RAMAN EXCITATION-SPECTRA OF EXCITON-PHONON MODES OF AGGREGATED 2,2'-CYANINE USING AN INTERNAL RAMAN STANDARD

Intramolecular vibrational bands in the Raman spectrum of aggregated, adsorbed 2,2'-cyanine are shown to be appropriate internal Raman stand ards to normalize excitation spectra of two low-frequency intermolecul ar (exciton-phonon) modes to surface coverage and excitation frequency changes. It is postulated that the relative widths of the low-frequen cy intermolecular Raman bands are determined by their energies and tha t k-state dephasing, which depopulates the substates from which allowe d transitions occur, is faster the higher the energy. Frequency maxima at 575.5 and 577.5 nm for Raman excitation spectra of the two exciton -phonon modes are interpreted as indicating the peak absorption wavele ngths for the two J-aggregates of 2,2'-cyanine. The widths of the exci tation profiles are rationalized in terms of the relative energies of the two J-bands. It is suggested that the cis aggregate is of higher e nergy and has fewer excitonic sublevels (k-states) with allowed transi tions to the ground state. Measurements associated with the present ex periment have been facilitated through the use of a charge-coupled dev ice camera, and nearly insurmountable difficulty would accompany the u se of a single-channel detector.