Experimental evidence of the oxidation state through resonant Raman scattering

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

LNLS Activity Report 2007

Año: 2008 vol. 2007 p. 1 - 2

1518-0204

When atoms are irradiated by x-ray photons different kindsof interactions take place: the photn can be absorbed by thephotoelectric effect or can suffer a Rayleigh or Compton scattering.However, under resonant conditions other low probabilityinteractions can occur. One of these interactions is theresonant Raman scattering (RRS) [1].The x-ray resonant Raman scattering is an inelastic processwhich presents fundamental differences compared to otherscattering interactions between x-rays and atoms; when theenergy of the incident photon approaches from below to theabsorption edge of the target elemet, a strong resonant behaviortakes place contributing to the attenuation of x-rays inmatter. According to the absorption-emission model the RRSprocess can be represented by three steps [2]: 1) the initialstate consists of an incident photon with an energy below, forexample, the K threshold 2) a hole is produced in the K shelland the excited electron is ejected to an unoccupied state; anelectron from a higher shell fills the vacancy and a photon isemitted 3) the final state consists of a hole in a higher shell, ascattered photon and an excited electron in the continnum orin a bound excited stateThe resonant character of the process and the existence ofan onset energy in the RRS spectrum enable the probing ofthe edge structure characteristics by tunning the incident energytowards the edge. In the case of oxide compounds, theRaman peak changes the maximum energy (corresponding toan excited electron with kinetic energy equal to zero); the Ramanpeak shifts to energies lower by a few elctron volts due tothe change of the absorption edge energy [3]. By employing alow resolution detecting system, changes in the RRS structurecan be observed, providing the possibility of identifying theelements of a sample and their oxidation state [4] [5]. Thisis an important fact since the chemical bonding of an elementaffects the emission of its characteristic x-rays; the formationof chemical bonding causes a migration of the valence electronsamong participant atoms, reducing screenig effects andincreasing their inner shell binding energies.