			15min:

PROSPECTS FOR PRECISION MEASUREMENTS ON AMMONIA MOLECULES IN A FOUNTAIN.

HENDRICK L. BETHLEM, Laser Centre Vrije Universiteit, De Boelelaan 1081, NL-1081HV Amsterdam, The Netherlands.

The recent demonstration of cooling and manipulation techniques for molecules offers new possibilities for precision measurements in molecules. At the LCVU, we are constructing a molecular fountain based on a Stark decelerated molecular beam. In this fountain, ammonia molecules are decelerated, cooled, and subsequently launched upwards some 10-50 cm before falling back under gravity, thereby passing a microwave cavity or laser beam twice -- as they fly up and as they fall back down. In this way, it will be possible to measure the inversion transition in ¹⁵NH₃ around 22.6~GHz with an fractional accuracy better than 10^-12 . Besides serving as a proof-of-principle, these measurements may be used as a test of the time-variation of fundamental constants using the sensitivity of the tunneling motion to a change of the proton-electron mass ratio. We present first experimental results of the molecular beam decelerator and focusing lenses.