ESR of triplet excitons in 1.4 dibromo-naphthalene. Linear excitons

Abstract

The ESR spectrum of metastable triplet excitons in single crystals of 1.4 dibromo-naphthalene (1.4 DBN) at 1.4°K and 35 GHz was investigated. Optical measurements have established previously that in this system excitons are free even at 1.2°K. Exciton motion in these crystals takes place mainly by hopping between translationally equivalent molecules along the crystal c axis (AA jumps). This fact is due to the crystal structure of 1.4 DBN and the excitons are therefore called linear excitons. The ESR spectra were observed both directly (standard microwave detection) and optically, using the phosphorescence intensity of individual Zeeman components of phosphorescence lines as indicator. Using optical detection one can easily distinguish between the free exciton ESR and ESR lines which are due to isolated trap molecules in the crystal. The ESR spectra show clearly the one-dimensional character of excitons in these crystals. In addition one can measure the inter-chain jump rates between translationally inequivalent chains (AB jumps), by studying the line shape and line width of the ESR lines. These rates are three orders of magnitude smaller than those of in-chain jumps. The inter-chain jumps are also responsible for the spin-lattice relaxation time T 1. T 1 was measured and calculated using the methods which were developed earlier for excitons in naphthalene and anthracene.