A Zeeman decelerator is the magnetic counterpart to a Stark decelerator. The working principle of the two devices is very similar, except that a Zeeman decelerator is based on the interaction between paramagnetic atoms/molecules and magnetic fields.
In the true sense of the word, our Zeeman decelerator is the younger sibling of the Stark decelerator, both in age and in size: whereas the Stark decelerator was built in 2005, the Zeeman decelerator is still under construction. With its 131 electrode pairs, the Stark decelerator is more than 1m in length. Our Zeeman decelerator consists of merely 12 solenoid coils, with a total length of less than 13cm.
Due to its favourable magnetic-moment-to-mass ratio, even this rather small number of deceleration stages is sufficient to decelerate H atoms to a standstill.
Seeding in Ar and pre-cooling the valve to 250 K yields an initial beam velocity of 510 m/s, i.e., a translational energy of 11 cm^-1 that needs to be removed in total. Apart from the hyperfine splitting at low magnetic fields, the Zeeman energy in atomic hydrogen increases linearly with the applied magnetic field (see figure below). At a current of 300 A, a magnetic field of 2.2 T is generated in the centre of each deceleration coil. This corresponds to a maximum kinetic energy of 1 cm^-1 that can be removed per deceleration stage. Since we use 12 coils in succession, we are indeed able to remove all the initial kinetic energy of a bunch of particles.
Figure: Zeeman energy diagram for ground-state hydrogen atoms.
Figure: Magnetic field generated in our solenoid coils at a current of 300 A. The beam direction is along the z axis.
As in a Stark decelerator, the only real cooling (in the sense of phase-space compression) takes place during the supersonic expansion. In the decelerator itself, only a part of the initial phase-space distribution is shifted to lower kinetic energies. However, the phase-space volume itself is maintained.
Figure: A packet of H atoms is decelerated from an initial velocity of 520 m/s to 170 m/s (\phi_0 = 30°).
The amount of energy removed in each deceleration stage depends on how far the particles move into the increasing magnetic field of the coils. The further they travel towards the centre of the coil, the more energy is removed. However, the accepted phase-space volume decreases.
The energy removal is usually expressed in terms of a phase angle. For that, a synchronous particle is defined that is always exactly at a position \phi_0 when the coil is switched off. A number of other particles, denoted as non-synchronous particles, will oscillate around the position and velocity of the synchronous particle. Others, whose relative position and/or velocity lies outside the phase-stable region will get out of phase with the synchronous particle, i.e., they will not be decelerated.
In the following figure, this is illustrated for phase angles of 0° and 30°. The dynamics become more complicated when the radial motion and the finite switching times of the coils (approx. 9 \mus) are also taken into account.
Figure: Longitudinal phase-space acceptance for phase angles of 0° and 30°. While the amount of deceleration increases at higher phase angles, the decelerated phase-space volume is decreased.
Our experimental setup consists of a pulse valve that is used to expand a NH_3/Ar mixture into the vacuum. The hydrogen atoms are produced by photolysis with an ArF excimer laser (193 nm) at the end of a glass capillary which is attached to the end of the valve. Before entering the Zeeman decelerator, the beam passes through a skimmer to differentially pump the source chamber and the experimental chamber. Behind the Zeeman decelerator, the H atoms are ionised in a (2+1)-REMPI scheme via the ^2P state (frequency-doubled output of a dye laser, 243 nm) and subsequently accelerated towards an MCP detector in a Wiley-McLaren-type time-of-flight spectrometer.
Our Zeeman decelerator is built in collaboration with Frédéric Merkt’s group in Zurich .