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.     

Measurements of differential cross sections fore-Ar,Kr, Xe scattering atE = 0.05 − 2 eV

Differential scattering experiments have been performed for the systemse-Ar, Kr, Xe in the energy rangeE=0.05 ? 2 eV and the angular range ?=20–100° using a crossed-beam arrangement. The measurements cover the region of the Ramsauer-Townsend minimum (for the first time in differential scattering using modern techniques) and show the drastic variation of the cross section over this energy range. For these experiments, a new electron spectrometer has been developed which is especially designed to allow measurements at very low collision energies under well controlled conditions. In the present work, the measurements could be extended down to 50 meV. The energy resolution was ΔE = 10 meV (FWHM). Absolute cross sections and the correct transmission properties of the systems are determined usinge-He scattering as a reference system. The experimental data are analysed by the MERT method which allows to extrapolate the cross sections to zero energy. On the basis of the MERT parameters, detailed comparison is made with other work.     

Triple-differential cross sections of helium for (e, 2e) processes

The triple differential cross sections for electron impact ionization of helium in a symmetric coplanar energy-sharing geometry at incident energies from 45–500 eV and an angle of 45° are calculated by use of the modified BBK model. A comparison with other theoretical results has been performed. It has been found that the present results give an excellent agreement with the absolute experimental measurement for the energy range considered.     

Differential, partial and total electron impact ionization cross sections for SF(6)

Single and double differential ionization cross sections for the production of ions resulting from dissociative, single and double ionization of SF(6) by electron impact have been calculated using a semiempirical formulation based on the Jain-Khare approach. In addition, triple differential cross sections have been obtained for some of the doubly charged fragment ions at an incident electron energy of 100, 150, and 200 eV, respectively, and a fixed scattering angle of 30 degrees. As no previous data seem to exist for differential cross sections we have derived from these differential cross sections corresponding partial and total ionization cross sections from threshold up to 900 eV and compared those with the available theoretical and experimental data.     

Cross sections for electron impact excitation of the vibrationally resolved A 1Pi electronic state of carbon monoxide

The authors report new differential cross section measurements for electron impact excitation of the A (1)Pi(v(')) states of carbon monoxide. The energy range is 20-200 eV. They also reanalyze the A (1)Pi(v(')) manifold cross sections of Middleton et al. [J. Phys. B 26, 1743 (1993)] in order to provide a basis for comparison with our new vibrationally resolved differential cross sections. Excellent agreement is found between the two sets of measurements at all common energies. From 20 to 200 eV the present differential cross sections are extrapolated and integrated, and the corresponding integral excitation cross sections determined. New scaled Born integral cross sections, calculated as a part of the present study, are compared against these experimental integral cross sections, with excellent agreement being found for all the A (1)Pi(v(')=0-7)<--X (1)Sigma(g) (+)(v(")=0) transitions. In addition our scaled Born integral cross sections are found to be in excellent agreement between 300 and 1500 eV with those derived from the previous experiments of Lassettre and Skerbele [J. Chem. Phys. 54, 1597 (1971)] and of Zhong et al. [Phys. Rev. A 55, 1799 (1997)] and from near threshold to 15 eV with those derived from Zobel et al. [J. Phys. B 29, 813 (1996)] and Zetner et al. (J. Phys. B 31, 2395 (1998)].     

Proposed standard inelastic differential electron scattering cross sections; Excitation of the 21 P level in He

Differential and integral cross section data for electron-impact excitation of the 2¹ P level in He have been critically reviewed. Experimental and theoretical results have been compared and a set of differential cross sections at 20° scattering angle in the 25 to 500 eV impact energy range has been deduced based on all available information. It is proposed that this set of data represents the most accurate inelastic differential cross sections available at the present time and could be used as a secondary standard for normalization of cross sections.     

Dissociation of hydrogen molecules in collisions with light alkali ions. II. Li+-H2

The dissociation of a ground state H₂ molecule in single collisions with a Li⁺ ion has been studied using a time of flight technique over a large range of center of mass scattering angles (30° ? υ ? 180°) and collision energies (16 eV < E_cm < 55.5 ev).The results have been transformed into the center of mass system to obtain inelastic differential cross sections (contour maps). In contrast to most other scattering experiments on collision induced dissociation, the results at high energies (E_cm > 40 eV) cannot be explained by a two-step mechanism. Instead dissociation appears to occur in a time comparable to the collision time. The results are consistent with several collision models. Of these the spectator model in which only one of the atoms of the molecule is struck by the incident ion is favored since it is in good agreement with the differential cross sections for backward scattering.     

Elastic and inelastic cross sections for low-energy electron collisions with pyrimidine

We present theoretical elastic and electronic excitation cross sections and experimental electronic excitation cross sections for electron collisions with pyrimidine. We use the R-matrix method to determine elastic integral and differential cross sections and integral inelastic cross sections for energies up to 15 eV. The experimental inelastic cross sections have been determined in the 15-50 eV impact energy range. Typically, there is quite reasonable agreement between the theoretical and experimental integral inelastic cross sections. Calculated elastic cross sections agree very well with prior results.     

Measurements of differential cross sections for vibrational quantum transitions in scattering of Li+ on H2

A pulsed monoenergetic ⁷Li⁺ ion beam (lab. energy 10–40 eV) is scattered from a highly collimated (= 1.5°) H₂ nozzle beam. The time-of-flight spectrum of the ions scattered in the forward laboratory direction shows both a fast peak corresponding to forward center-of-mass scattering and a slow peak corresponding to wide-angle center-of-mass scattering. These peaks have been further resolved to show contributions from individual vibrational quantum transitions. From an analysis of the time-of flight spectra the differential inelastic cross sections for a wide range of angles and energies between 2 eV <E_cm < 9 eV have been determined. The spectra also contain information on rotational inelastic cross sections.     

Dissociative excitation of benzene by electron impact

In the wavelength region 1850–9000 Å radiation from H, C and CH fragments is observed as a result of the dissociative excitation of benzene by electron impact (0–1000 eV). Emission cross sections and threshold energies have been determined for the Balmer series of the hydrogen atom and the A²Δ - X²Π emission of the CH fragment.     

Total scattering cross sections for ethylene by electron impact for incident electron energies from 1 to 2000 eV

We report total scattering cross sections for C₂H₄ molecule by electron impact. Calculations are performed by using two different quantum mechanical methods and they cover the energy range from 1 to 2000 eV. For low energy calculations up to 15 eV, UK molecular R‐matrix code through QUANTEMOL‐N software is used, while intermediate to high energy (15–2000 eV) calculations were carried out by applying spherical complex optical potential formalism. Comparison is made with earlier measurements and theoretical data wherever available. A shape resonance is detected around 2 eV due to the ²B_2g symmetry of an electronic state that corresponds to the temporary negative ion formation of ethylene. The differential cross sections are also calculated for the energy range from 1 to15 eV for the scattering angles between 0º and 180º. © 2013 Wiley Periodicals, Inc.     

Cross section for Ly-α emission by electron impact on methane

Utilizing Ly-α emission cross sections for H₂ as secondary standards, absolute values of Ly-α emission cross sections for CH₄ have been obtained for electron impact energies varying from threshold to 100 eV. A crossed electron beam—molecular beam geometry was employed and the Ly-α radiation was detected at 90° and 45° with respect to the incident electron beam by a solar blind photomultiplier in tandem with an oxygen filter. The results are compared with previous measurements. Appreciable differences among the various experimental data are found.     

Dissociative excitation of water by electron impact

Absolute emission cross sections and threshold energies have been measured for radiation (1850–9000Å) from excited fragments (OH, O and H) produced by electron impact (0–1000 eV) on water vapour. The results are compared with previous experiments and the discrepancies are discussed. The measurements indicate that hydroxyl radicals excited in the A² ∑⁺ state originate from excitation of both singlet and triplet states of the water molecule. Excited atomic fragments arise partly from predissociation of Rydberg states of the water molecule converging to the third ionization potential.     

Differential cross sections for elastic scattering and electron detachment in collisions of halogen anions with noble gas atoms

Electron detachment in collision of halogen anions and noble gas atoms was investigated in the collision energy range of 500-3000 eV. Differential cross sections were determined both for elastic scattering and for the electron detachment process in the angular range of 0-3°. It was found that direct electron detachment, due to the interaction of the discrete ion-atom state with a continuum of states increases with increasing collisional energy. Furthermore, the experimental evidence indicates that direct detachment primarily occurs in a limited range of impact parameters. A model, based on angular coupling, is described which qualitatively explains these two experimentally observed features.     

Surface excitation parameter for 12 semiconductors and determination of a general predictive formula

The surface excitation parameter (SEP) is theoretically calculated for 12 semiconductors (GaN, GaP, GaSb, GaAs, InSb, InAs, InP, SiC, ZnSe, ZnS, Si and Ge) and for Ni (which is usually used as a reference in experiments) for electron energies between 300 eV and 3400 eV, and for angles between 0° and 70° to the surface normal. We use our previous definition of SEP, as the change in excitation probability, for an electron, caused by the presence of the surface in comparison with an electron moving the same distance in an infinite medium. The calculations are performed within the dielectric response theory by means of the QUEELS‐ε(k, ω)‐REELS software determining the energy‐differential inelastic electron scattering cross‐sections for reflection‐electron‐energy‐loss spectroscopy (REELS), and for which the only input is the dielectric function of the medium. By fitting to these SEP values as well as our previous ones, i.e. from 27 materials, including metals, oxides, polymers and semiconductors, we also establish a simple equation depending on the generalized plasmon energy and the energy band gap of the material which allows to estimate the SEP when the dielectric function is not available. Copyright © 2009 John Wiley & Sons, Ltd.     

Classical trajectory study on an ab initio CI vibrotor potential energy surface for Li+-CO differential cross sections

A previous rigid rotor potential surface for Li⁺-CO has been improved by computing surface points for two additional CO bond lengths at three different angles of orientation. The CI calculations including all single and double excitations which can be generated within the Hartree-Fock SCF molecular orbital basis have been improved by taking certain quadruple excitations into account in an approximate way. Classical trajectories computed on this surface have been used to determine differential cross sections at scattering angles of 37.1°, 43.2° and 49.2°, and for a relative kinetic energy of 4.23 eV. Comparison with experiment shows that inclusion of CO vibrations does not account for the discrepancy found previously between the classical rigid rotor and the experimental results. When summed over all final vibrational levels the vibrotor results are nearly identical to the rigid rotor cross sections.     

Differential measurements on Ne*(2p3sP0.2)-Ne collisions in the hyperthermal energy range

Differential cross sections for collisions of metastable neon atoms with ground state neon atoms have been measured in the energy range 0.247–0.551 eV in a crossed nozzle beam experiment using heating and seeding techniques. At large angle, the cross sections exhibit a rainbow feature due to a hump in the 0_u⁻ and 1_u potentials. The present data are in good agreement with calculations based on potential energy curves deduced from previous experiments at thermal energy.     

Experimental and theoretical elastic cross sections for electron collisions with the C3H6 isomers

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.     

Effect of Different Electron Elastic-Scattering Cross Sections on Inelastic Mean Free Paths Obtained from Elastic-Backscattering Experiments

Inelastic mean free paths (IMFPs) of electrons with energies between 100eV and 5,000eV have been frequently obtained from measurements of elastic-backscattering probabilities for different specimen materials. A calculation of these probabilities is also required to determine IMFPs. We report calculations of elastic-backscattering probabilities for gold at energies of 100eV and 500eV with differential elastic-scattering cross sections obtained from the Thomas-Fermi-Dirac potential and the more reliable Dirac-Hartree-Fock potential. For two representative experimental configurations, the average deviation between IMFPs obtained with cross sections from the two potentials was 11.4%.     

Experimental and theoretical investigation of the triple differential cross section for electron impact ionization of pyrimidine molecules

Cross-section data for electron impact induced ionization of bio-molecules are important for modelling the deposition of energy within a biological medium and for gaining knowledge of electron driven processes at the molecular level. Triply differential cross sections have been measured for the electron impact ionization of the outer valence 7b(2) and 10a(1) orbitals of pyrimidine, using the (e, 2e) technique. The measurements have been performed with coplanar asymmetric kinematics, at an incident electron energy of 250 eV and ejected electron energy of 20 eV, for scattered electron angles of -5°, -10°, and -15°. The ejected electron angular range encompasses both the binary and recoil peaks in the triple differential cross section. Corresponding theoretical calculations have been performed using the molecular 3-body distorted wave model and are in reasonably good agreement with the present experiment.