Fig. 7-1 Difference between resonance and "ordinary" Raman scattering

Fig. 7-2 Exciting wavelength dependency of the uranyl ion symmetric stretching vibration at 835cm-1

Studies on the molecular structure of actinide compounds in excited electronic states are in progress through the use of resonance Raman scattering. As shown in Fig. 7-1, resonance Raman scattering (or resonance Raman effect) occurs when the exciting frequency approaches or enters the region of electronic absorption of this system. The Raman scattering intensity of the sample molecule is then strongly enhanced as compared with that of ordinary Raman scattering. Recently, such resonance Raman scattering has been observed for uranyl nitrate (UO2(NO3)2), one of the well-known compounds in the actinide series. In the electronic ground state, the uranyl ion ( UO22+) is linear and has three fundamental vibrations: asymmetric stretching, symmetric stretching and degenerate bending. The uranyl ion also has a strong electronic absorption band near 430 nm in the visible region. It has been found that the resonance Raman effect occurs in the symmetric vibration (O-U-O), observed at 835 cm-1. The Raman scattering intensity changes markedly depending upon the wavelength of the laser-exciting light as shown in Fig. 7-2. From the analysis of exciting-light dependency of the symmetrical vibration, the conclusion can be drawn that the uranyl ion in the excited electronic state has an equilibrium conformation linearly distorted along the symmetric stretching mode and its bond distance lengthens by approximately 0.014 nm in the direction of the uranyl (O-U-O) axis.

Reference K. Owada et al., Excitation Profile of the Resonance Raman Effect of Uranyl Nitrate in Acetone, Spectrochim. Acta, 52A (2), 149 (1996).

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