By D.R. Bates (ed.), Benjamin Bederson (ed.)
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Extra resources for Advances in Atomic and Molecular Physics, Vol. 14
Both of these calculations have failed to show any of these additional structures as has the transmission experiment of Sanche and Schulz (1972a). The results of Golden and Zecca (1970) were obtained by using a signal averaged over about 80 sweeps of the energy domain studied and required about 20 hours running time in order to obtain an energy spectrum. Thus, the results of Golden and Zecca (1970) were possibly subject to long-term drift and consequent systematic error. Therefore, the same energy domain was studied by Golden et al.
The matrix element corresponding to elastic scattering is, from Eq (1l), s0 -- e 2 i d o E - E, - i r / 2 E - E, ir/2 where 6, is the nonresonant S-wave phase shift r = ky2K and the scattering matrix is continued across the threshold according to the prescription iK = k , . Since y 2 k is almost constant near the threshold, r N TK, where is a constant. Then the elastic scattering cross section is given by r2K2/4 ( E - E,)2 + T2K2/4 -K -r-( 2 ( E - E,) sin 26, + r K sin26, ( E - E,)2 + f2K2/4 T 2 k : sin2 6 , r"k$/4 ( E - E , Tk,/2)2 ( E - E , fk,/2)' + + The cusp is due to the third term in Eq.
However, when the electron detector is placed at very small (or very large) angles with respect to the incident beam direction, (as shown in Fig. lob) the electron detector views a larger interaction length, which may be a complicated function of scattering angle studied. When one designs an apparatus to be able to study very small (and very large) scattering angles, it is tempting to move the detector further from the interaction region. However, this makes the situation worse. The effect of scattering from the background gas for electrons not scattered from the target beam results in a measurement of too ELECTRON ATOM AND MOLECULE SCATTERING 25 much scattering at the smaller (or larger) scattering angles but from places other than the scattering center.