Polarization measurements of He i singlet transitions following beam-tilted-foil excitation

The Stokes parameters of light emitted following beam-foil excitation have been measured as a function of foil tilt angle from 0^∘{} to 70^∘{} for the 2s¹S-3p¹Pᵒ, 2s¹S-4p¹Pᵒ, 2p¹Pᵒ-3d¹D, and 2p¹Pᵒ-4d¹D transitions in He i. The precision of the measurements has been established using the 2p¹Pᵒ-4s¹S transition. A new method is described for analyzing beam-tilted-foil data. The results are compared with those forecast by the theory suggested by Band, and good agreement is found for the transitions from ¹Pᵒ terms but somewhat poorer agreement is found for the transitions from ¹D terms. In particular, the M/I polarization is shown to become negative for a foil tilt angle of around 60^∘{} in the case of the ¹Pᵒ terms and 45^∘{} for the ¹D terms, while the total polarization fraction remains nearly constant at all tilt angles observed for the ¹Pᵒ terms but shows a pronounced minimum at about 45^∘{} for the ¹D terms. A simple procedure is also outlined for the accurate determination of the phase of a retarder plate at any required wavelength.