J. Diaz et al., Analysis of the pi* and sigma* bands of the x-ray absorption spectrum of amorphous carbon - art. no. 125204, PHYS REV B, 6412(12), 2001, pp. 5204
The same method used to determine sigma* excitations in the x-ray-absorptio
n spectra of organic molecules is applied to analyze the structure of the s
igma* 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 differ
ent sigma* components is based on the dependence of the sigma* binding ener
gy with bond distance. The sigma* band is built up by summing the component
s resulting from the possible different types of sigma bonds in a-C. This m
ethod serves to separate the pi* states from the sigma* states and to ident
ify 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 theo
retical 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 spectroscopi
es help in the interpretation of the x-ray absorption spectra and the assig
nment of the sigma* components.