Shape of the π-luminescence band in NaCl: Adiabatic potential surface at the lowest state of the self-trapped exciton

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Abstract
The shape of the pi -luminescence band due to the self-trapped excitons in NaCl has been measured in a temperature range between 4.2 and 80 K. It is found that the band shape deviates from a Gaussian shape, that the deviation is enhanced with increasing temperature and that the peak of the band shows a blue shift. The experimental results including the Stokes shift are analysed and explained satisfactorily using two-dimensional configuration coordinate potential surfaces having a first-order coupling with a hard mode and first- and second-order couplings with a soft mode. It is suggested that the soft mode is the transitional mode of the halogen molecular ion, and hence the configuration of the lowest-triplet self-trapped exciton is of an off-centre type.