Low-temperature absorption and resonance Raman spectra of the MnO4- ion doped in a KClO4 crystal.

Abstract

We have performed polarized absorption and resonance Raman experiments on a permanganate ion doped in a potassium perchlorate single crystal at temperature T=15 K. At this low temperature the m(${\mathit{C}}_{\mathit{s}}$) site splitting of the excited degenerate $^{1}$${\mathit{T}}_{2}$ electronic level of the permanganate ion is well resolved and the amount of splitting is about 40 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Due to the electronic configuration, one would expect that non-Condon terms have to be considered in the description of the absorption spectrum. For the theoretical simulation of our experimental results we have used expressions derived from the time-correlator formulation for the optical absorption. These are much easier to handle and they cause significant shorter calculation times than the usual sum-over-states expressions. In order to determine the symmetries and the wave-number positions of the site-split permanganate vibrations, we have performed resonance Raman experiments. The results obtained from these experiments form the basis for the interpretation of the absorption spectrum. The applied model includes the linear and quadratic electron-phonon and linear non-Condon coupling. Within this model we describe the multimode system and we show how a normal vibration, which apparently has no significant effects in the absorption spectrum, influences the discussion of the model system. For the fully symmetric breathing mode of the permanganate ion, we have calculated the change of the Mn-O equilibrium bond length in the electronic excited state from the corresponding linear electron-phonon coupling constant to be 4.6\ifmmode\pm\else\textpm\fi{}0.4 pm.