  We present a first calculation of the rate for plasmon production in
semiconductors from nuclei recoiling against dark matter. The process is
analogous to bremsstrahlung of transverse photon modes, but with a longitudinal
plasmon mode emitted instead. For dark matter in the 10 MeV - 1 GeV mass range,
we find that the plasmon bremsstrahlung rate is 4-5 orders of magnitude smaller
than that for elastic scattering, but 4-5 orders of magnitude larger than the
transverse bremsstrahlung rate. Because the plasmon can decay into electronic
excitations and has characteristic energy given by the plasma frequency
$\omega_p$, with $\omega_p \approx 16$ eV in Si crystals, plasmon production
provides a distinctive signature and new method to detect nuclear recoils from
sub-GeV dark matter.
