Crossing and anticrossing of atomic levels and their use in atomic spectroscopy

The phenomena discussed can arise in sufficiently rarefied vapors of atoms at definite values of the external magnetic field. They are accompanied by interference of the corresponding atomic states, and this affects noticeably the character of the polarization and the angular distribution of the resonant emission of the atoms. The study of the microstructure of the resonant emission of atoms situated in an external constant field and a radiofrequency field can yield valuable information on the structure of the atomic states, on the character of the interatomic interactions, and on the phenomena associated with them. New methods greatly extend the capabilities of the interference technique and have an exceedingly high resolving power (which can amount to a fraction of a hertz for certain gas systems). The phenomena in question have found various scientific and technical applications, and serve in particular as a basis for the operation of high-sensitivity magnetometers, which make it possible to perform absolute measurements of ultraweak magnetic fields (to 10$^{-11}$ Oe). The physical principles of the phenomena are considered, a theory is developed for them, and certain possible scientific and technical applications are indicated. The bibliography covers all the principal papers published before 1972.