In a foundational 1969 study, Mooradian documented the first account of interband PL (Figure when exposing gold and copper to intense light ranging from 300 nm to 515 nm. This emission pathway consists of the excitation of electrons from the d -band to the conduction band (violet arrow), followed by the radiative direct interband recombination of the remaining holes with conduction band electrons below the Fermi level (blue arrow). Interband PL is characterized by a peak near the excitation wavelength that decreases in intensity and levels out at longer wavelengths. The fast non-radiative recombination of d -band holes, reflected in their short lifetime in the order of ∼30 fs, renders interband emission highly inefficient. As a result, the reported photoluminescence quantum yields (PLQY) are typically around 10 -10 . While interband emission requires excitation with energies above the interband threshold, electrons (and holes) can, in principle, also be excited with photon energies below the interband threshold by intraband transitions (Figure , orange arrow). In this case, momentum conservation must be achieved by scattering of the electron with a phonon, a defect, or a surface. Subsequently, a fraction of the excited charges relaxes via momentum-assisted radiative recombination (red arrow), causing flat, broadband emission with a cutoff at the excitation wavelength. The low density of sp-band electrons and the necessity of an addi-tional scattering process render intraband PL in a bulk metal at least one order of magnitude weaker compared to the already extremely weak interband PL. These low efficiencies effectively relegated the PL of metals to a footnote in solid-state research throughout much of the 20th century.