Analysis of the π* and σ* bands of the x-ray absorption spectrum of amorphous carbon -: art. no. 125204

The same method used to determine sigma* excitations in the x-ray-absorption spectra of organic molecules is applied to analyze the structure of the sigma* band in the x-ray absorption spectra of amorphous carbon (a-C) films. The analysis assumes that only a sigma bond interaction to first neighbors is relevant to explain the structure of the sigma* band in a-C films. This is justified by the local character of the x-ray absorption probe and the short-range order existing in these films. The identification of the different sigma* components is based on the dependence of the sigma* binding energy with bond distance. The sigma* band is built up by summing the components resulting from the possible different types of sigma bonds in a-C. This method serves to separate the pi* states from the sigma* states and to identify the kind of chemical bonds existing between carbon atoms. This analysis yields a proportion of sp(3)-bonded atoms of 60% in a film with a density of about 2.9 g/cm(3), which is a value closer to what is expected from theoretical calculations. The analysis identifies a component at about 288.5 eV which is associated to strained sigma bonds of the type of the existing in sp(3) bonded rings like cyclopropene. Raman and photoemission spectroscopies help in the interpretation of the x-ray absorption spectra and the assignment of the sigma* components.