Entire phonon spectrum of molecular crystals by the localized exciton sideband method: Naphthalene

The complete one-phonon density-of-states of a molecular crystal can be mapped out by a carefully chosen exciton sideband involving a localized molecular internal excitation in a dilute isotopic mixed crystal. A theoretical derivation shows that the necessary criteria are: absence of localized in-band phonons; applicability of a phonon amalgamation limit for the mixed crystal; weak exciton-phonon, guest-lattice coupling and weak guest-host exciton interaction. Experimentally, at least two exciton-phonon sidebands should be investigated, so as to exclude trap and defect transitions, and at least one of the guest transitions should belong to the deep-trap limit and have sidebands weak enough to minimize multiphonon transitions. The above is demonstrated for the naphthalene crystal by high-resolution fluorescence and phosphorescence spectra of 0.25% C₁₀H₈ in C₁₀H₈ and by phosphorescence of 0.14% 2-DC₁₀H₇ in C ₁₀D₈ at 2°K. The phosphorescence phonon sidebands correlate surprisingly well with Pawley's "atom-atom" calculated phonon density-of-states, and seems to give a much better phonon density map than the very recent high-resolution inelastic, incoherent neutron scattering data. New Raman data are also presented, showing defect (isotopic impurity) induced bands, which are interpreted in terms of the above phonon density-of-states peaks. Applications to reversible and irreversible thermodynamics are mentioned.