Abstract

As discussed by Heitler, ¹⁾ the secondary emission process in gaseous atoms (or molecules) under nearly resonant light excitation can be classified into coherent and incoherent processes in the impact-limit case where the duration of the collision is much shorter than the reciprocal detuning frequency and the time between the collisions. Namely, when the atom is undisturbed by collisionsl perturbation, the second-order optical process should be considered as a single coherent quantum process. On the other hand, when the atom is disturbed during the photo-absorption and emission sequence, the coherence between the two processes are destroyed, and the second-order optical process must be regarded as a two-step process consisting of independent absorption and emission.

© 1988 Optical Society of America