Probing a trapped molecule's dynamics.

TM0037

An alien molecule lodged in a crystal lattice responds to the nudges of its neighbors as they shift in position or flip from one molecular configuration to another. By carefully monitoring the light given off by a single, trapped molecule, two researchers have made the first direct observations of these local motions within a crystal.

The single-molecule spectroscopic technique, pioneered by WE. Moerner and William P Ambrose of the IBM Almaden Research Center in San Jose, Calif., paves the way for detailed analyses of molecular dynamics in both crystals and glasses. "You can see new things - new physics," Moerner says. The IBM researchers describe their results in the Jan. 17 NATURE.

Moerner and Ambrose studied the light given off by single molecules of the organic compound pentacene embedded in the crystal lattice of another organic compound known as para-terphenyl. They used laser light tuned to a specific wavelength to excite certain pentacene molecules, which then fluoresced, giving off light at a particular wavelength. By using a crystal only I to 10 microns thick and cooled to 1.5 kelvins, and by keeping the laser beam tightly focused and selecting the appropriate exciting wavelength, the researchers could ensure they were dealing with only one pentacene molecule at a time.

"As we changed the laser [wavelength], we could pick out different, single molecules in the same volume," Moerner says.

He and Ambrose discovered that a pentacene molecule's fluorescence wavelength can shift abruptly by small amounts.

"We saw that [a molecule] would stay at a particular wavelength for a certain time period - seconds to minutes - and then jump to another wavelength nearby" Moerner says. "It would stay there for a while, then maybe jump back to the original or to a new wavelength."

Such jumps apparently result from local changes in the structures or positions of the molecules surrounding the impurity Because para-terphenyl molecules consist of three molecular rings linked by single bonds to form a string, one possible cause for the jumps may be sudden shifts in the central ring's tilt with respect to the two outer rings.

Conventional spectroscopic techniques, which collect and analyze light absorbed or emitted by large collections of molecules, would wash out such subtle effects. Only by focusing on single molecules can researchers begin to study what happens within a solid material on a microscopic level.