Very Cool Chemistry

The first indication that chemical reactions can occur at nano-Kelvin temperatures has raised the tantalizing possibility of a new realm of quantum-controlled, ultracold chemistry (Science 2010, 327, 853).

“Being able to carry out chemical reactions at virtually absolute zero is a completely new regime for chemistry and chemical reactions,” Jeremy M. Hutson, a chemistry professor at the University of Durham, in England, says about the new work. “It’s a fabulous demonstration of how quantum mechanics can be manifested at the most detailed level.”

The experiments involved using laser techniques to create an ultracold, dense gas of KRb molecules in their ground state. Under such circumstances, one might think that chemistry would turn off, says Deborah S. Jin, a researcher at JILA, a precision physics lab run jointly by the National Institute of Standards & Technology and the University of Colorado, Boulder. Jin led the work with colleague Jun Ye.

In fact, chemistry still happens, although understanding it requires a decidedly different view of reactivity, Jin says. Gone is the classical picture of a collision between two billiard balls, with the ensuing interaction of electrons. Instead, quantum mechanics takes over, and the molecules must be thought of as waves. Whether and how those waves interact determines chemical reactivity. Consequently, molecules can react even if they’re barely moving and are at a distance from each other.

In one example, Jin, Ye, and colleagues prepared KRb molecules with identical electronic, vibrational, rotational, and nuclear spin states at 250 × 10^–9 K. In that situation the molecules are barely mobile and won’t react because the Pauli principle forbids them from interacting. When the researchers flipped the orientation of the nuclear spins of some molecules so the waves of the molecules could overlap, however, pairs of KRb molecules with different nuclear spins reacted to produce K₂ and Rb₂, even when the original molecules were as far as 1 μm apart.

The ability to create ultracold gases is currently limited primarily to alkali and alkaline earth metals, which have a limited chemistry, Hutson says. Researchers are developing new methods to cool a wider range of molecules, which would further open the possibilities of precisely controlled chemistry and understanding the role of quantum mechanics in reactivity.