Interaction of Isotopic Modes with Polaritons

Weak phonon modes at 2032,5; 2040; 2046; and 2 0 5 3 c m 1 in K 3 Cu(CN) 4 have been assigned to originate from 13C = 14N and 12C = 15N vibrations. The assignments have been carried out using measurements at He-temperatures and in aqueous solutions. Furthermore calculations have been used concerning the frequency shifts and relative intensities with respect to the 12C = 14N vibrations. The coupling of polariton modes associated with the A^phonon at 2074 and the E-phonon at 2080 c m 1 with the isotopic modes has been experimentally investigated. A comparison of calculated dispersion branches with the experimental data has shown that the isotopic modes can not simply be treated as fundamentals because the coupling observed is much weaker than expected from the general polariton dispersion theory for first order phonons.