Scattering Cross Sections For He Research Articles

Influence of screening length modification on the scattering cross section in LEIS

Scattering cross sections for He + ions in the energy range of 100 eV to 100 keV and for Al, Cu and Au target atoms were calculated. Employing the Thomas–Fermi–Molière model the potential strength was tuned by variation of the screening length. The resulting change in scattering cross section was analyzed and the absolute value is compared to cross sections obtained from potentials commonly employed in the medium-energy ion scattering (MEIS) regime. A large influence on the scattering cross section is observed for targets with large atomic number in the very low energy range. For instance, the scattering cross section for 100 eV He +-ions scattered from Au by 129° changes by a factor of 2.5 between different potential strengths claimed in the literature to be suitable for low-energy ion scattering (LEIS) energies. An experiment to determine electronic energy loss of very slow ions in metals is presented. It shows how uncertainties in the scattering potential strength can lead to systematically wrong results, although perfect agreement between experimental data and simulations is found. The impact of these results on quantitative surface structure and composition analysis is discussed.

Total Electron Scattering Cross Sections of He, Ne, Ar, Kr and Xe in the Energy Range 100 eV to 10 000 eV

The total cross sections for electron scattering from He, Ne, Ar, Kr and Xe in the energy range from 100 eV to 10 000 eV have been calculated based on the optical-model potential. Our theoretical results are compared with the available experimental data. The consistency between them is also discussed. At higher energies (over 2000 eV for He, over 5000 eV for Ne, Ar, Kr and Xe), the total cross sections of electron scattering from these atoms are scarce, so our calculations will give a reference for further experimental and theoretical studies.

Absolute Total Scattering Cross Section of He–H2 over the Relative Velocity Range of 2000–3000 m/sec

Absolute helium-hydrogen molecule total cross sections for thermal energy collisions were measured using molecular beams derived from nozzle sources and crossed at right angles. Both nozzles were adjustable in temperature over limited ranges to provide variation in the relative velocities. The absolute density of the target gas (H2) was measured with a capacitance manometer. Over a relative velocity range of 2000–3000 m/sec the cross section, corrected for finite angular resolution, was found to be nearly constant in magnitude averaging 48 Å2. A spherical potential which best fits the experimental data [Lennard-Jones (12, 6), ε=0.7 meV, rm=3.57 Å] was obtained for atom-molecule separations of 2.7–3.3 Å. The range was determined from the response of the cross section to piecewise variation of the theoretical potential. The results are consistent with previously reported theoretical and experimental works.

The Rayleigh Scattering Cross-Sections of He, C, N and O

The Rayleigh scattering cross-sections of He, C, N and 0 have been computed by quantum defect method. Fairly good agreement has been obtained between the results of dipole length and dipole velocity formalisms. The wavelength independent part of the polarizabilities obtained in the calculation have been compared with the static polarizabilities given by others. At large wavelengths, the Rayleigh scattering cross-sections of He, C, N and 0 relative to H, have been found to be approximately in the ratio of 0.1 : 7.9 : 4.I : 1.4. The effect of wavelength dependent polarizabilities on Rayleigh scattering cross-sections has also been investigated. The importance of Rayleigh scattering vis-a-vis pure absorption has been discussed.

Calculated Scattering Cross Sections for He–He at Thermal Energies

Elastic scattering cross sections were calculated for He- He interactions at thermal energies. The calculation is based on a potential constructed by joining a short-range repulsion via a Morse function to a long- range attraction. The results are illustrated graphically. The experimental Q's were higher than the predicted values. (P.C.H.)