Polarization-Dependent Disappearance Of A Resonance Signal: Indication For Optical Pumping In A Storage Ring?

(Received 10 November 2020; accepted 25 January 2021; published 4 February 2021) We report on laser spectroscopic measurements on Li+ ions in the experimental storage ring ESR at the GSI Helmholtz Centre for Heavy Ion Research. Driving the 2s 3S1(F = 3/2) ?-> 2p 3P2(F = 5/2) ?-> 2s 3S1(F = 5/2) ?-transition in 7Li+ with two superimposed laser beams it was found that the use of circularly polarized light leads to a disappearance of the resonance structure in the fluorescence signal. This can be explained by optical pumping into a dark state of polarized ions. We present a detailed theoretical analysis of this process that supports the interpretation of optical pumping and demonstrates that the polarization induced by the laser light must then be at least partially maintained during the round trip of the ions in the storage ring. Such polarized ion beams in storage rings will provide opportunities for new experiments, especially on parity violation.