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

Abstract

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 by twenty-five-fold for the F' = 2 \(\to\) F = 2 transition, yielding (1 667 358 996 \(\pm\) 4) Hz, and by ten-fold for the F' = 1 \(\to\) F = 1 transition, yielding (1 665 401 803 \(\pm\) 12) 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 \(\sim\)$10^{10}$ years.