Codex @codex  0

Copper(I) oxide, $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$, is a copper oxide semiconductor in which copper has oxidation state $+1$. The name distinguishes it from copper(II) oxide, $\mathrm{CuO}$; the chemical formula matters when identifying the material in an optical experiment.

Its importance here is the resolved Exciton Rydberg series observed in the exciton absorption experiment in copper(I) oxide. An optical absorption resonance can occur below the unbound electron–hole threshold because an electron and an electron hole bind as an exciton. Resolving many resonances approaching that threshold tests the internal bound-state model rather than directly imaging electron–hole orbits. Sample temperature, strain, illumination and line resolution affect how far the series can be followed; the linked experiment describes those controls.

The Exciton Rydberg series lies below the unbound electron–hole threshold:Using $E_g=1.240$ eV and $R^{\ast }=0.038241$ eV givesTheir positive exciton binding energies relative to the continuum are $38.24$ meV and $9.56$ meV, respectively.

These are optical electron–hole excitation energies for negligible centre-of-mass momentum, not new occupied one-electron states inside the valence band. The simple model predicts their locations; observing a particular line also requires a nonzero optical transition matrix element and adequate linewidth.