Slowing down molecules
It all started in the women’s toilet at the Radboud University. Across the hallway from
where velocity map imaging was developed a year before, in a lab that used to be the
women’s toilet, Rick Bethlem, Giel Berden and Gerard Meijer first decelerated neutral
molecules with electric fields. The Molecular and Laser Physics group at the Radboud
University has a long tradition of using electric lenses and deflectors to manipulate the
transverse motion of molecular beams. In 1998 it was realized that time-varying electric
fields could be used to manipulate the forward velocity of beams. Using a series of 63
electrode pairs to which voltage differences of 20 kilovolts were pulsed on and off, they
managed to decelerate a beam of CO molecules from 225m/s to 98m/s [1]. This
experiment opened a route towards the creation of samples of cold molecules and study
their properties at low temperature. In later experiments, this type of decelerator,
dubbed ‘Stark decelerator’, was used to bring molecules to a complete standstill and
keep them confined in traps and storage rings for several seconds [2,3]. The dynamics
of the molecules inside the decelerator was studied in great detail, which led to the
design of Stark decelerators with improved stability [4,5]. Stark decelerators are now
used by many groups throughout the world including three groups in the Netherlands;
the group of Bethlem in Amsterdam uses Stark deceleration to launch molecules in a
fountain and inject molecules in a synchrotron, the group of Van de Meerakker in
Nijmegen uses Stark deceleration to create controlled beams with tunable velocity for
performing crossed beam scattering experiments and the group of Hoekstra in
Groningen has build a 5 meter long traveling wave decelerator to decelerate heavy
paramagnetic molecules for testing Particle Physics Models.
[1] “Decelerating neutral dipolar molecules,” H.L. Bethlem, G. Berden, and G. Meijer,
Phys. Rev. Lett. 83, 1558-1561 (1999).
[2] “Electrostatic trapping of ammonia molecules,” H.L. Bethlem, G. Berden, F.M.H.
Crompvoets, R.T. Jongma, A.J.A. van Roij, and G. Meijer, Nature (London) 406, 491-494
(2000).
[3] “A prototype storage ring for neutral molecules,” F.M.H. Crompvoets, H.L. Bethlem,
R.T. Jongma, and G. Meijer, Nature (London) 411, 174-176 (2001).
[4] “Higher-order resonances in a Stark decelerator,” S.Y.T. van de Meerakker, N.
Vanhaecke, H.L. Bethlem, and G. Meijer, Phys. Rev. A 71, 0543409 (2005).
[5] “Transverse stability in a Stark decelerator,” S.Y.T. van de Meerakker, N. Vanhaecke,
H.L. Bethlem, and G. Meijer, Phys. Rev. A 73, 023401 (2006).
Source: Wim Ubachs (VU University)