Experiments with Supersonic Beams as a Source of Cold Atoms
Experiments with Supersonic Beams as a Source of Cold Atoms
Experiments with Supersonic Beams as a Source of Cold Atoms
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
The apertures set the transverse temperature, and hotter atoms are removed from the<br />
beam by the apertures, leaving only the colder atoms in the beam. For this re<strong>as</strong>on,<br />
it is appropriate to concentrate on the parallel component <strong>of</strong> the velocity, giving the<br />
Gaussian velocity pr<strong>of</strong>ile<br />
<br />
m<br />
p(v) = e<br />
2πkBT<br />
− m(v<br />
<br />
−w) 2<br />
2kB T<br />
=<br />
σ<br />
1<br />
√ e<br />
2π −(v <br />
−w) 2<br />
2σ 2 , (2.25)<br />
where σ is the standard deviation in the velocity parallel to the beam axis. This<br />
velocity distribution is compared to the Maxwellian velocity distribution for a few<br />
relevant g<strong>as</strong> species and reservoir temperatures in figure 2.1. As can be seen in<br />
the figure, the width <strong>of</strong> the velocity pr<strong>of</strong>ile is significantly narrower for the c<strong>as</strong>e <strong>of</strong><br />
a supersonic beam. Expressed in terms <strong>of</strong> temperature, beams <strong>of</strong> a few tens to a<br />
few hundred milliKelvin are common. This is significantly colder than the reservoir<br />
temperatures, and the supersonic expansion h<strong>as</strong> effectively cooled the beam.<br />
The cooling achieved by supersonic expansion is extremely general and can be<br />
used to cool any desired species. In the above discussion on isotropic expansion, it is<br />
<strong>as</strong>sumed that the g<strong>as</strong>es were truly ideal g<strong>as</strong>es. <strong>Supersonic</strong> beams are generally created<br />
<strong>with</strong> a noble carrier g<strong>as</strong> that generally behaves <strong>as</strong> an ideal g<strong>as</strong> at higher pressures<br />
and lower temperatures than many other g<strong>as</strong> species. Other species <strong>of</strong> interest are<br />
mixed in the reservoir before the expansion, or added just outside the nozzle during<br />
the expansion. If the species <strong>of</strong> interest is a g<strong>as</strong> at the reservoir temperature, it is<br />
typically seeded into the carrier g<strong>as</strong> in the reservoir. A species that is not a g<strong>as</strong> at the<br />
reservoir temperature is typically entrained into the beam at the exit <strong>of</strong> the nozzle.<br />
L<strong>as</strong>er ablation can be used to produce an atomic sample <strong>of</strong> a solid at the exit <strong>of</strong> a<br />
nozzle, or alternatively, an oven can produce an atomic beam that is then entrained in<br />
the supersonic beam. In either c<strong>as</strong>e, the added species is cooled by thermalizing <strong>with</strong><br />
the dominant carrier g<strong>as</strong> and is carried along <strong>with</strong> the supersonic beam, providing a<br />
15