Precision Tests using Molecules

The structure and symmetry of polar molecules make them uniquely suitable for a number of precision tests of fundamental physics theories. At LaserLaB Amsterdam we are performing two experiments aimed at testing the time-invariance of the fundamental constants of nature. Although the ultimate goal of the two experiments is the same, they employ rather different strategies.

In the first experiment, we are measuring microwave transitions in a beam of CO molecules. Due to an accidental degeneracy between rotational levels of two different spin-orbit ladders in CO, the microwave transition between these levels is very sensitive to the proton-to-electron mass ratio. If this ratio changes by a certain fraction over time, then the frequency of the transition will change 500 times this fraction. By accurately measuring this transition over time, we hope to detect a variation of the proton-to-electron mass ratio or set an upper limit to it.

The second experiment aims at measuring the inversion frequency of ammonia in a molecular fountain. In this fountain, ammonia molecules are decelerated, cooled, and subsequently launched upwards some 10-50 cm before falling back under gravity, thereby passing a microwave cavity twice - as they fly up and as they fall back down. The sensitivity of the inversion frequency to a variation of the proton-to-electron mass ration is smaller than the transition in CO, but this is compensated by the much higher precision that can be obtained in a fountain.

Contact: Dr. H.L. Bethlem: e-mail:
Physics of Light and Matter; Dept. of Physics and Astronomy
                                                                                              Photo courtesy of Hans van Schoot.

Fig. 1: Photograph showing the last stages of the Stark decelerator at LaserLaB Amsterdam that is used to make a slow beam of ammonia molecules.