du principe électrostatique
par l'expérience d'un groupe hollandais
technique used by the Dutch group both to cool and to trap
molecules relies on the Stark shift of the energy levels
induced by the interaction of the molecule's electric dipole
moment with an electric field. Meijer told us that in the
1950s, John King of MIT had tried a similar electrostatic
method to slow ammonia to make an ammonium maser, but he
never got it to work. In the 1960s, Lennard Wharton of the
University of Chicago tried the reverse technique to accelerate
molecules, also without success. More recently, researchers
from the Lawrence Berkeley National Laboratory and the University
of California, Berkeley, have applied a similar technique
to slow neutral atoms. 3 Bretislav Friedrich of Harvard
points out that electric fields, unlike their magnetic counterparts,
can be switched quickly on the time scale of molecular translation.
The corresponding dynamics of electrical trapping exhibit
different and, at this point, novel phenomena. The principle
of the deceleration in time-varying electric fields is shown
schematically in figure 1 .
fields slow molecules
A dipolar molecule (green)
experiences a Stark shift as it enters the electric field
(red lines) between electrodes.
As the molecules near the center, those with
dipoles antiparallel to the field are decelerated, as
shown by the shorter velocity vector (blue).
Electrodes are turned off when the dipole reaches
the center, so that the molecule does not gain velocity
as it leaves the field. Connections to a high voltage
source are labeled by ±HV. (Adapted from ref. 4.)
pulse of the molecules to be slowed travels toward a pair
of electrodes. Those molecules having their electric dipoles,
on average, oriented antiparallel to the electric field
are attracted toward regions where the electric field intensity
is low. An electric field is switched on just as the molecules
approach the electrodes, putting a brake on the low-field-seeking
molecules. These molecules must climb a potential hill as
they move into the high-field region at the center of the
electrodes, consequently losing kinetic energy.
course, if the field remained on, the molecules would lose
potential energy (gaining back kinetic energy) as they moved
out of the field. But the experimenters turn off the field
when the pulse of molecules is near the center, preserving
the slow speeds. The molecular pulse then heads toward a
second pair of electrodes, where the cycle is repeated for
a succession of stages (the Dutch experiment had 63 stages).
The timing of the electric fields is critical, and only
molecules in a chosen energy level will be in phase with
the switching on and off of the field at each stage. Figure
fields trap molecules
As dipolar molecules (blue
cloud) approach the trap, they face a steep gradient in
the electric field (red lines), which slows those with
dipoles opposite to the electric field, as sketched in
the upper figure. Graph at bottom shows the potential
energy as a function of position.
Once molecules are in the cell, the field configuration
(top) creates a potential well (bottom), trapping them
inside. Energy is given in units of wavenumber. (Adapted
from ref. 1.)
the electrostatic stages must be turned on and off sequentially,
the electrostatic trapping method works only with pulses
of molecules rather than with a steady stream. Meijer and
company formed their pulse by expanding a gas containing
1% ammonia molecules and 99% xenon atoms through a cooled
solenoid valve. Collisions with the heavy rare gas atoms
removed much of the energy of the ammonia molecules, so
that the molecular beam streaming through the valve had
a mean translational speed of 280 m/s, with a fairly narrow
velocity spread (about 15%).
year, the Dutch team used the Stark decelerator to slow
molecules of metastable carbon monoxide, but did not trap
them. 4 In June, the same group presented a mathematical
description of the process, showing that the phase-space
density can be kept constant in the deceleration process.
the recent work on trapping, Meijer and his colleagues turned
to deuterated ammonia molecules (ND 3)--the first molecule
with more than two atoms to be trapped. (The deuterated
version of ammonia was preferred because the normal version
has a nonlinear Stark effect at the most convenient values
of electric fields.) The particular state selected to be
slowed was the upper inversion level in the vibrational
ground state. About one-eighth of the ammonia molecules
in each pulse were in this state.
trap the slowed molecules, the Dutch researchers directed
the pulse of molecules into an electrostatic trap. As the
molecules approached the trap, the experimenters applied
an inhomogeneous field, which got stronger the farther the
molecules traveled into the trap, as seen in figure 2 .
was like making the molecules travel uphill: Eventually
most were stopped or even turned around. At that point,
the electric field was changed to a symmetric configuration
with a minimum at the center. By ionizing and detecting
a small volume of the trapped molecules, the Dutch experimenters
have determined the density to be 10 6/cm 3, and they estimate
their volume to be about 0.25 cm 3. So far the team does
not have a direct measurement of the temperature of the
trap, but team members suspect it is appreciably less than
the depth of the trap's potential well, or 350 mK, corresponding
to a few tens of meters per second. (The maximum speed of
molecules contained in the trap is 17 m/s.) Meijer adds
that the temperature might be as low as 2 mK, corresponding
to the narrow velocity spread of molecules loaded into the
trap from the beam. The 1/ e decay time of the
trapped species was 0.24 s.
demonstrated the promise of their new technique, the Dutch
experimenters hope to improve it in several ways. Meijer
thinks they can achieve trap densities of 10 9/cm 3by such
measures as increasing the intensity of the beam, raising
the electric field strength and adding more stages to handle
higher initial velocities, and moving the trap closer to
the end of the decelerator for more efficient loading. He
thinks they can extend the trap lifetime by reducing the
vacuum pressure. Furthermore, he plans to try evaporative
cooling to further reduce temperatures. He and his coworkers
have yet to devise ways to load the trap with successive
pulses. The team has also been working on a storage ring
for neutral molecules, based on their decelerator ideas.
As Meijer remarked, "Every trick people played with charged
particles, we can now do with dipole molecules ."
1. H. L. Bethlem, G. Berden, F. M. H. Crompvoets, R. T.
Jongma, A. J. A. van Roij, G. Meijer, Nature 406 , 491 (2000).
2. For a review, see J. T. Bahns, P. L. Gould, W. C. Stwalley,
Adv. At., Mol., and Opt. Phys. 42 , 171 (2000).
3. J. A. Maddi, T. P. Dinneen, H. Gould, Phys. Rev. A 60
, 3882 (1999).
4. H. L. Bethlem, G. Berden, G. Meijer, Phys. Rev. Lett.
83 , 1558 (1999).
5. H. L. Bethlem, G. Berden, A. J. A. van Roij, F. M. H.
Crompvoets, G. Meijer, Phys. Rev. Lett. 84 , 5744 (2000).