  The nature of enhanced photoemission in disordered and amorphous solids is an
intriguing open question. A point in case is light emission in silicon, which
occurs when the material is porous or nanostructured, but the effect is absent
in the bulk crystalline phase, a phenomenon that is still not fully understood.
In this work, we study structural photoemission in a heterogeneous cross-linked
silicon glass, a material that represents an intermediate state between the
amorphous and crystalline phases, characterized by a narrow distribution of
structure sizes. This model system shows a clear dependence of photoemission on
size and disorder across a broad range of energies. While phonon-assisted
indirect optical transitions are insufficient to describe observable emissions,
our experiments suggest these can be understood through electronic Raman
scattering instead. This phenomenon, not commonly observed in crystalline
semiconductors, is driven by structural disorder. We attribute photoemission in
this disordered system to the presence of an excess electron density of states
within the forbidden gap (Urbach bridge), where electrons occupy trapped
states. Transitions from gap states to the conduction band are facilitated
through electron-photon momentum matching, which resembles Compton scattering,
but observed for visible light and driven by the enhanced momentum of a photon
confined within the nanostructured domains. We interpret the light emission in
structured silicon glass as resulting from electronic Raman scattering. These
findings emphasize the role of photon momentum in the optical response of
solids that display disorder at the nanoscale.
