Search for Time Variation of Fundamental Constants in Nonpolar Molecular Ions

Trapped and sympathetically cooled O$_2^+$ ions provide several paths to performing precision measurements of the time variation of the proton-to-electron mass ratio. We describe one such scheme, two-photon vibrational overtone transitions. The nonpolar nature of O$_2^+$ as well as its $^2\Pi_g$ ground state suppress many systematic effects. We discuss plans and progress towards a short-term goal on $\dot{\mu}/\mu$ that would match current limits set in molecular systems. The O$_2^+$ vibrational overtones have long-term prospects below the most stringent limit to date in any system.