Laser-Cooled Atoms: Chromium

Element: Chromium (Cr)

Atomic Number: 24

Mass: Four "stable" isotopes between 50 and 54 amu. Chromium-50 is technically radioactive, with a half-life considerably longer than the age of the universe, so...

Laser cooling wavelength: 425nm, but see below.

Doppler cooling limit: 120 μK.

Chemical classification: Transition metal, smack in the middle of the periodic table. Shiny.

Other properties of interest: Has a fairly large magnetic moment in its ground state, 6 Bohr magnetons, which means it has strong magnetic interactions. For this reason, it's kind of an interesting system to study-- you can conceivably use it to study quantum magnetism in a regime that's hard to access with alkali metals.

History: Most of the atoms we've talked about laser cooling have clustered in the outer columns of the periodic table. There's a good reason for this, namely that those atoms have relatively simple electronic structure: the alkalis have only one electron in their outermost shell, and the alkaline earths only two. The rare gases have a full outer shell, but if you excite one of those to a metastable state, you get something that looks kind of like an alkali if you tilt your head and squint.

This is nice if you're doing laser cooling, because the simple electronic structure means a relatively simple set of energy levels, and a good chance of finding a transition where the atoms just go up to one excited state and back down to the ground state where they started. when you move toward the middle part of the table, though, things start to get messier, and there are lots and lots of other states the atoms can fall into.

Chromium is actually reasonably good as such things go-- it has a single outer shell electron-- making it a fairly easy target by transition metal standards, which is why people were pushing it around with light by the early 1990's (there was a group doing lithography experiments with a chromium atomic beam at NIST when I was in grad school, using standing waves of light to deposit neat rows of atoms on surfaces). It's still a good deal more complicated than cooling alkalis, though, because the array of extra states means you need additional lasers to get the atoms back to the state where they'll absorb the laser cooling light. In particular, you need both 425nm cooling light (the sort of purple light in the middle image above) and a "repumper" laser at around 660nm (you can make out traces of red light in that picture as well; the green is for an optical lattice. Larger images can be found on their pictures page).

If you're willing to deal with the hassle of making multiple lasers work at the same time, though-- and the hassle increases as some high power of the number of lasers-- you can make a MOT for chromium, and collect large numbers of atoms. The relatively simple (for a transition metal) level scheme and the large magnetic moment make this worthwhile, so several groups have cooled and trapped chromium, and even made BEC in chromium via laser cooling. (John Doyle's group at Harvard tried to get chromium BEC starting with buffer-gas-cooled samples, but ran into a problem with collisional losses that kept it from working.)

More recently, people have cooled, trapped, and reached quantum degeneracy with erbium and dysprosium, which have even bigger magnetic moments, and we may get to those later. Chromium was the first in this area, though, and remains important for that.

Random fun things: The chromium sources used at NIST operated at around 2000K, which was the largest difference from initial to final temperatures that I had heard of at the time. As I recall, they had to take some care to ensure that there wasn't a straight-line path from the source to any glass windows that they were hoping to pass light through, as the deposition of chromium on the glass would rapidly make a very shiny mirror, and since you can't heat vacuum windows to 2000K, that rendered them pretty much useless.

Art: The cartoon version of chromium is a shiny greaser. The Comic Book Periodic Table includes a gloriously tacky collectible gimmick issue from 1993.

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