Origin of spin-dependent effects

The cross sections for scattering of the two beams with opposite polarisation differ from each other because of the spin—orbit part of the scattering potential, which is proportional to the scalar product L S of the 

With careful measurements and analysis, Fletcher et al. (1986) were able to obtain a best value for the uncertainty in S of +5%. 

The role of exchange scattering can be observed most clearly when polarised electrons are scattered from a target of polarised light atoms. If one observes a spin-flip process like 

If explicitly-spin-dependent forces, such as the spin—orbit interaction, are negligible, the cross section for scattering of electrons of polarisation Pe by a one-electron target of polarisation Pa is given in terms of the cross section for an unpolarised beam by (McClelland, Kelley and Celotta, 1987), 

Here / -I- g and / — g are the singlet and triplet amplitudes respectively. The exchange asymmetry is given by (Kessler, 1985) 

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