Measurement of differential cross sections

Without the target gas, the vacuum in the chamber is usually maintained at 10 —10 torr by a suitable pump, which is usually either a well trapped diffusion pump or a turbo-molecular pump. When the gas target is introduced the vacuum is generally maintained at about 10 torr. 

The apparatus of Ramsauer and Kollath (1932) is primitive by today s standards. Excellent energy resolution and satisfactory beam intensity can be achieved by utilizing energy-dispersing and direction-focussing 

The most frequently-used lenses in electron optics are aperture lenses or tube (cylindrical) lenses. There is a substantial literature on the design and characteristics of electron lenses and on charged-particle optics. Harting and Read (1976), Hawkes and Kasper (1988) and Wollnik (1987) are especially useful. 

As an example a spectrometer with 12 meV resolution constructed by Linder s group in Kaiserslautern (Weyhreter et al, 1988) is shown schematically in fig. 2.5. This instrument, which can operate down to 50 meV incident electron energy, is constructed of stainless steel and ceramics. It employs double tandem hemispheres for energy selection and analysis. A real aperture is placed between the two hemispheres to eliminate the 

The entrance optics to the analyser is usually chosen so that it can be tuned to nearly constant transmission over a large energy range. The gun optics is also usually chosen so that the beam can be focussed at the same position with the same image size over a wide energy range. This is necessary in order to avoid distortion of peak shapes and resonance features and inaccuracies in the cross-section measurements. After transmission through the analyser a simple lens transfers the electrons to the surface of an electron multiplier, usually a channeltron, which is operated in the pulse count mode. 

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Coleman, P.G. and McNutt, J.D. (1979). Measurement of differential cross sections for the elastic scattering of positrons by argon atoms. Phys. Rev. Lett. 42 1130-1133. 

There are only isolated measurements of integrated cross sections, but there are absolute measurements of differential cross sections. We adopt the procedure of using the coupled-channels-optical calculation of Bray et al. (1991c) to interpolate and extrapolate these measurements, since it agrees quite well with differential cross sections in figs. 8.4 and 8.5. 

Fig. 8.28 Schematic diagram for the measurement of differential cross sections in crossed molecular beams...

Scattering of electrons, fast atoms, or ions with laser-excited atoms A can result in elastic, inelastic, or superelastic collisions. In the latter case, the excitation energy of A is partly converted into kinetic energy of the scattered particles. Orientation of the excited atoms by optical pumping with polarized lasers allows investigations of the influence of the atomic orientation on the differential cross sections for A -f-B collisions, which differs for collisions with electrons or ions from the case of neutral atoms [13.124]. An example of the measurement of differential cross sections of the reactive collisions Na - -HF is given in [13.125]. 

Measurements of Differential Cross Sections in Crossed Molecular Beams... 

The various spectroscopic techniques discussed in the previous sections allowed the measurement of absolute rate constants of collision-induced transitions from which mean integral cross sections could be deduced, averaged over the thermal velocity distribution and over all directions of the relative velocity of the collision partners. Much more detailed information can be obtained from measurements of differential cross sections in crossed molecular beams. In particular nonspherical interaction potentials, which are for instance responsible for rotational transitions in collisions between atoms and molecules and which cannot be obtained from data averaged over all directions, can be deduced from differential cross sections. 

Fig. 12.14a,b> Measurement of differential cross sections for inelastic atom-molecule collisions in crossed beams, (a) Schematic level scheme, (b) experimental arrangement [12.31]... 

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