Electromagnetic processes in a stationary field

Within the framework of a consistent quantum-mechanical approach, a universal expression for the rate of photoprocess in a stationary electromagnetic field is derived. It is used to derive the Einstein coefficient for stimulated emission. Two limiting cases are considered: photoprocess in a thermal radiation field and in a monochromatic field. Using the obtained formula, the rate of photoionization of Rydberg states of an atom by thermal radiation is calculated.