Knowledge Bank

Time resolved spectroscopy is well suited to study the coherence evolution of a gas submitted to transient e.m. pulses. In the case of a large inhomogeneous broadening, the macroscopic coherence is rapidly destroyed by dipole dephasing processes : a photon echo experiment $(\pi /2,\pi$ sequence) allows a restoration of the macroscopic coherence. In the microwave domain, a prevailing inhomogeneous broadening is obtained when the absorption is switched on/off by an inhomogeneous Stark field : a strong attenuation of photon echoes, related to the molecular free flight through the inhomogeneous Stark field, occurs. We propose a 3-pulse photon echo experiment $[\pi /2,\pi ,\pi$ sequence) in which the molecular free flight effects are avoided, and the observation duration is lengthened. Such an experiment seems suitable to the study of velocity changing collisions. In this purpose we have studied the various effects leading to an echo damping : strong collisions, Stark and e. m. field distributions, e. m. field spectral purity.