Peer-Reviewed Egoboo: The Metastable Xenon Project

I've been slacking a bit, lately, in terms of putting science-related content on the blog. Up until last week, most of my physics-explaining energy was going into working on the book, and on top of that, I've been a little preoccupied with planning for the arrival of FutureBaby.

I'd like to push things back in the direction of actual science blogging, so I'm going to implement an idea I had a while back: I'm going to go back through the papers in my CV, and write them up for ResearchBlogging.org.

This offers a couple of nice benefits from my perspective. First of all, I already know what's in these papers, so I don't have to work as hard at it as I would to blog new stuff. It also lets me add a few personal comments and insider notes about the experiments, how we did them, and what part I played, which is good life-in-the-lab information. And, of course, I'm just vain enough to enjoy shameless self-promotion.

The first half-dozen of my papers were all done in the same lab, as part of my thesis research, so they share a lot of common background. The experiment in question is no longer at NIST, but the Internet Archive conveniently contains a copy of the metastable xenon project web site that I made back in 1998 or so, and there are preprint copies of some of the papers available there for free, if you'd like to follow along. I'll kick things off with a bit of background explanation, though.

So. Metastable xenon-- what's that all about? Well, xenon is one of the rare gases, element number 54, sitting between iodine and cesium on the periodic table. Trace amounts of it are present in the atmosphere, and it's used for a bunch of applications in lighting and material processing. Weirdly, it also acts as an anesthetic. But what's the "metastable" part about?

Well, if you took high-school chemistry, you may remember that the special thing about the rare gases (or "noble gases") is that they have a full outer shell of electrons, and thus are quite happy as single atoms, thankyewverymuch. They don't form chemical bonds with much of anything-- xenon compounds have been produced, but only with great effort.

That same feature means that it's extremely difficult to excite an electron from the full outer shell to a higher-lying excited state. The amount of energy needed to excite a rare-gas atom to its lowest excited state would ionize a bunch of more reactive atoms. The lowest excited state of xenon, for example, is 8.4 eV above the ground state (that's 793 kcal/mol, if you like godawful chemist units), which is enough energy to strip one electron clean off anything in the first two columns of the periodic table.

8.4 eV corresponds to a wavelength of about 149 nm, which is well into the vacuum ultraviolet-- it won't propagate very far through air. Nobody has managed to make a convenient laser at these wavelengths, so laser-cooling xenon in its ground state is right out.

This is where the "metastable" part comes in. It turns out that if you do manage to get xenon excited into its first excited state, that state is "metastable," meaning that an atom will stay there effectively forever-- the lifetime, measured in the first experiments I was part of at NIST (as a "thanks to" rather than an author-- citations are here), is about 43 seconds, compared to about 34 nanoseconds for the next excited state. Once they're in that state, there's a "closed transition" to another higher-energy excited state, which decays only back to the metastable state. The wavelength of light that excites this transition is about 882 nm, conveniently located in the near-infrared portion of the spectrum, where there are laser sources readily available. Thus, we can use the metastable state as an effective ground state, and laser cool atoms in that state.

This works brilliantly, as it turns out. The figure at right shows the amount of light scattered by the metastable Xe trap as a function of the frequency of the lasers, with the various peaks indicating the trapping of each of the nine stable isotopes of xenon (the transition frequencies for each isotope are very slightly different-- the tight group of peaks in the center spans a little more than one one-millionth of the absolute frequency of the laser)-- when atoms are trapped, light comes out, and when there are no atoms, there's no light. The big peak in the middle represents xenon-132, the most abundant of the nine stable isotopes, and that peak probably corresponds to something like a million trapped xenon atoms at a density of about 10¹⁰ atoms per cubic centimeter (which sounds high, but is something like a billion times less dense than air). The temperature of the sample was roughly 100 microkelvin, or 100 one-millionths of a degree (Celsius) above absolute zero, corresponding to an average velocity of about 10 cm/s.

My thesis was titled "Ultra-Cold Ionizing Collisions in Metastable Xenon," and all of the experiments I did used the metastable xenon system to look at what happens when atoms collide at very low temperatures. This might not sound terribly exciting, but in fact, collisions in the microkelvin regime are dramatically different than the billiard-ball collisions you talk about in the room-temperature theory of gases-- the atoms are moving slowly enough to allow modification of the collisions by the absorption or emission of photons, and even quantum effects.

Xenon (and metastable gases in general) is a good system for studying collisions because each of the trapped atoms is carrying that metastable energy with it. Each metastable xenon atom is like a little bomb carrying 8.4 eV of stored energy, and when two of them collide, they have enough energy between them to ionize one of the two. The "bomb" goes off, leaving behind either an ion, a ground-state atom, and a free electron (in the process called "Penning Ionization") or an ionized xenon molecule and a free electron (in the process called "Association Ionization"). Free electrons and ions are really easy to detect, so we can monitor the collision rate in a sample of metastable atoms just by parking a charged-particle detector close to the trap, and looking at what comes out.

So, that's the essence of the first four papers I'll be talking about: we excited xenon atoms into the metastable state, cooled them to very low temperature, and looked at what happened when they ran into each other, while we did various things to modify the rate of collisions. AS you'll see, we found some interesting stuff along the way.

In a series of forthcoming posts, I'll go through the four experimental papers that formed the core of my thesis research, and explain what we did to modify the collision rate, and what that tells us about xenon and the universe in general. I'll also try to tell some stories about what it was like to do those experiments-- the personal nitty-gritty details that you don't get from the papers themselves.

We'll start tomorrow, with the paper "Optical Control of Ultracold Collisions in Metastable Xenon." Unless, of course, we have to run off to the hospital before I get a chance to type it up...