Laser spotlights molecular choreography.

Laser spotlights molecular choreography

Molecules pack more dancing action into 1 second than the cast of "A Chorus Line" could muster in a century's worth of performances. In less time than it takes to say "off-off-Broadway," a two-atom molecule such as iodine can perform about 10 billion rotations. Moreover, during each high-speed somersault, the atomic partners oscillate toward and away from each other about 1,000 times.

But even these dance steps aren't too fast for the scientific eyes of three physical chemists of the California Institute of Technology in Pasadena. Using some of the world's fastest laser pulses as though they were flash bulbs for freeze-framing molecular motions, Ahmed H. Zewail, Marcos Dantus and Robert M. Bowman have captured in detail the individual rotations and vibrations of iodine molecules. The studies provide basic experimental data to check the accuracy of theoretical pictures of molecular motions, which rely on quantum mechanical calculations.

The laser observations work like photography, notes chemist Ian W.M. Smith of the University of Birmingham in England, whose commentary accompanies the researchers' latest report in the Feb. 22 NATURE. Just as a photographer has to use a shutter speed of one-thousandth of a second or less to snap clear pictures of a sprinter moving at 10 meters per second, chemists seeking clear "pictures" of fleeting molecular rotations and oscillations must use light pulses that are shorter than the time required for the motions, Smith explains.

Zewail's group uses laser pulses that last about 10 femtoseconds, or 10 quadrillionths of a second. A femtosecond is to a second as a second is to roughly 32 million years.

Since 1987, Zewail has used femtosecond laser observations to observe simple reactions in which a chemical bond breaks (SN: 9/24/88, p.203). The new work represents the first observations of atomic motions in bound systems like iodine molecules. Since so much of the natural arena involves dancing molecules, "to be able to see molecular motions as they happen is as fundamental as it gets," Zewail told SCIENCE NEWS. Practical applications aren't even on a back burner.

"It's the first time we've ever been able to see that kind of thing," adds chemist James L. Kinsey of Rice University in Houston. "I think it will influence the way people think about these processes for a long time."

To observe and monitor molecular vibrations, the Caltech researchers first use a "pump pulse" to inject energy into the molecules. Then these primed pairs of atoms in vibrating molecules can absorb a second pulse, called a probe pulse, lasting several femtoseconds. After soaking up this extra energy, they fluoresce (emit photons) as they relax to lower energy states. By monitoring the laser-induced fluorescence over picoseconds of time (trillionths of seconds), the researchers can assemble a detailed picture of the molecules' simple or complicated vibrations.

Even though molecular rotations are a thousand times slower than the vibrations, observing them requires more finesse. To get fluorescence signals clean enough for reliable interpretation, the first laser pulse must both excite the molecules and align their spins in a common spatial plane. To do this, the researchers use polarized laser pulses, which energize only those molecules whose axes happen to coincide with the angle of polarization. Jolting the molecules with the second laser pulse yields a fluorescence pattern that reveals, among other things, the number of distinct rotational speeds present in the population of aligned spinning molecules.

Zewail predicts the development of even speedier laser pulses in the future, and notes that sub-femtosecond observations may reveal yet another realm of molecular behavior. Nonetheless, he says, touring chemical reactions and molecular motions with femtosecond pulses should prove a fruitful pastime for many years to come.