Theoretical investigation of the effect of irreversible relaxation on light-echo-type signals during multipulse excitation

A relatively complete and detailed classification of the various light-induction and echo-type responses produced in large material specimens as a result of multipulse excitation is developed by allowing for irreversible relaxation. Responses arising at intermediate stages of the excitation process are taken into account along with those that result after the excitation process is terminated. The excited specimens are considered to be quantum systems consisting of large numbers of identical noninteracting particles with a discrete nondegenerate finite equidistant energy-level spectrum that may be described by spin-energy concepts. The pulse-field interaction operator is linear in spin energy; transverse and longitudinal irreversible relaxation are accounted for by using phenomenological spin operators governed by time differential equations from which Bloch-type equations are obtained by averaging. A matrix method is suggested for analyzing solutions to such equations.