Unusual intensities in the resonance Raman spectra and excitation profiles of an intervalence metal-to-metal charge transfer complex

The resonance Raman spectra of the metal-to-metal charge transfer (MMCT) mixed-valence complex [(OC)(5)Cr-0-CN-Os-III(NH3)(5)](CF3SO3)(2) are measured and analyzed. The excitation profiles (the Raman intensities as a function of excitation wavelength) are obtained using the SO3 stretch of the triflate ion as an internal standard. The ratios of the Raman intensities are not constant with different excitation wavelengths. This unusual observation is attributed to interference between two overlapping electronic states of the molecule. The electronic states are observed in the low-temperature polarized single crystal absorption spectrum. The resonance Raman intensities and excitation profiles are calculated with the time dependent theory of spectroscopy and include the interference between the electronic states. The origin of the unusual intensities is quantitatively explained in terms of the real and imaginary parts of the Raman cross sections from the two states.