Laser-Cooled Atoms: Sodium

Element: Sodium (Na)

Atomic Number: 11

Mass: one stable isotope, 23 amu

Laser cooling wavelength: 589 nm

Doppler cooling limit: 240 μK

Chemical classification: Alkali metal, column I of the periodic table. Like the majority of elements, it's a greyish metal at room temperature. Like the other alkalis, it's highly reactive, and bursts into flame on contact with water. For this reason, all physicists working with sodium have True Lab Stories about accidentally blowing stuff up with it.

Other properties of interest: Scattering length of around 80 a0; Feshbach resonance at around 900 G.

History: Most of the early laser-cooling experiments with neutral atoms (note: Dave Wineland did laser cooling of ions well before this, but that's a slightly different game) were done in sodium in the early-mid 1980's. This was largely technological, I think: at the time, the best tunable laser sources were dye lasers, and the rhodamine 6G laser dye allows reasonably reliable production of light at the sodium wavelength without being incredibly noxious (by the standards of laser dyes, at least). The first demonstrations of Zeeman slowing of a beam and sub-Doppler temperatures in optical molasses were made in sodium (and you can read the original papers via this Physics Focus article). Sodium was also the first cold atom magnetically trapped, and the atom used for the first MOT.

It was not, however, the first atom cooled to Bose-Einstein Condensation-- rubidium has that honor. It was close behind, though, with the Ketterle group successfully achieving a BEC of sodium a few months after Cornell and Wieman got BEC in rubidium (the NIST group, where I was working at the time, got BEC in sodium a couple of years later). Ketterle earned his share of the 2001 Nobel by racking up a whole bunch of "firsts" with the sodium BEC machine-- interference of two separated condensates, in-situ imaging of a condensate, demonstration of Feshbach resonances-- mostly because in the early days, they were able to make larger condensates than the Rb folks were, so certain kinds of measurements were much easier for them.

Sodium is considerably less popular these days, because while rhodamine 6G relatively well-behaved for a laser dye, it's still a gigantic pain in the ass to work with, and there are much more user-friendly solid-state laser sources-- diodes and Ti:Sapph lasers-- available now. (I'm not aware of anyone doing sodium cooling with doubled IR lasers, but I'd be a little surprised if nobody's tried it...) Other atoms also offer greater versatility, with multiple stable isotopes and more complicated level structures-- sodium's a pretty vanilla atom, other than the explode-in-water thing. But sodium's place in the history of physics is forever assured due to its central role in the development of laser cooling and BEC.

Random fun things: A long time ago, Bill Phillips was (mis)quoted by the New York Times as saying "Of course, there are no two-level atoms, and sodium is not one of them."

Art: The cartoon version of sodium is a chubby dude in a bathing suit. Most of the comic book treatments play off the explosiveness (greatly overstating the danger, by the way...).

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