Multilevel theory of time-delay four-wave mixing with incoherent light in absorption bands

Abstract

A homogeneous broadening multilevel theory of time-delay four-wave mixing with incoherent light is developed for absorption bands that consist of a series of vibrational transitions. A formula for the delay-time dependence of the signal intensity I(τ) is obtained. The differences between I(τ) derived from multilevel theory and from two-level theory are pointed out. It has been demonstrated experimentally that the multilevel theory is more suitable than the two-level theory for this type of absorption band. It is shown, in particular, that when we apply the two-level theory to the analysis of experiments in this type of absorption band, what we determine is not the dephasing time of an individual transition but the effective dephasing time, which reflects the behavior of the total absorption spectrum of the band.