Rick Bethlem

Dr. Hendrick L. Bethlem
Section Atomic Molecular and Laser Physics
Department of Physics
VU University Amsterdam
De Boelelaan 1081
1081 HV Amsterdam
The Netherlands

Room T2.64
Phone +31-(0)20-598 7951

Laser Lab
Room Kb1.42
Phone +31-(0)20-598 7954



- 3-7-2017: Our paper on studying cold collisions in a molecular synchrotron is out;
see arXiv1706.10236.
- 22-4-2017: The dutch Journal of Physics features a story on our fountain (in dutch);
see NTvN-mei2017.
- 7-4-2017 Aernout van der Poel wins Young Speaker Contest at Fysica 2017;
see News_archive.
- 19-01-2017: Cunfeng Cheng wins poster prize at Physics@Veldhoven;
see Poster Prize Ceremony.
- 13-12-2016: Our paper on the fountain is published;
see Cheng et al. PRL117_253201(2016).
- 15-11-2016: Our programme on "physics beyond the standard model with cold molecules" is funded by NWO.
- 14-11-2016: Volkskrant features a story on our fountain (in dutch);
see volkskrant 14-11-2016.


A molecular fountain

The resolution of any spectroscopic or interferometric experiment is ultimately limited by the total time a particle is interrogated. We have recently demonstrated the first molecular fountain, a development which permits hitherto unattainably long interrogation times with molecules. In our experiments, ammonia molecules are decelerated and cooled using electric fields, launched upwards with a velocity between 1.4 and 1.9 m/s and observed as they fall back under gravity. A combination of quadrupole lenses and bunching elements is used to shape the beam such that it has a large position spread and a small velocity spread (corresponding to a transverse temperature of <10μK and a longitudinal temperature of <1μK) when the molecules are in free fall, while being strongly focused at the detection region. The molecules are in free fall for up to 266 milliseconds, making it possible to perform sub-Hz measurements in molecular systems and paving the way for stringent tests of fundamental physics theories.

A molecular synchrotron

Rather than trapping particles in a trap, as is common in atomic physics, particles can also be stored in rings. We have developed a synchrotron for low-energy neutral molecules composed of 40 hexapoles arranged in a circle. By switching the voltages applied to the hexapoles every time the molecules pass through a gap, the molecules are kept in a tight bunch. We use this synchrotron to study collisions between stored ND3 molecules and beams of argon atoms. The advantage of using molecules stored in a synchrotron is two-fold: (i) The collision partners move in the same direction as the stored molecules, resulting in a low collision energy (down to 10cm-1); (ii) by storing molecules many roundtrips, the sensitivity to collisions is greatly enhanced.

Test of the time variation of Mp/Me in methanol

Methyl alcohol is one of the simplest molecules that exhibits internal rotation; the methyl (CH3) group rotates with respect to the alcohol (OH) group. In addition, the molecule rotates as a whole. Microwave transitions that convert the internal rotation to overall rotation -- and vice versa -- are very sensitive to the proton-to-electron mass ratio. When the proton-to-electron mass ratio changes by a certain fraction, the resulting fractional frequency change in methyl alcohol is up to 50 times this fraction. This is an order of magnitude larger than the transitions used so far in searches for possible spatial of temporal variations of the proton-to-electron mass ratio.

List of publications and theses from my group

Section Atomic, Molecular and Laser Physics
Department of Physics
Faculty of Sciences

Section Atomic, Molecular and Laser Physics - Vrije Universiteit Amsterdam
Address: De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands
Telephone:+31(0)205987892 Fax:+31(0)205987992