Prospects for precision measurements on ammonia molecules in a fountain

• Published in: The European physical journal. ST, Special topics Vol. 163; no. 1; pp. 55 - 69
• Main Authors: Bethlem, H. L., Kajita, M., Sartakov, B., Meijer, G., Ubachs, W.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01.10.2008
• Subjects: Atomic; Classical and Continuum Physics; Condensed Matter Physics; Materials Science; Measurement Science and Instrumentation; Molecular; Optical and Plasma Physics; Physics; Physics and Astronomy; Atomic Clock; Stark Shift; European Physical Journal Special Topic; Fundamental Constant; Blackbody Radiation
• ISSN: 1951-6355; 1951-6401
• DOI: 10.1140/epjst/e2008-00809-5

The recent demonstration of cooling and manipulation techniques for molecules offer newpossibilities for precision measurements in molecules. Here, we present the design of a molecularfountain based on a Stark decelerated molecular beam. In this fountain, ammonia molecules aredecelerated to a few meter per second, cooled to sub microKelvin temperatures and subsequentlylaunched. The molecules fly upwards some 30 cm before falling back under gravity, thereby passing amicrowave cavity twice – as they fly up and as they fall back down. The effective interrogationtime in such a Ramsey type measurement scheme includes the entire flight time between the twotraversals through the driving field, which is on the order of a 1/2 second. We present numericalsimulations of the trajectories through the decelerator and estimate the expected count rate. Wepresent an evaluation of the expected stability and accuracy for the inversion transition in 15 NH 3 around 22.6 GHz. The estimated frequency instability is , with τ being the measurement time in seconds. With a careful design ofthe interogation zone, systematic frequency shifts are kept below 10 -14 . Besides serving as aproof-of-principle, these measurements may be used as a test of the time-variation of fundamentalconstants using the sensitivity of the tunneling motion to a change of the proton-electron massratio.