How to Make Slow Atoms and Molecules 2

Consider the air around you, which is hopefully at something like “room temperature”– 290-300 K (60-80 F). That temeprature is a measure of the kinetic energy of the moving atoms and molecules making up the gas. At room temperature, the atoms and molecules in the air around you are moving at something close to the speed of sound– around 300 m/s (give or take a bit, depending on the mass).

If you’re a physicist or chemist looking to study the property of these atoms and molecules, that speed is kind of a nuisance. For one thing, the atoms and molecules tend not to stick around long enough to interact with a given atom or molecule for very long– in ten milliseconds, they move 2-3 meters, which is about as big as atomic and molecular apparatus gets. If you’re studying their properties via spectroscopy (which is the best tool we have for looking at what’s going on with atoms and molecules), that speed causes another problem: the Doppler shift. The frequency of the light absorbed or emitted by a moving atom or molecule is shifted from the value for a stationary one, and a gas of atoms will contain atoms and molecules moving at a variety of different speeds and directions. This tends to wash out fine details of the spectra.

In order to study atoms and molecules in detail, then, we need some way to make them slow the hell down. Physicists and physical chemists have developed a wide range of techniques for making atoms and molecules move slowly, many of which are quite ingenious. In the previous post, we looked at methods using cold liquids or mechanical rotators to slow atoms down. In this post, we’ll look at a variety of ways to use fields to do the job.

Magnetic Fields: My second-favorite bit of jargon in this subfield (after “swatting limit” in the previous post) is “atomic coilgun,” another technique employed by Mark Raizen’s group at Texas. The coilgun is much less violent than it sounds, though it’s still pretty cool.

As you might guess, the coilgun involves, well, coils. A whole series of them, in fact, arranged one after another on the path the beam of atoms to be slowed will follow. These coils can carry a great deal of current, which generates a magnetic field that is strongest at the center of the coil, and drops off from there. An atom with a magnetic moment– which most of the atoms in the periodic table have– will be repelled by this field (if you choose your polarities properly), and slow down as it enters the region of the coil.

If the current in the coil is left on, though, the atom will speed back up as it passes through the center and out the other side, so you don’t gain anything. If you turn the current off very quickly, though, at the exact moment when the atoms reach the center of the coil, you get all the slowing down, and none of the speeding up. If you do this with many coils in sequence, each timed to turn off at exactly the right moment, you can bring a beam of atoms to a complete stop, as the Raizen group did with a 64-stage coilgun in the paper linked above.

This is an approach with some technical challenges– the coils carry 750 amps of current, which needs to be switched on and off very quickly– the current switches off in about 20 microseconds, which takes some doing. When something goes wrong with that much current, the results can be messy. This has the advantage, though, of being a very general method of slowing things down, which will work on a wide range of atoms or even molecules. It also gives you a good deal of control of the final speed, by adjusting the field strength and switching sequence for the individual coils.

Electric Fields: If you don’t like working with hundreds of amps of current, you can always go with lots and lots of voltage. The Stark deceleration method (Johannes, not Tony) is used by a whole community of people, a convenient example of which is provided by Ed Hinds’s group at Imperial College (I had that paper bookmarked from my Physics World article). The electrostatic method is very similar to the coilgun technique, using high-voltage electrodes (10 kV) rather than current-carrying coils. The basic principle of operation is the same, though– the electrodes produce a field that slows the beam down until it reaches the center, at which point, the field is switched off before it can speed the beam back up.

The decrease reported in that paper is more modest than the coilgun paper– they slowed a beam of YbF molecules from 298 m/s to 287 m/s with 12 stages of deceleration– but it’s a good proof of principle. Also, they’re using really heavy molecules, not neon atoms, which has to count for something.

This looks like a “pick your poison” sort of thing to me– both coilgun and Stark deceleration techniques require a good deal of work on timing electronics, and the use of some fairly unpleasant electrical signals. There are probably good reasons to prefer one over the other, that each group would be happy to spell out, but the basic ideas are very similar.

High-Intensity Lasers: Since electric and magnetic fields separately can slow beams of atoms or molecules down, what about a combination of the two? In other words, what about light?

You can pull a very similar trick using an intense laser beam. The combination of oscillating electric and magnetic fields can also produce a field that decelerates a beam of atoms– a group in Edinburgh has slowed benzene molecules using a 15ns laser pulse. The slowing was again fairly modest– only 25 m/s– but, again, one 15ns pulse. Of course, it’s a really intense pulse– the abstract says it’s 10¹²W/cm², or roughly two trillion times the intensity of a good laser pointer– but high-power lasers are awesome…

Note that this is not the traditional laser cooling. Laser cooling relies on the absorption and emission of photons by the target species, while this just treats the laser as a honkin’ big electromagnetic field. The pulsed-laser technique is far more general– the paper linked above is slowing benzene molecules, which are much too complex for standard laser cooling techniques.

And, of course, this brings us to:

Laser Cooling: This refers to techniques that slow the motion of atoms through the resonant absorption of photons from a laser beam. I’ve written about laser cooling at length before, and you can also play with the spiffy Java applets at the Physics 2000 site at Colorado. Laser cooling is the most effective of these techniques in terms of final velocity– laser cooling has been used to slow and stop beams of atoms since the early 1980’s, and laser cooling techniques can bring a sample of gas from room temperature down to microkelvin temperatures, or in velocity terms, from the speed of sound down to a centimeter per second or so.

Given that, why are there all these other techniques? Well, because laser cooling has limits. In order to use resonant photons to bring atoms to rest from room temperature velocities, you need them to absorb and re-emit something like a hundred thousand photons in a row. Most atoms, and essentially all molecules have too many states for this to work– after a few cycles of absorbing photons and returning back to the initial state, they’ll end up dropping into one of the zillions of other possible states, one that does not interact with the original laser beam. You can fix this by adding more lasers to “pump” the atoms back where they started, but that gets prohibitively expensive in a hurry. The vast majority of laser cooling experiments use atoms from the first two columns of the periodic table, or from the last column, because those atoms have the right level structure to allow lots of photon absorptions.

The other issue with laser cooling techniques is that they’re not very flexible. To laser cool a sample of atoms, you need to tune your laser very close to the transition frequency for that particular atom. If you want to switch atoms, you need to switch frequencies, and in practice, this usually means a completely different laser system. Sometimes, it means replacing all of the optics, if the difference between the transition frequencies is big enough (a good mirror for light at 1064 nm looks like a clear piece of glass for light in the visible range).

So, while laser cooling of the sort that won the 1997 Nobel Prize in Physics is the ultimate tool for reaching really slow velocities, it’s limited in its applicability. The other techniques described in this post and the previous one may not reach the same ridiculously low temperatures, but they work for more different kinds of things, which is why they’re all still being actively investigated.