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Issue Eur. Phys. J. Special Topics
Volume 163, October I 2008
Atomic Clocks and Fundamental Constants
Page(s) 55 - 69
DOI 10.1140/epjst/e2008-00809-5
Published online 25 October 2008

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. Bethlem1, 2, M. Kajita3, B. Sartakov4, G. Meijer2 and W. Ubachs1

1  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 $7\times 10^{-12}~\tau^{-1/2}$, 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.



© EDP Sciences, Springer-Verlag 2008


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