Eur. Phys. J. Special Topics 163, 55-69 (2008)
DOI: 10.1140/epjst/e2008-00809-5

Prospects for precision measurements on ammonia molecules in a fountain

H.L. Bethlem^{1, 2}, M. Kajita³, B. Sartakov⁴, G. Meijer² and W. Ubachs<sup>1

1</sup>  Laser Centre Vrije Universiteit, De Boelelaan 1081, 1081HV Amsterdam, The Netherlands
²  Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
³  National Institute of Information and Communications Technology, Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan
⁴  General Physics Institute RAS, Vavilov Str. 38, 119991 Moscow, Russia

rick@few.vu.nl

Abstract
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 ¹⁵NH₃ around 22.6 GHz. The estimated frequency instability is , with 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.

© EDP Sciences, Springer-Verlag 2008