Decay of the triplet exciton in ferroelectric AgNa(NO2)2at low temperatures

The decay of the triplet exciton in AgNa(NO 2 ) 2 generated by excitation of the triplet state of NO 2 - ions with short laser pulses is investigated between 3 and 180K by measuring the decay function of the luminescence. Below 7K the radiative decay is exponential with a time constant of 221 mu s. Between 10 and 30K and the decay function is composed of an exponential with a nearly constant time constant of 663 mu s and an exponential with rapidly decreasing time constant. Above 30K the slow decay term prevails, with a decreasing time constant. The results are discussed using a simple model which takes into account transitions from one of the triplet sublevels and thermal transfer among the levels. The time constants of thermal transfer and of the decay at higher temperatures can be related to one-phonon-induced processes involving optical phonons at 52 and 152 cm -1 , respectively.