Optically-addressable organic molecules for precision tests of fundamental constant variation

We identify a set of organic molecules as promising candidates for the measurement of variations in the proton-to-electron mass ratio, μ. Since rotational and vibrational energies of molecules are inherently sensitive to μ, oscillations in μ induced by certain dark matter models would cause oscillation in the resonance frequency of rovibrational transitions. One candidate molecule, glyoxal (C2H2O2), is discussed in detail. The near-degenerate symmetric and antisymmetric O-C stretching vibrational states serve as "science states" that can be probed with low-power and low-frequency microwave radiation in a Ramsey measurement. With a vapor pressure of >250 Torr at 25 C, purified glyoxal can be seeded with high flux into a cryogenic molecular beam source for large statistical sensitivity. The science states can be populated using quantum cascade lasers, and convenient optical transitions at visible wavelengths enable efficient, state-selective laser-induced fluorescence readout. Due to its high degree of symmetry and closed-shell electronic structure, glyoxal is robust against systematic errors associated with electromagnetic fields. Extensions of the proposed method can satisfy additional experimental criteria with the suitable choice of an alternative molecule. For example, benzonitrile-d5 possesses a large molecular dipole moment, which makes it suitable for electrostatic focusing or deceleration which may further increase statistical sensitivity.