Past exam of the condensed matter physics major option course of the University of Oxford / 2019 / 3 / d

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 $hc\tilde{\nu }\simeq 33.5\mathrm{meV}$, 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.