Cold Molecules - New avenue to the 5th phase of matter

Using a method usually more suitable to billiards than atomic physics, researchers from Sandia and Columbia University have created extremely cold molecules that could be used as an improved first step in creating molecular Bose-Einstein condensates (BECs). BECs, predicted in the 1920s but not actually made until 1995, represent a special fifth phase of matter -- like liquids, solids, gases, and plasmas -- but can only exist at extremely low temperature. Their existence was first proposed by Satayendra Nath Bose, then formalized by Albert Einstein.

Future applications, assuming BECs can be usefully harnessed, range from improved atomic clocks to individual yes/no switches (Q-bits) in computers; from precision gravity detectors that could perhaps locate underground caverns to a model black hole in which light can enter, but cannot escape.

Researchers from Sandia and Columbia made a serendipitous discovery while studying collisional energy transfer between a beam of atoms intersecting a beam of molecules. They noted that some collisions occurred -- as they might between two billiard balls -- at exactly the right velocity for molecules to become motionless.

Zero Kelvin

The definition of a cold molecule is one that is moving slowly, and this experiment produced some very slowmoving molecules. (Zero Kelvin, or absolute zero, is defined as a molecule or atom being stationary.)

The study led to a new technique for cooling molecules to tens of milliKelvin temperatures -- tens of thousandth of a Kelvin. In a paper, the team reported that single collisions among molecules in two beams of nitric oxide molecules with argon atoms can produce nitric oxide molecules with speeds no greater than 15 meters per second, equal to a maximum temperature of 0.4 degrees Kelvin.

"Our technique has promise to be developed into a first step in the cooling process needed for a molecular Bose- Einstein condensate," says Sandia researcher and principal investigator Dave Chandler. The work is co-authored by Sandia post-doctorate Mike Elioff and James Valentini of Columbia University.

The main method used to achieve atomic ultra-cooling to the microKelvin temperature range -- the same preliminary cooling range as the Sandia technique -- makes use of laser beams that intersect at a point. An atom, possessing the appropriate absorption characteristics, passing through that point in effect stands still, like a kid in a dodgeball game struck from all sides with balls. Transfixed by pressure from the beams, the atom becomes almost motionless.

The problem in cooling molecules by the laser method is that while some atoms possess characteristics that can be harmonically matched by a laser frequency, like the same note played by two pianos, molecular energy frequencies are more complex. This complexity makes them unsuitable for this type of laser cooling.

This leaves the field open for other techniques to be developed for the preliminary cooling of molecules. Four or five other techniques, published recently, had some level of success at cooling molecules. The most successful method to date has been the welding of ultracold atoms together to make ultracold molecules.

An open field

"We only manage to cool one molecule in a million," says Chandler. "But -- inefficient or efficient -- we generate cold molecules. With some improvements, we hope to be able to make substantial numbers of them."

Molecules are cheap, he says, so getting one in a million cooling collisions out of a quadrillion total collisions per second the molecules undergo in the beams doesn't bother him.

This first-step method -- the only one to rely solely on the masses of the atoms and molecules involved -- could be useful in slowing down the speed of a variety of molecules sufficiently such that magnetic or electrical traps can be used to cool molecules further. Without prior slow-down, molecules would escape these relatively weak traps, like molecules of water rising from the surface of hot coffee.

Instruments in Chandler's lab, working at their resolution limit, show selected molecules in the intersecting beams slowing from 600 meters/second to 15 meters/second. The group's calculations indicate an average speed to be on the order of 4 meters/second. This average speed is equivalent to a temperature in the milliKelvin range, still several thousandths of a degree above the universe's absolute zero of -273 Celsius. The last 99 yards, so to speak, are the hardest: Bose-Einstein condensates exist in the nanoKelvin range, still six orders of magnitude colder.

The work, funded by DOE's Basic Energy Sciences, focuses on understanding how energy flows between molecules for a better understanding of heat transfer.

Technical Contact: Dave Chandler chand@sandia.gov, 925-294-3132

Media Contact: Neal Singer nsinger@sandia.gov, 505-845-7078