= Copper(I) oxide
{title2=$\mathrm{Cu_2O}$}
{wiki}
= Cuprous oxide
{synonym}
Copper(I) oxide, $\mathrm{Cu_2O}$, 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.
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