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 standards to normalize excitation spectra of two low-frequency intermolecular (exciton-phonon) modes to surface coverage and excitation frequency changes. It is postulated that the relative widths of the low-frequency intermolecular Raman bands are determined by their energies and that k-state dephasing, which depopulates the substates from which allowed 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 wavelengths for the two J-aggregates of 2,2'-cyanine. The widths of the excitation profiles are rationalized in terms of the relative energies of the two J-bands. It is suggested that the cis aggregate is of higher energy and has fewer excitonic sublevels (k-states) with allowed transitions to the ground state. Measurements associated with the present experiment have been facilitated through the use of a charge-coupled device camera, and nearly insurmountable difficulty would accompany the use of a single-channel detector.