  Sub-GeV dark matter (DM) which interacts with electrons can excite electrons
occupying molecular orbitals in a scattering event. In particular, aromatic
compounds such as benzene or xylene have an electronic excitation energy of a
few eV, making them sensitive to DM as light as a few MeV. These compounds are
often used as solvents in organic scintillators, where the de-excitation
process leads to a photon which propagates until it is absorbed and re-emitted
by a dilute fluor. The fluor photoemission is not absorbed by the bulk, but is
instead detected by a photon detector such as a photomultiplier tube. We
develop the formalism for DM-electron scattering in aromatic organic molecules,
calculate the expected rate in p-xylene, and apply this calculation to an
existing measurement of the single photo-electron emission rate in a
low-background EJ-301 scintillator cell. Despite the fact that this measurement
was performed in a shallow underground laboratory under minimal overburden, the
DM-electron scattering limits extracted from these data are already approaching
leading constraints in the 3-100 MeV DM mass range. We discuss possible next
steps in the evolution of this direct detection technique, in which scalable
organic scintillators are used in solid or liquid crystal phases and in
conjunction with semiconductor photodetectors to improve sensitivity through
directional signal information and potentially lower dark rates.
