adapted cluster-configuration interaction method, reproduced observed vibrational excitations specific to the individual Rydberg states well and enabled quantitative assignments. Geometric changes in the terminal nitrogen N-t 1s and the central nitrogen N-c 1s excited states with respect to the 3p pi, 3p sigma, and 4s sigma transitions were analyzed. The coupling of these valence and Rydbergs states was examined based on the second moment analysis. Irregular Rydberg-state behavior in the N-c 1s(-1) 4s sigma state was observed.

Low energy electron attachment to gas phase carbonyl fluoride, F2CO,has been performed by means of a crossed electron-molecular beam experiment in an electron energy range from 0 to 30 eV with an energy resolution of similar to 0.5 eV. The most intense signal is observed at 19 amu due to F- and two other anionic species with lower intensities at 38 and 47 amu assigned to F-2(-) and COF-, respectively. Anion efficiency curves of the three anions have been measured. Product anions (F- and COF-) are observed mainly in the low energy region arising from simple bond breaking, while F-2(-) is being produced from two bond cleavages with further structural and electronic rearrangement. Quantum chemical calculations on the electronic properties of F2CO have been performed in order to complement the experimental results. (C) 2011 Elsevier B.V. All rights reserved.

We present results from a joint experimental and theoretical study of elastic electron scattering from atomic iodine. The experimental results were obtained by subtracting known cross sections from the measured data obtained with a pyrolyzed mixed beam containing a variety of atomic and molecular species. The calculations were performed using both a fully relativistic Dirac B-spline R-matrix (close-coupling) method and an optical model potential approach. Given the difficulty of the problem, the agreement between the two sets of theoretical predictions and the experimental data for the angle-differential and the angle-integrated elastic cross sections at 40 eV and 50 eV is satisfactory.

We report results from measurements for differential and integral cross sections of the unresolved B-1(1u) and E-3(2g) electronic states and the E-1(1u) electronic state in benzene. The energy range of this work was 10-200 eV, while the angular range of the differential cross sections was similar to 3 degrees-130 degrees. To the best of our knowledge there are no other corresponding theoretical or experimental data against which we can compare the present results. A generalized oscillator strength analysis was applied to our 100 and 200 eV differential cross section data, for both the B-1(1u) and E-1(1u) states, with optical oscillator strengths being derived in each case. The respective optical oscillator strengths were found to be consistent with many, but not all, of the earlier theoretical and experimental determinations. Finally, we present theoretical integral cross sections for both the B-1(1u) and E-1(1u) electronic states, as calculated within the BE f -scaling formalism, and compare them against relevant results from our measurements. From that comparison, an integral cross section for the optically forbidden E-3(2g) state is also derived. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3575497]

Cross sections for positron scattering from krypton have been measured with an energy resolution of ∼60 meV over the energy range 0.5-60 eV. Absolute values of the grand total (σGT), positronium formation (σPs), and grand total minus positronium formation (σGT-σPs,) cross sections are presented. Theoretical estimations of σGT and σGT- σPs are also performed for this target using the convergent close-coupling method and the relativistic optical potential approach. We also provide experimental and theoretical results for elastic differential cross sections, for selected energies both below and above the Ps threshold. Where available, the present results are compared to both experimental and theoretical values from the literature. © 2011 American Physical Society.

Cross sections for positron scattering from krypton have been measured with an energy resolution of similar to 60 meV over the energy range 0.5-60 eV. Absolute values of the grand total (sigma(GT)), positronium formation (sigma(Ps)), and grand total minus positronium formation (sigma(GT) - sigma(Ps),) cross sections are presented. Theoretical estimations of sGT and sigma(GT) - sigma(Ps) are also performed for this target using the convergent close-coupling method and the relativistic optical potential approach. We also provide experimental and theoretical results for elastic differential cross sections, for selected energies both below and above the Ps threshold. Where available, the present results are compared to both experimental and theoretical values from the literature.

Experimental results are reported for elastic differential and integral cross sections for electrons scattering from iodomethane (CH(3)I). These measurements were made at seven incident electron energies in the range 5-50 eV, with a scattered electron angular range of 20 degrees-135 degrees. Corresponding calculations using the independent atom method plus screened additivity rule (IAM-SCAR), both with and without a dipole correction, are also reported as a part of this study. Where possible, comparison is made to the only other set of experimental results available in the literature (Kato et al 2010 J. Chem. Phys. 132 074309) and to calculated cross sections from the Schwinger multichannel approach at the static exchange level (Natalense et al 2001 Braz. J. Phys. 31 15). In general, good agreement is found between the present measurements and IAM-SCAR computations, and between our results and the earlier investigations.

We have measured the total cross sections for positron scattering from He using an electrostatic high-brightness slow positron beam apparatus. Our results are in reasonable agreement with the data by Jaduszliwer et al in the energy region below 15 eV, which have been obtained under the influence of the lower axial magnetic field than other groups.

The O 1s and N 1s excited states of N2O (nitrous oxide, N t-Nc-O) have been investigated by the symmetry-adapted cluster-configuration interaction (SAC-CI) method. Our approach in this series of works using high-resolution angle-resolved ion-yield (ARIY) spectroscopy and the SAC-CI method is reviewed for the O 1s excited states of N2O. The vibrational structure observed by ARIY spectroscopy was interpreted by two-dimensional ab initio potential energy surfaces (2D PESs). The valence-Rydberg coupling was analyzed by the electronic part of the second moment, 〈r2〉. The thermal effect in the core-electron excitation spectrum was examined by the PES calculations in the bending coordinate. The 2D PESs of the Nt and Nc excited states have also been calculated by the SAC-CI method and are discussed in detail.

An experimental technique for the measurement of the total cross section for electron scattering from atoms and molecules at high resolution is described. The total cross sections for electron scattering from Kr in the energy range from 14 meV to 20 eV obtained with the technique are also reported. The present technique employs a combination of the penetrating field technique and the threshold photoionization of rare-gas atoms using synchrotron radiation as an electron source in order to produce a high-resolution electron beam at very low energy. The characteristics of the electron sources were determined by measuring the ionizing photon energy dependence of photoelectron yield. Absolute total cross sections for electron scattering are obtained by the attenuation method. The measured absolute values of the total cross sections for electron scattering from Kr agree with those obtained by other groups down to 175 meV, above which several experimental works have been reported. Below 175 meV, the present results generally agree with theoretical cross sections down to 14 meV. The resonant structures in the total cross sections due to Kr- (4p(5)5s(22)P(3/2)) and the Kr- (4p(5)5s(22)P(1/2)) Feshbach resonances are also reported. The resolution of the present setup has been estimated from a fit of the measured profile of the Kr- (4p5(5)s(22)P(3/2)) resonance by the theoretical curve obtained from the resonant scattering theory.

We have measured the initial vibrational-state-specific, symmetry-resolved ion-yield spectra of CO2 and N2O in the shape resonance regions above the K-shell ionization thresholds. For both molecules, significant diminution of the shape resonances by bending excitation in the initial electronic ground state is observed. The measured ion-yield spectra are well reproduced by calculations employing the Schwinger variational principle. The observed changes in the spectra are seen to be due to a shift of the resonances to lower energy and an initial increase in width with bending.

Recoil-induced rotational excitation accompanying photoionization has been measured for the X, A, and B states of N-2(+) and CO+ over a range of photon energies from 60 to 900 eV. The mean recoil excitation increases linearly with the kinetic energy of the photoelectron, with slopes ranging from 0.73 x 10(-5) to 1.40 x 10(-5). These slopes are generally (but not completely) in accord with a simple model that treats the electrons as if they were emitted from isolated atoms. This treatment takes into account the atom from which the electron is emitted, the molecular-frame angular distribution of the electron, and the dependence of the photoelectron cross section on photon energy, on atomic identity, and on the type of atomic orbital from which the electron is ejected. These measurements thus provide a tool for investigating the atomic orbital composition of the molecular orbitals. Additional insight into this composition is obtained from the relative intensities of the various photolines in the spectrum and their variation with photon energy. Although there are some discrepancies between the predictions of the model and the observations, many of these can be understood qualitatively from a comparison of atomic and molecular wavefunctions. A quantum-mechanical treatment of recoil-induced excitation predicts an oscillatory variation with photon energy of the excitation. However, the predicted oscillations are small compared with the uncertainties in the data, and, as a result, the currently available results cannot provide confirmation of the quantum-mechanical theory. (C) 2010 American Institute of Physics. [doi:10.1063/1.3503658]

An experimental technique for the measurement of the total cross section for electron scattering from atoms<br /> and molecules at high resolution is described. The total cross sections for electron scattering from Kr in the<br /> energy range from 14 meV to 20 eV obtained with the technique are also reported. The present technique<br /> employs a combination of the penetrating field technique and the threshold photoionization of rare-gas atoms<br /> using synchrotron radiation as an electron source in order to produce a high-resolution electron beam at very low<br /> energy. The characteristics of the electron sources were determined by measuring the ionizing photon energy<br /> dependence of photoelectron yield. Absolute total cross sections for electron scattering are obtained by the<br /> attenuation method. The measured absolute values of the total cross sections for electron scattering from Kr<br /> agree with those obtained by other groups down to 175 meV, above which several experimental works have been<br /> reported. Below 175 meV, the present results generally agree with theoretical cross sections down to 14 meV.<br /> The resonant structures in the total cross sections due to Kr− (4p55s2 2P3/2) and the Kr−

Absolute electron-impact cross sections for molecular targets, including their radicals, are important in developing plasma reactors and testing various plasma processing gases. Low-energy electron collision data for these gases are sparse and only the limited cross section data are available. In this report, elastic cross sections for electron-polyatomic molecule collisions are compiled and reviewed for 17 molecules relevant to plasma processing. Elastic cross sections are essential for the absolute scale conversion of inelastic cross sections, as well as for testing computational methods. Data are collected and reviewed for elastic differential, integral, and momentum transfer cross sections and, for each molecule, the recommended values of the cross section are presented. The literature has been surveyed through early 2010. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3475647]

Excitation of the A-band low-lying electronic states in the methyl halides, CH(3)I, CH(3)Br, CH(3)Cl, and CH(3)F, has been investigated for the (n -&gt;sigma*) transitions, using electron energy loss spectroscopy (EELS) in the range of 3.5-7.5 eV. For the methyl halides, CH(3)I, CH(3)Br, and CH(3)Cl, three components of the Q complex ((3)Q(1), (3)Q(0), and (1)Q(1)) were directly observed, with the exception of methyl fluoride, in the optically forbidden EELS experimental conditions of this investigation. The effect of electronic-state curve crossing emerged in the transition probabilities for the (3)Q(0) and (1)Q(1) states, with spin-orbit splitting observed and quantified against results from recent ab initio studies. (C) 2010 American Institute of Physics. [doi:10.1063/1.3464483]

In this paper, we present integral cross sections (ICS) for electron impact excitation of the n=2 levels in helium in the impact energy range of 23.5 eV to 35 eV. The ICS of each final state, 2(3)S, 2(1)S, 2(3)P and 2(1)P, has been determined by integration of the angular differential cross sections (DCS) over all of 0 to 180, where those DCS were obtained from both our previous experiments and the extrapolation using the convergent close coupling calculation. The present experimental ICS for the optically allowed 2(1)P transition state are also compared with those obtained from the BEf-scaling method. Very good agreement between the experimental and BEf-scaled 2(1)P ICSs is generally found in the measured impact energy region.

We report differential cross sections for electron impact excitation of the a (1)Pi(g), C (3)Pi(u), E (3)Sigma(+)(g), a '' 1 Sigma(+)(g), b(1)Pi(u), c(3) (1)Pi(u), o(3) (1)Pi(u), b' (1)Sigma(+)(u), G(3)Pi(+,)(u) and F-3 Pi(+)(u) electronic states in N-2. The incident electron energies are 20, 30, and 40 eV, while the scattered electron angles are 10 degrees and 20 degrees. These kinematic conditions were specifically targeted in order to try and shed new light on the worrying discrepancies that exist in the literature for the a (1)Pi(g), C (3)Pi(u), E (3)Sigma(g)+ and a '' 1 Sigma(+)(g) cross sections, and in general the present measurements confirm that those from the more recent results of the University of California, Fullerton, and the Jet Propulsion Laboratory [M. A. Khakoo, P. V. Johnson, I. Ozkay, P. Yan, S. Trajmar, and I. Kanik, Phys. Rev. A 71, 062703 ( 2005); C. P. Malone, P. V. Johnson, I. Kanik, B. Ajdari, and M. A. Khakoo, Phys. Rev. A 79, 032704 ( 2009)] are reliable. In addition, we provide a rigorous cross-check for the remaining seven electronic states, where the only recent comprehensive study is from Khakoo and colleagues [Phys. Rev. A 77, 012704 ( 2008)]. Here, however, some of those cross sections are confirmed and others are not, suggesting that further work is still needed.

Differential cross sections (DCSs) for excitation of the C-H stretch vibrations for CH3F, CH3Cl, CH3Br and CH3I were measured by electron impact. Resonance enhancement was observed for the C-H stretch at 6.6, 5.9, 5.5 and 4.7 eV respectively. For CH3F this is attributed to a (2)A(1) shape resonance formed through temporary occupation of the LUMO and for the heavier halogens to a E-2 shape resonance formed through temporary occupation of the LUMO+1. DCSs for excitation of the C-X stretch were also measured for CH3F and CH3Cl. The shift in the resonance energy is discussed in context to the substitution effect of the halogens.

We report absolute elastic differential, integral, and momentum transfer cross sections for electron interactions with the series of molecules CH3X (X=F, Cl, Br, I). The incident electron energy range is 50-200 eV, while the scattered electron angular range for the differential measurements is 15 degrees-150 degrees. In all cases the absolute scale of the differential cross sections was set using the relative flow method with helium as the reference species. Substitution effects on these cross sections, as we progress along the halomethane series CH3F, CH3Cl, CH3Br, and CH3I, are investigated as a part of this study. In addition, atomic-like behavior in these scattering systems is also considered by comparing these halomethane elastic cross sections to results from other workers for the corresponding noble gases Ne, Ar, Kr, and Xe, respectively. Finally we report results for calculations of elastic differential and integral cross sections for electrons scattering from each of the CH3X species, within an optical potential method and assuming a screened corrected independent atom representation. The level of agreement between these calculations and our measurements was found to be quite remarkable in each case.

An electrostatic slow positron beam system has been developed for positron scattering studies. The apparatus uses a remoderator in reflection geometry in conjunction with an electrostatic hemispherical energy analyzer for the brightness enhancement. It is possible to determine absolute total scattering cross section values under magnetic field-free conditions down to a few mG. Measurement of the energy distribution of the positrons that are reemitted from the remoderator and total cross sections for positron-He scattering are presented.