Experimental evidence of the oxidation state through resonant Raman scattering

H. J. SÁNCHEZ; M.C. VALENTINUZZI; J. J. LEANI; J. ABRAHAM; C. A. PÉREZ

Campinas, Brasil

Congreso; Reunión Anual de Usuarios de LNLS RAU 2009; 2009

Laboratorio Nacional de Luz Sincrotrón

When atoms are irradiated by x-ray photons different kinds of interactions take place: the photon can be absorbed by the photoelectric effect or can suffer a Rayleigh or Compton scattering. However, under resonant conditions other low probability interactions can occur. One of these interactions is the resonant Raman scattering: when the energy of the incident photon approaches from below to the absorption edge of the target element, a strong resonant behavior takes place contributing to the attenuation of x rays in matter. The resonant character of the process and the existence of an onset energy in the Raman spectrum enable the probing of the edge structure characteristics by tunning the incident energy towards the edge. In the case of oxide compounds, the Raman peak changes the maximum energy, the peak shifts to energies lower by a few electron volts due to a change in the absorption edge energy. By employing a low resolution detecting system, changes in the Raman structure can be observed, providing the possibility of identifying the elements of a sample and their oxidation state. This is an important fact since the chemical bonding of an element affects the emission of its characteristic x-rays; the formation of chemical bonding causes a migration of the valence electrons among participant atoms, reducing screening effects and increasing their inner shell binding energies. The analyzed samples consisted on pure foils (> 99.99%) of Cu and oxide foils (> 99%) of CuO and Cu2O. Samples were irradiated with monochromatic syncrotron radiation below the K absorption edge of Cu to inspect the Raman emissions. As it should be expected, the Raman scattering process is resonantly enhaced as the energy of the incident photons is closer to the K absorption threshold and the Raman peak dominates the background. In the case of oxides, the Raman peak shifts to lower energies a few electron volts due to the change of the absorption edge energy. However peak areas of pure elements were quite similar to those of oxides and no signicant changes were observed. After fitting, the residuals were calculated. A clear difference was observed among the oxidations states of Cu. The results suggest the possibility of structural characterization by means of  resonant Raman scattering using an energy dispersive system combined with synchrotron radiation.