Cold Molecule Spectroscopy for Constraining the Evolution of the Fine Structure Constant

Abstract

We report precise measurements of ground-state, $\ensuremath{\Lambda}$-doublet microwave transitions in the hydroxyl radical molecule (OH). Utilizing slow, cold molecules produced by a Stark decelerator we have improved over the precision of the previous best measurement 25-fold for the ${F}^{\ensuremath{'}}=2\ensuremath{\rightarrow}F=2$ transition, yielding $(1\text{ }667\text{ }358\text{ }996\ifmmode\pm\else\textpm\fi{}4)\text{ }\text{ }\mathrm{Hz}$, and by tenfold for the ${F}^{\ensuremath{'}}=1\ensuremath{\rightarrow}F=1$ transition, yielding $(1\text{ }665\text{ }401\text{ }803\ifmmode\pm\else\textpm\fi{}12)\text{ }\text{ }\mathrm{Hz}$. Comparing these laboratory frequencies to those from OH megamasers in interstellar space will allow a sensitivity of 1 ppm for $\ensuremath{\Delta}\ensuremath{\alpha}/\ensuremath{\alpha}$ over $\ensuremath{\sim}{10}^{10}\text{ }\text{ }\mathrm{yr}$.