The Modulated Absorption of Light in an Optical Pumping Experiment in 4He

Optical pumpmg experiments have been performed on helium 4 atoms in the metastable level 2 with the object of studying the modulation of light in absorption in the longitudinal and transverse beams. Magnetic resonance curves were obtained similar to those found, in emission in double-resonance experiments. With a part~cular geometrical arrangement employing circular polarizers, the Bloch magnebc resonance curves were obtamed. Modulation at four t i m e s the applied frequency was obtained with a linear polarizer 111 the pumping beam. This modulation is ascribed to the circulation of coherence in the optical pumping cycle. Fiom the behaviour of the resonance signals as a function of the frequency and amplitude of the stimulating field, it is inferred that a substantial part of the signal arises from coherence which has survived two complete cycles. The observations are satnfactorily explained by the theory developed in the previous paper.