Dr. Hendrick L. Bethlem
Section Atomic Molecular and Laser Physics
Department of Physics
VU University Amsterdam
De Boelelaan 1081
1081 HV Amsterdam
Phone +31-(0)20-598 7951
Phone +31-(0)20-598 7954
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
) 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