Splitting of the interband absorption edge in Au

Very accurate ellipsometric measurements were made on very pure bulk samples of Au at room temperature in the 1.0-3.5-eV energy region. A model calculation has been developed for $d$ bands to Fermi-surface transitions occuring near the $X$ point in the Brillouin zone and it is shown that for topological reasons the line shape of the imaginary part of the dielectric constant ${\ensuremath{\epsilon}}_{2}$ is considerably different from what has been calculated for $L$ transitions. Using this model to fit our experimental data it has been possible to show that the absorption edge in Au is "split" into two separate contributions. The onset of $X$ transitions has been located at 1.94 eV while the onset of the $L$ transitions has been confirmed at 2.45 eV in excellent agreement with previous determinations.