Laser-Cooled Atoms: Lithium

Element: Lithium (Li)

Atomic Number: 3

Mass: Two stable isotopes, masses 6 and 7 amu

Laser cooling wavelength: 671 nm

Doppler cooling limit: 140 μK.

Chemical classification: Alkali metal, column I in the periodic table. Yet another greyish metal. We’re almost done with alkalis, I promise. Less reactive than any of the others, so the explosions in water aren’t very impressive.

Other properties of interest: Lithium-7 is a boson, but has a negative scattering length, meaning that BEC’s of lithium-7 tend to implode unless you modify the collisional properties. Lithium-6 is a fermion, and much nicer to work with.

History: Lithium has been laser cooled for a long time– since the early 1990′s at least– because it has a relatively convenient cooling wavelength in the red, which can be accessed with dye lasers and red diodes. The middle image up above, from Zurich by way of LaserFest shows the approximate color.

To the extent that there was any controversy at all about the 2001 Nobel Prize for BEC, it involved lithium. In 1995, when Wieman and Cornell announced that they had achieved BEC in rubidium, Randy Hulet at Rice also announced that his group had gotten BEC in lithium. The problem was, nobody was entirely convinced. At the time, nobody knew what the best kind of magnetic trap for making BEC was going to be, and the Rice group had opted for a trap design that incorporated a permanent magnet. This had some nice features– it made for a rather tight trap, as I recall, and didn’t have the magnetic field zero that leads to Majorana losses– but it has one major drawback: it can’t be turned off. That meant that essentially all of their imaging needed to be done in situ, so where Wieman and Cornell could turn off their trap and detect the BEC through letting the cloud expand to a substantial size, the Rice group was stuck looking at really tiny clouds inside their trap, and there weren’t any good techniques for that at the time.

Combine this with the poor collisional properties, which lead to a sudden and dramatic collapse when the condensate gets too large, and they never had really solid evidence that they had BEC. They most likely did have very small condensates, but probably jumped the gun on the announcement in a way that made a lot of people think they were making an unjustified publicity grab. They kept at it for another year or two, presenting re-analyses of their imaging system to argue that they really had seen BEC, but eventually let it drop. The third share of the 2001 Nobel went to Wolfgang Ketterle, and even if he was the third to get BEC, not the second, he had done so much amazing stuff by that point that it wasn’t terribly controversial.

Lithium saw a bit of a resurgence a few years later, when interest switched to degenerate Fermi gases, where lithium-6 offers some nice properties, and a fair bit of work has been done with it. When I started Googling things for this post, I ran across this nifty all-optical route to degenerate Fermi gas, using basically the same idea as the all-optical BEC in strontium, doing cooling to extremely low temperatures using an ulraviolet laser transition to a state with a very long lifetime. I had missed that when it first came out, but thought that was cool.

Random fun things: Lithium is no longer available on credit.

Art: The cartoon version of lithium is a Goth with a knife and bulky headphones? The Comic Book Periodic Table includes a Scrooge McDuck comic, of all things.