Experimental Study of Zeeman Light Shifts in Weak Magnetic Fields

According to the quantum theory of the optical-pumping cycle, one can describe the effect of a nonresonant light beam on the different Zeeman sublevels of an atomic ground state by an effective Hamiltonian ${\mathcal{H}}_e$ which depends on the polarization and spectral profile of the incident light. In order to check the structure of ${\mathcal{H}}_e$, we have performed a detailed experimental study of the ground-state energy sublevels of various kinds of atoms perturbed by different types of nonresonant light beams. Attention is paid to the modification of the Zeeman structure due to ${\mathcal{H}}_e$. We have been able to obtain experimentally in the ground state of ${}_{}{}^{199}{}_{}{}^{}\mathrm{Hg}$, ${}_{}{}^{201}{}_{}{}^{}\mathrm{Hg}$, and ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$, Zeeman light shifts that are larger than the width of the levels due to the thermal relaxation. The removal of the Zeeman degeneracy in zero external field, due to a nonresonant light irradiation, is observed by different optical-pumping techniques; the full Zeeman diagram of the perturbed atoms is also determined when a static field ${\overrightarrow{\mathrm{H}}}_0$ is added. We have checked that the effect of ${\mathcal{H}}_e$ can be described in terms of fictitious static electric or magnetic fields. These fictitious fields can be used to act selectively on a given atomic level.