Prospects for precision measurements on ammonia molecules in a fountain

The recent demonstration of cooling and manipulation techniques for molecules offer new possibilities for precision measurements in molecules. Here, we present the design of a molecular fountain based on a Stark decelerated molecular beam. In this fountain, ammonia molecules are decelerated to a few meter per second, cooled to sub microKelvin temperatures and subsequently launched. The molecules fly upwards some 30 cm before falling back under gravity, thereby passing a microwave cavity twice - as they fly up and as they fall back down. The effective interrogation time in such a Ramsey type measurement scheme includes the entire flight time between the two traversals through the driving field, which is on the order of a 1/2 second. We present numerical simulations of the trajectories through the decelerator and estimate the expected count rate. We present an evaluation of the expected stability and accuracy for the inversion transition in (NH3)-N-15 around 22.6 GHz. The estimated frequency instability is 7 x 10(-12) tau(-1/2), with tau being the measurement time in seconds. With a careful design of the interogation zone, systematic frequency shifts are kept below 10(-14). 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.