= Solution
{c}
A possible assignment is a zone-centre transverse <optical phonon> of <gallium arsenide>: the two atomic sublattices oscillate oppositely, producing a narrow lattice <Raman scattering> line near $270\,\mathrm{cm}^{-1}$. Its energy is $hc\widetilde\nu\simeq33.5\,\mathrm{meV}$, a reasonable optical-phonon scale. Unlike the broad carrier feature, its frequency is set mainly by interatomic force constants and ionic masses.
Whether this transverse <optical phonon> is allowed in an ideal first-order <Raman scattering> geometry depends on the surface orientation and incident and scattered <polarizations>; disorder or imperfect geometry can relax a forbidden selection rule. Those experimental details are unspecified, so this is a possible explanation rather than an exclusive assignment.
Back to article page
