Calculation of the initial density matrix for He i in beam–tilted-foil experiments

Abstract

We investigate the validity of the surface electric-field interaction model in beam-foil experiments where the polarization of the emitted light is measured. After summarizing the theory for expanding the density operator into state multipoles and linking these to the Stokes parameters of the light, we show also how to use symmetries to reduce the size of the calculations. A very simple model is chosen for the electric field: it is perpendicular to the foil and constant over a finite distance, then zero. This model is used to find the initial density matrix for the n=3 singlet states of He i and the electric-field parameters from tilted-foil experiments. Reasonable results are obtained given the unrealistic electric-field model used.