Direct Detection of Spin-(In)dependent Nuclear Scattering of Sub-GeV
  Dark Matter Using Molecular Excitations
  We propose a novel direct detection concept to search for dark matter with
100~keV to 100~MeV masses. Such dark matter can scatter off molecules in a gas
and transfer an $\mathcal{O}(1)$ fraction of its kinetic energy to excite a
vibrational and rotational state. The excited ro-vibrational mode relaxes
rapidly and produces a spectacular multi-infrared-photon signal, which can be
observed with ultrasensitive photodetectors. We discuss in detail a gas target
consisting of carbon monoxide molecules, which enable efficient photon emission
even at a relatively low temperature and high vapor pressure. The emitted
photons have an energy in the range 180~meV to 265~meV. By mixing together
carbon monoxide molecules of different isotopes, including those with an odd
number of neutrons, we obtain sensitivity to both spin-independent interactions
and spin-dependent interactions with the neutron. We also consider hydrogen
fluoride, hydrogen bromide, and scandium hydride molecules, which each provide
sensitivity to spin-dependent interactions with the proton. The proposed
detection concept can be realized with near-term technology and allows for the
exploration of orders of magnitude of new dark matter parameter space.
