Frequency is the most accurately measured quantity in physics. High-resolution spectroscopic measurements on atoms and molecules serve therefore as stringent tests of various fundamental physics theories. Ultimately the resolution of any measurement is limited by the time a molecule spends in the measuring device. At the LCVU a molecular fountain is being set up. In this device a
beam of molecules is decelerated and cooled using switched electric fields and subsequently directed upwards. The molecules will fly upwards some 30 cm before falling back under gravity thereby passing a microwave cavity twice; as they move up and down. The effective interrogation time in such a scheme includes the entire flight time between the two traversals through the driving
field and is typically a second. This long interaction time allows for a very high accuracy.
One of the molecules that will be investigated is ammonia. The microwave transition in ammonia is a measure for the rate at which the protons tunnel through the barrier between the two equivalent configurations of the molecule and is exponentially dependent on the ratio of the proton mass to the electron mass (μ). The inversion frequency is therefore a very sensitive probe for a possible variation of this ratio.
Experimental scheme of the fountain clock that will be used to measure the inversion
frequency of ammonia.