Optically driven spin nutations in the ground state of atomic sodium

We report detailed studies of the dynamic response of spin coherence to a step input of optically resonant narrow-band laser radiation in the presence of a static transverse magnetic field. The evolution of the spins is observed with a cw optical probe beam using polarization-selective detection of the transmitted light. In extension of previous optical-pumping experiments, the dynamics of the system are studied systematically as a function of optical-resonance detuning and intensity. Our experiments are performed on the 3s ${}_{}{}^2{}_{}{}^{}S_{1/2}^{}{}_{}{}^{}$ ground state of sodium, using the $D_1$ line for optical excitation. The experimental data are compared with theoretical predictions based on a Bloch-type equation of motion for an optically driven spin system in a J=(1/2 ground state.