Optical pumping dynamics and spin relaxation in gaseous He_

Abstract

The first part of this investigation is concerned with the dynamics of the optical polarization process in He3 gas subjected to an electrical discharge. The characteristic time for the build-up of polarization under the influence of pumping radiation is measured as a function of pressure and temperature, as is the polarization decay time when the pumping radiation is removed. These times, together with the measured values of attainable polarization, allow a test of the phenomenological theory customarily used in describing optical pumping dynamics in helium. The gross disagreement obtained between theory and experiment is attributed in part to the over-simplified model employed in development of the theory. The theory is therefore extended and modified on the basis of a more realistic model and account is taken of the effects of P-state mixing and of re-absorption of radiation in optically thick samples. The agreement of experiment with theory is thereby substantially improved, though sane unexplained discrepancies remain. If the exciting electrical discharge required for optical pumping is turned off, the characteristic decay time for the polarization is greatly increased. Decay times are measured under these conditions at both room temperature and liquid nitrogen temperature. A single measurement at 4.2°K established an upper limit on the decay time at that temperature. Under the conditions of this experiment it can be shown that Tr is determined by wall relaxation. The observed temperature dependence is Interpreted in terms of a phenomenological model for wall relaxation, and various microscopic models for the relaxation mechanism are investigated.