3-PHOTON-EXCITATION SPECTROSCOPY FOR THE EXCITON RELAXATION IN ALKALI-HALIDES

The yields of sigma and pi luminescence due to self-trapped excitons h ave been measured upon the three-photon excitation (TPE) which gives r ise to the resonance on the upper branch of the lowest exciton-polarit on in NaCl and KI. Both the sigma and pi luminescence are induced by t he TPE in the whole wavelength range of the resonance, and the yield o f the sigma luminescence relative to that of the pi luminescence is as high as that by the conventional band-to-band excitation, showing str ong contrast to the result of the one-photon excitation (OPE) spectros copy. The different feature of the sigma-luminescence excitation in TP E from that in OPE has been discussed.