Spin echo generated by resonance transitions between laboratory coordinate frame levels and quasi-energy levels

The general rules governing the transition to a new representation in which the Hamiltonian of interaction with an alternating field contains constant terms are formulated. Diagonalization of this Hamiltonian leads to the formation of quasi-stationary energy levels. The possibility of the existence of a temperature at the quasi-energy levels is considered separately. A three-level spin system is used as an example to show that another specially selected alternating magnetic field can excite resonance transitions between the energy levels in the laboratory coordinate frame and the energy levels in the new representation. Expressions for free-precession and spin echo signals, which carry information concerning quasi-stationary energy levels, are derived.