An optical pumping-induced damping of Rubidium Larmor frequency

We study a transversal spin dynamics considered to be important in optically-pumped alkali atomic magnetometers, specifically a cubic Rb vapor cell exposed under a constant external B-field and a circularly polarized D1-line pumping beam incident perpendicularly to the external field. The transient evolution of the magnetization of Rb during optical pumping exhibits a lowered frequency compared to the Larmor frequency ?B, which indicates a damping effect induced by optical pumping. This phenomenon is measured under various intensities of the optical pumping beam and various temperatures of the cell. The effect can partly be explained by analyzing the density matrix equation of the system.