Quantitative Analysis of Modulated Excitation X-ray Absorption Spectra: Enhanced Precision of EXAFS Fitting

The sensitivity of X-ray absorption spectroscopy (XAS) can be increased by using the modulated excitation approach, where the sample is excited with a periodic external stimulation (e.g., changes in temperature, gas/liquid concentration, etc.) and the corresponding spectra are filtered with the frequency of the excitation. The resulting demodulated spectra contain similar spectral features as difference spectra but to much higher k values because of the isolation of the structural change that occurs with the same frequency as that of the excitation. Furthermore, multiple intermediate species that occur with a phase delay can uniquely be differentiated. We present a robust approach for the quantitative analysis of demodulated extended X-ray absorption fine structure (EXAFS) spectra. Simulations show that the proposed fitting approach resolves small changes in the sample's structure with greatly enhanced precision. Experimentally, this is demonstrated by the formation of ruthenium oxide species upon partial oxidation of a Ru metal particle, which cannot be detected in a standard EXAFS experiment.