Multiple-order Raman scattering in MnO42−-doped CsI

Multiple-order Raman scattering was observed in CsI doped with Mn$\mathrm{O}_4^{}{}_{}{}^{2-}$. The Raman spectra consist of a series of sharp, equally spaced lines due to multiple-order scattering from the totally symmetric mode of Mn$\mathrm{O}_4^{}{}_{}{}^{2-}$. Each sharp line in the Raman spectra exhibits a side band which is composed partly of low-frequency CsI phonons and partly of high-frequency internal vibrations of the Mn$\mathrm{O}_4^{}{}_{}{}^{2-}$ molecule. The intensity of the Raman scattering was measured as a function of laser frequency. In general, the Raman scattering was seen to be enhanced each time the laser frequency was tuned to one of the vibrational sublevels of the excited electronic state. An exception occurred at the third vibronic level at which a minimum in the scattered intensity was observed. The Raman spectra, resonance enhancement spectra, and optical absorption spectra can be explained, quantitatively, using a simple model involving a ground electronic state and one excited electronic state each with vibronic structure corresponding to one localized mode of vibration. Since the frequency of this localized mode is shown to be different in the ground and excited electronic states, we can easily distinguish between resonant Raman scattering and hot luminescence.