Influence of an electromagnetic field on relaxation processes in a two-level atom

The Keldysh diagram technique is used in the problem of a two-level atom interacting with an external field and a thermostat. A system of kinetic equations is obtained for an arbitrary external field. The special cases of the kinetic equations in weak and strong fields are derived by going to the appropriate limits. In the case of some specific cases new formulas are found for the absorption, resonance fluorescence, and polarization of a two-level atom in an external electromagnetic field.