The Stark decelerator is a device which accepts a collimated beam of molecules entering at one end, and slows down a fraction of the molecules as they pass through a sequence of ~ 100 high voltage dipoles. It is capable of producing gas phase molecular samples with an effective temperature of order 10 mK.
The slow-moving bunch of molecules ejected by the decelerator not only has a narrow distribution of velocities, but also is selected in a single vibration-rotation quantum state, and the molecules are spatially oriented.
In an electric field the degeneracy of the energy levels of a molecule may split as shown on the left (the Stark effect). This generally requires that the molecule has a dipole moment. The linear Stark effect, shown left, is displayed by symmetric top molecules such as ammonia. Some molecules will exist in states whose energy increases with field; these are known as 'Low field seekers' because they will experience a force away from a high-field region of space as a consequence of the increasing potential energy (i.e., they behave as if moving up a hill). Likewise molecules whose energy decreases with field are known as 'High field seekers', because they will tend to move towards the high field region where their energy is lower.
Ammonia energy levels in a field for J=+1, K=+1
A low-field seeking molecule approaching the first pair of dipoles in the Stark decelerator will slow down a little as it experiences an increasing field. When it reaches a position close to the top of the energy hill (at (2), on the left) we need to switch the fields to prevent it running down the hill the other side (and hence accelerating). The voltages of the first and second pairs of electrodes are then swapped over so that the hill moves along suddenly, as shown in grey. The molecule thus instantaneously finds itself at the bottom of another hill and continues climbing, slowing down a little more. By repeating this sequence 100 times over with a carefully timed field it is possible to slow a subset of the molecules down to any arbitrary velocity.
After passing through the decelerator and through an additional series of focusing and bunching devices, the velocity of the molecules is measured by ionizing them using a laser and accelerating them to an ion detector. The time of flight relative to their entry into the decelerator is measured.
The picture to the above shows time-of-flight measurements for a beam of ND3 travelling initially at 360 ms-1. A bunch of slower molecules is separated out travelling at a lower velocity, which can be controlled by varying the timing sequence of the pulsed fields.
Properties of decelerated molecules
Arbitrarily adjustable velocity down to a few ms-1
Narrow velocity distribution corresponding to temperature of ~ 10 mK
Quantum state selected
Selected molecules are oriented in space.