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

We report precise measurements of ground-state, $\Lambda$-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 $F^{\prime }=2\to F=2$ transition, yielding $(1667358996\pm 4)\mathrm{Hz}$, and by tenfold for the $F^{\prime }=1\to F=1$ transition, yielding $(1665401803\pm 12)\mathrm{Hz}$. Comparing these laboratory frequencies to those from OH megamasers in interstellar space will allow a sensitivity of 1 ppm for $\Delta \alpha /\alpha$ over $\sim {10}^{10}\mathrm{yr}$.