STIMULATED LASER-INDUCED EMISSION NEAR THE D//1 LINE OF SODIUM AND FOUR-WAVE MIXING BY A THREE-LEVEL SYSTEM.

When a high-density (approximately 10**1**5-cm** minus **3 sodium vapor is illuminated by an intense pulsed dye laser detuned to the blue of the D//2 line, the forward radiation contains a coherent emission to the blue of the D//1 line. It is shown that none of the existing conventional models for this emission is consistent. A four-wave mixing (FWM) experiment was performed to estimate and ascertain the origin of the high gain at blue peak frequency omega //B. On the basis of these experiments and an analysis of other work, it was determined that the process responsible for the blue peak generation differs from FWM and has a gain of greater than 10**3 cm** minus **1. A downconversion process in which a laser photon splits into two photons is proposed: a photon at omega //B and another one related to the magnetic dipole transition 3**2P//3/////2-3**2P//1/////2; the high gain is due to waveguiding effects.