# Synopsis: Potassium atoms feel a distant attraction

The atoms in highly excited vibrational states of a diatomic molecule can be quite far apart near their maximum excursion. Physicists are now using laser spectroscopy to carefully measure the long-range effective interaction between potassium atoms in these states—an essential parameter to understanding ultracold atomic collisions.

To understand ultracold collisions between neutral atoms it is necessary to have accurate knowledge of the interatomic interaction potentials at extremely long range, out to many times the atomic radius. One approach is to study diatomic molecules in highly excited vibrational states, since there is a high probability of finding the atoms at fairly large distances from one another as they reach their maximum excursion. However, state-of-the-art techniques of laser spectroscopy must be used to excite and measure these states.

In the July 11th issue of Physical Review A, Stephen Falke and coworkers at the Leibniz Universität Hannover and Christian Lisdat at the Physikalisch-Technische Bundesanstalt have measured, with MHz precision, the energy gaps between several asymptotically high vibrational levels in the electronic ground state of the ${}^{39}{\text{K}}_{2}$ molecule. By combining this data with earlier measurements, the authors can fit new ground-state potential functions and calculate accurate scattering lengths for each of the isotopic combinations of ${}^{39}\text{K}$, ${}^{40}\text{K}$, and ${}^{41}\text{K}$ atoms. Their findings are sufficiently accurate that they can observe small deviations from the Born-Oppenheimer (adiabatic) approximation that is conventionally used to describe neutral atom collisions.

The exquisitely detailed experimental and analytical work illustrates the influence of advances in atomic and molecular physics that have been highlighted in several recent Nobel Prizes, including femtosecond frequency combs, iodine-stabilized lasers, and quantum degenerate gases. It will also contribute to continuing progress in molecular Bose-Einstein condensates and degenerate Fermi gases, which is opening new vistas in molecular and chemical physics. - Keith MacAdam

### Announcements

More Announcements »

## Subject Areas

Atomic and Molecular Physics

## Previous Synopsis

Quantum Information

Mesoscopics

## Related Articles

Atomic and Molecular Physics

### Synopsis: Losing Light in a BEC

The index of refraction in a gas of bosons is enhanced relative to its value in a classical gas, a predicted quantum effect that has now been observed in ultracold sodium atoms. Read More »

Atomic and Molecular Physics

### Focus: Atoms As Thermometers

A small number of atoms in repeated trials can accurately measure the temperature of an ultracold gas cloud—a step toward measuring temperature on the micrometer scale. Read More »

Optics

### Synopsis: Bad Cavities for Precise Lasers

The frequency of a laser based on trapped ultracold atoms can be made insensitive to fluctuations in the laser cavity’s length. Read More »