Fine structure at the x-ray absorption pi* and sigma* bands of amorphous carbon: distorted trigonal and tetragonal bonding?

X-ray absorption near edge spectroscopy (XANES) of amorphous carbon films shows a fine structure at the pi* and sigma* bands that is not present in the reference crystalline graphite and diamond materials. Thermal treatments of the amorphous carbon films in a vacuum induce clear changes in the fine structure, hence indicating that it is due to the presence of multiple bonding states. The intensity and energy position of the multiple pi* and sigma* states is directly correlated, confirming its origin as different bonding states of sp2 hybridized carbon atoms. The presence of the fine structure in amorphous carbon is interpreted as due to the presence of distorted trigonal and tetragonal bonding structures, in addition to the ideal hexagonal and cubic structures.