University of Rochester

Laser Spotlight Interrogates Individual Molecules

October 4, 1994

Scientists are developing a new type of spectroscopy to look at the molecular world in a way no one has done before.

They're studying how a single molecule absorbs and emits light by isolating it with a laser for seconds or even minutes at a time, pumping it full of energy, and analyzing the re-emitted light. These molecular signals provide a new-found window into molecular neighborhoods, since different molecules of the same material absorb and emit light differently depending on location. Such work should give scientists a better understanding of a material's properties.

"Measuring the spectra of single molecules is like polling each individual voter," says Anne Myers, associate professor of chemistry at the University of Rochester. "With traditional spectroscopy you get one overall result, with contributions from thousands of molecules, but with this method you know the exact behavior of each individual.

"The very fact that a single molecule can send out a measurable signal is remarkable."

Myers will describe her work on single-molecule spectroscopy, done in collaboration with scientists at IBM's Almaden Research Center in San Jose, Calif., at an invited talk at the annual meeting of the Optical Society of America Friday, Oct. 7 in Dallas.

To use the technique, scientists use liquid helium to cool to near absolute zero an ultra-thin sample of a mixed crystal or polymer. The material includes only a few thousand molecules that absorb and emit visible light, held in place by a scaffolding of molecules that do not react to light.

A tightly focused laser with a beam only a few microns wide illuminates a small portion of the material, and scientists "sweep" the laser, changing the color of light it emits very slightly (fractions of a nanometer). When the laser light matches the light that a particular light-sensitive molecule absorbs, that molecule can become excited and emit photons, which are detected by a sensitive photodetector.

"It's like tuning your radio," says Myers. "When you're driving far from a large city, you get large regions of static, punctuated by a few stations that cut through the darkness. Here you slowly tune the laser and see nothing, and then suddenly a molecule will begin to resonate."

Usually scientists cannot observe a particular molecule because its atoms are moving about, and even the tiniest vibrations cause the type of light the molecules absorb to change very rapidly. But by selecting the material carefully and cooling to very low temperatures, scientists can find some molecules whose absorption frequencies hold still for several seconds -- long enough to emit thousands of photons. A few stay put for several minutes, even up to half an hour, under laser probing.

"This allows you to look at one molecule for a prolonged period of time -- to interrogate it for as long as possible before moving on to another," says Myers.

The technique was developed by W.E. Moerner of IBM, with whom Myers worked during a sabbatical last year. While Moerner is trying to generate images with single-molecule resolution and to develop single-molecule optical "switches," Myers is studying the characteristics of the emitted light for clues about a material.

There are a variety of techniques to look at molecules and atoms in great detail, but few actually send in signals and analyze responses to learn about the effect of a particular molecule's submicroscopic location in a solid or liquid.

In one recent set of experiments with polymers, Myers found that the spectra of molecules split into two distinct groups. The distinction suggests a fundamental difference, she says, perhaps distinguishing molecules found in an amorphous portion of the material from crystalline molecules. Eventually, understanding the precise consequences of molecular arrangements may help scientists develop better materials.

Myers hopes to apply the technique to proteins, polymers that change shape extraordinarily quickly by shifting their energy. Such molecular motion underlies many biochemical processes basic to life, such as oxygen transport.

A review article on the technique by Myers, Moerner, Paul Tch‚nio and Marek Zgierski will appear in an upcoming issue of the Journal of Physical Chemistry. This work was supported by IBM and the Packard Foundation. tr