The crystal spectra of very weak transitions

Abstract

The weak absorption system of crystalline naphthalene at 3200 å has been measured at 4°K in polarized light. Previous work at 20°K is supplemented by a number of newly recorded absorption lines, and by re-measurement of the frequencies of all lines at the lower temperature. The spectrum is compared with the vapour spectrum and corresponding bands identified so far as possible. Exciton resonance effects are analysed and discussed. The analysis discloses that the important b _{3 g} vibrations of upper state frequencies 438 and 911 cm^-1 are both reduced by 5 per cent in the crystal, and smaller changes are identified in other vibrations. Comparison with the vapour spectrum shows that vibrationally induced bands, even when relatively strong, are little influenced by intermolecular forces, whereas very weak pure electronic bands give evidence of much greater effects.