Direct detection of 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 O(1) fraction of its kinetic energy to excite a vibrational and rotational state. The excited rovibrational 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 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.