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Cracking the Collision Code

Published: 02-25-2006

When molecules smash into each other, things happen on the quantum level. Electrons get shoved around. They may even jump from one atom to another. Spin directions can change. A chemical reaction may even take place. Since it's not possible to directly observe this kind of electron behavior, scientists want to be able to probe it with novel spectroscopies. Now, thanks to a recent theoretical study, ultracold spectroscopy looks particularly promising for elucidating electron behavior during molecular impacts.

Fellow John Bohn and Chris Ticknor, who received his Ph.D. in physics from CU in 2005, performed the analysis. Their study shows that whatever happens during ultracold molecular collisions gets imprinted on the collision spectra because how hard molecules hit each other depends upon the electric field. Bohn says the challenge is to determine what the resulting spectra mean in terms of what is actually occurring between the colliding molecules.

"We need a quantum Rosetta Stone to help us decipher these spectra," Bohn explained, noting that scientists already understand a lot about the long-range interactions of ultracold molecules and something about their short-range interactions. Bohn said the new work is equivalent to decoding the letter "A" in an as-yet-unknown language that describes how (and why) ultracold short-range interactions produce specific spectral patterns as a function of changes in the electric field. Researchers still need to decipher the rest of the alphabet and learn how the letters combine to form words. And, as occurred with the decoding of the three languages on the original Rosetta Stone, they need to understand the connection of the new language with what's already known about the quantum languages of long- and short-range interactions of ultracold molecules.

Ultracold molecules don't move or vibrate much when temperatures hover near absolute zero. This characteristic of having a fixed energy is the key to the spectroscopy explored by Bohn and Ticknor. In analyzing changes in the spectra of ultracold molecules as a function of changing electric field, they specifically looked at what happens to molecular collisions. To their surprise, the field had a big influence on how hard the molecules hit each other.

The researchers were able to build a simple model that explained the appearance of a somewhat regular series of peaks in the collision spectra. The peaks were well correlated with different values of the electric field. Bohn said these characteristic patterns in the spectra of ultracold collisions are the first step in deciphering a new language of short-range interactions. More detailed patterns almost certainly contain volumes of information about the quantum behavior of ultracold collisions-if only physicists understood what they are saying.


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