We show that when the graphene planes of graphite are uniformly expanded, thereby increasing the CC bond length to $1.7\mathrm{\AA }$, the ${\sigma }^{*}$ edge onset of the energy-loss near-edge structure (ELNES) spectrum shifts to lower energies by almost $5\mathrm{eV}$, meanwhile the ${\pi }^{*}$ edge shifts by less than $0.2\mathrm{eV}$. The shift of the ${\sigma }^{*}$ edge demonstrates that for bond lengths which are typical of some carbon systems such as amorphous carbon, it is possible to find ${\sigma }^{*}$ features in the ELNES spectra at energies as low as $286–288\mathrm{eV}$. Calculations on 64-atom amorphous carbon $\left(a\text{−}\mathrm{C}\right)$ and amorphous carbon nitride model structures characterized by a wide range of bond lengths confirm this. Most of the $s{p}^2∕s{p}^3$ quantification techniques that are available overlook this issue of ${\sigma }^{*}$ contamination of the ${\pi }^{*}$ region and assume that all features within this energy range are entirely of ${\pi }^{*}$ origin. We show that the effect of bond length variation on the ${\pi }^{*}$ spectrum of graphite and $a\text{−}\mathrm{C}$ is minor, thereby supporting the reliability of the former spectrum for $s{p}^2∕s{p}^3$ quantification purposes, as was recently demonstrated [see J. T. Titantah and D. Lamoen, Phys. Rev. B 70, 075115 (2004)].

• Received 18 April 2005

DOI:https://doi.org/10.1103/PhysRevB.72.193104