Helium scattering angles

Absolute electron-impact cross sections have been measured by Chamberlain, et al.111 for excitation of the 2 P and 2 5 states of helium at a scattering angle of 5° in the energy range 50-400 eV (these conditions are far from the optical limit). Measurements of this type are extremely difficult but with care may approach an accuracy of 5 to 10%. Such determinations are useful for normalizing relative measurements. 

Fig. 3.3. The energy dependence of the amplitude of sub-barrier scattering a(s,8) = -t(s, 8)/2n at two scattering angles 8 = 0, and, 8 = n for (a) singlet and triplet scattering off the hydrogen atom, beryllium atom and (b) helium and neon atoms.

Atom or helium diffraction AD Monoenergetic beams of thermal energy neutral atoms are elastically scattered off ordered surfaces and detected as a function of scattering angle. This gives structural information on the outermost layer of the surface. Atom diffraction is extremely sensitive to surface ordering and defects. Atomic structure... 

Most routine structure analyses are performed on data collected at room temperatures and pressures. Low temperatures " can be achieved with either a liquid nitrogen (Tmin 100 K) or a liquid helium (Tmin 10 K) cryostat, both of which cool the crystal by passing a very cold gas stream over it. Lower temperatures reduce thermal motion, so that more intense Bragg reflection data are found at the higher scattering angles than if the temperature has not been lowered. Such low temperatures also serve to maintain crystal stability. 

Helium scattering has been reported also from a Cu(lOO) crystal which was roughened electrochemieally. It was assumed that the surface consisted of randomly oriented 100 terraces separated by monatomic steps and the observed variation in scattered intensity as a function of incidence angle was attributed to interference between beams scattered from various terraces. It was concluded that atomic steps separated by terraces 50 unit meshes wide could be detected readily. 

Fig, 2, Projectile energy loss for protons colliding with helium as a function of the scattering angle for proton energies of 50 and 100 keV. 

Fig. 20. (a) Energy-loss spectrum of 3-5-keV N+ scattered from 02. The scattering angle was 0° and the target gas pressure was 5-5 mtorr. (b) Energy loss spectrum of 3-5-keV N+ scattered from helium at 0° and 6-0 mtorr. (c) Energy spectrum of the primary beam at 0° with no gas in collision chamber.96... 

This study used helium scattering to locate energy levels of bound states of He/LiF((X)l). If the angle and kinetic energy of the helium beam are varied, dips in the surface reflectivity will occur whenever the atom s kinetic energy perpendicular to the surface equals one of the discrete bound-particle energies of the surface-potential well. In the case in question, four bound levels were observed this information could compare to several different theoretical models. 

For this instrument the angle between the incoming beam and the detector axis is fixed at 90°. To reach the detector the helium atoms must scatter from the surface such that the incident and scattering angles measured with respect to the surface normal sum to 90° 6, -I- 6f = 90°. These atoms pass... 

DLS is used as a routine means of determining particle size in monodisperse polymer dispersions. Typical systems employ a red helium-neon laser (wavelength 633 nm) and a scattering angle of 90°. However, the resolution achievable with such systems when measuring polydisperse samples is generally quite low. As a rule of thumb, the particle diameters of two fractions must differ by a factor of 3 or 4 if they are to be clearly differentiated. 

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