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
He which depends on the polarization and spectral profile of the incident light. In order to check the structure of
He, 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
He. We have been able to obtain experimentally in the ground state of
199Hg,
201Hg, and
87Rb, 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 ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}}_{0}$ is added. We have checked that the effect of
He 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.
An analysis is made of the scattering and absorption of light by a hydrogenlike atom in the field of an electromagnetic wave which mixes resonantly an excited level (n = 2) with its neighbor (n = 3). The scattering and absorption cross sections are calculated as a function of the frequency of the wave and of the intensity of the electromagnetic field. The influence of low-frequency nonresonance field on the scattering and absorption of light by an atom in the ground state is considered. It is shown that the scattering (absorption and luminescence) spectra of a hydrogenlike atom may be frequency modulated by a low-frequency amplitude modulated field.