Superelastic scattering with imperfect laser polarisation

The formalism applied to superelastic electrons scattering from laser-excited atoms has to date assumed perfect polarisation of the laser light. We consider the effects of imperfect polarisation, deriving an expression for the superelastic scattering rate from target atoms optically pumped with elliptically polarised light, and show how imperfect polarisation effects can be measured and minimised under experimental conditions. In particular, we find that in measuring the scattering for linearly polarised light with a small circular component, the errors are proportional to the undesired circular polarisation component and to P-bar3, the scattering sensitivity to circular light. We also present in detail a new polarizer design which generates both linear and circular polarizations, allowing measurement of all scattering polarisation sensitivities without requiring interchange of optical elements. (authors)