Analysis of the π* and σ* bands of the x-ray absorption spectrum of amorphous carbon

The same method used to determine σ* excitations in the x-ray-absorption spectra of organic molecules is applied to analyze the structure of the σ* band in the x-ray absorption spectra of amorphous carbon (a-C) films. The analysis assumes that only a σ bond interaction to first neighbors is relevant to explain the structure of the σ* 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 σ* components is based on the dependence of the σ* binding energy with bond distance. The σ* band is built up by summing the components resulting from the possible different types of σ bonds in a-C. This method serves to separate the π* states from the σ* states and to identify the kind of chemical bonds existing between carbon atoms. This analysis yields a proportion of sp3-bonded atoms of 60% in a film with a density of about 2.9 g/cm3, 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 σ bonds of the type of the existing in sp3 bonded rings like cyclopropene. Raman and photoemission spectroscopies help in the interpretation of the x-ray absorption spectra and the assignment of the σ* components