Occurrence of Split Spectra in Doppler-free Two-photon Spectroscopy

Abstract

This paper discusses the origin of splitting of two-photon spectra of three-level cascade configurations having a near resonant intermediate level. A clear distinction between splitting mechanisms in homogeneously and inhomogeneously broadened systems is demonstrated. When one of the laser fields is a weak probe the velocity averaged two-photon response is primarily caused by two resonantly excited velocity groups; the absence of these implies a split spectrum as suggested by Chebotaev and collaborators. A simple derivation of the probe response is presented. When both fields have non-negligible intensities resonant excitation is inhibited and instead all level populations pulsate. Still a split spectrum can appear. A simple graph showing the regions where this occurs is given and analyzed.