Abstract
Energies of the coupled system of a spin 1/2 atom and a strongly oscillating RF field with an arbitrary direction are numerically calculated, and the size and direction of the power shifts of the multiphonon transitions are obtained. The intensity and shape of the multiphoton resonances appearing in optical pumping experiments are also calculated with the semiclassical theory. An optical pumping experiment is performed with caesium vapour to check the validity of the theoretical results, and good agreement is found.