Rayleigh scattering and redistribution of radiation in He I plasmas: measurements and calculations.

Conditions for which Rayleigh scattering and redistribution of radiation can be observed are examined. The competition between the relaxation processes and spontaneous emission is shown to determine the conditions for partial redistribution. We investigate specific cases in the singlet and triplet He I atomic system for plasma parameters reachable experimentally in a well diagnosed magnetic multipole source. Partial redistribution on the 2 1S-4 1P at 396.5 nm, 2 1P-5 1D at 438.8 nm, 2 3P-5 3D at 402.6 nm, 2 3P-4 3D at 447.2 nm and 2 3P-4 3S at 471.3 nm fine structure transitions is observed employing a dye laser pumping the He I plasma. We make use of a previously developed two-photon formalism based on the frequency fluctuation model to calculate radiative redistribution functions. Results of the partial redistribution measurements are presented, augmented by a comparison with calculations. This allows us to confirm available electron-atom elastic collision rates to within 20%.