Element: Xenon (Xe)
Atomic Number: 54
Mass: nine "stable" isotopes, masses from 124 to 136 amu. Xenon-136 is technically radioactive, but with a half-life of a hundred billion billion years, so, you know, it's pretty much stable.
Laser cooling wavelength: 882 nm
Doppler cooling limit: 120 μK
Chemical classification: Noble gas, part of column VIII of the periodic table. Doesn't react with anything, so poses much less danger to scientists than any of the alkalis, though it has, at times, been used as an anesthetic, and Will Happer's group at Princeton has a funny story about a student's reaction to breathing it for their polarized-gas MRI experiments. The colorless gas thing means you get a comic-book cover as part of the trading card image up top.
Other properties of interest: Like helium, xenon is not cooled in its ground state, but in the first excited state, which is metastable with a lifetime of tens of seconds, effectively forever in atomic physics terms. The internal energy of that state is much lower-- only 8.4 eV, less than half that of helium, so it's harder to get xenon atoms to knock electrons off things, but you still get tons of ionizing collisions.
History: Laser cooling of xenon is a topic near and dear to my heart, because it was the subject of my Ph.D. thesis research. It's never been a big thing, though-- there are probably around 20 people in the entire world who have done experiments with laser-cooled xenon. This is mostly a matter of the laser wavelengths being inconvenient-- the laser-cooling transition is at 882nm, which isn't close to any of the easy-to-make diode laser wavelengths, so you're looking at a Ti:Sapph laser to get it, and that prices it out of reach of a lot of groups.
The group at NIST started looking at xenon because there's a two-photon transition between the metastable state and a higher state that could serve as an optical clock, so the initial experiments were focused on that-- the apparatus was built by Steve Rolston and Matt Walhout, and when I got there, the first thing they were doing was measuring the metastable lifetime and doing spectroscopy of the upper state. The "clock" wavelength turns out to be inconvenient as well-- a bit over 2 microns, falling right in the gap between a couple of IR laser technologies-- and the upper state of the "clock" is very close to another state, close enough that room-temperature blackbody radiation makes a significant shift.
That pretty much rules it out as a competitive clock, though we continued to pay lip service to that. But since we had the xenon cooling apparatus, we found things to do with it, mostly looking at the really abundant ionizing collisions. That turned out to be a nice project-- four Phys. Rev. Letters and a Phys. Rev. A, plus a whole slew of conference talks and the like, and a career for me. As I was wrapping up, they shifted from collisions to the emerging field of ultra-cold neutral plasmas, where the xenon apparatus was used for some of the very first experiments making plasmas from cold atoms. They got another decade or so out of that, including a move from NIST to the new Joint Quantum Institute at Maryland when Steve became faculty there.
Sadly, when I saw Steve at DAMOP, he said that they're pretty much shutting the xenon experiment down-- a moment of silence, please. ... ... ... Anyway, while it's sad to see the end of the xenon era, it had a really good 20-year run, and I'll always have fond memories of it.
Random fun things: I put a literary flourish at the start of my thesis, using this quote:
"…Can you name the six noble gases?"
As this could be no poser for an economic geographer, I rattled them off in their proper aristocratic order. "Helium, Neon, Argon, Krypton, Xenon, and -er- Radon. They were raised to the peerage in the eleventh year of England’s George Fifth, and Neon was awarded the Order of the Seraphim by Gustav Sixth of Sweden for its compassionate service in guiding to bars and beaneries guys who roll into towns late at night."
– from The Moon’s Fire-eating Daughter by John Myers Myers
That got me a bit of hassle from the university, who didn't have a space in the Official Thesis Format for that sort of quote. I think I re-labeled it as a dedication or something in the copy I turned in to them. The book, by the way, is pretty forgettable other than that bit.