An ingenious method of taking snapshots of chemical reactions--with a "shutter speed" of a thousandth of a trillionth of a second--has won the 1999 Nobel Prize in Chemistry for California scientist Ahmed H. Zewail.

The physics prize, also announced in Stockholm yesterday, went to Gerardus 't Hooft and Martinus J.G. Veltman, Dutch theorists whose work in the 1970s helped make possible some of the most spectacular discoveries in modern physics, including the 1995 observation of the long-sought "top" quark.

The Royal Swedish Academy of Sciences awarded the $960,000 chemistry prize to Zewail, 53, of California Institute of Technology in Pasadena, for devising a laser technique in the 1980s that allows chemists to witness how individual atoms in molecules rearrange themselves step by step as chemical bonds break and re-form during even the most rapid chemical events.

The innovation, the Nobel announcement said, has "brought about a revolution in chemistry," including advances in understanding catalysts (compounds that prompt reactions), polymer formation, the workings of miniature electronic components and the details of such biological processes as photosynthesis and animal vision.

It literally sheds new light on many events that were previously invisible--such as which of several bonds breaks first in a reaction--as well as revealing unexpected intermediate products that arise briefly during the middle stages of a reaction.

The method was rapidly and widely adopted. It "takes us to the ultimate level of observation," said Ed Wasserman, president of the American Chemical Society. "These glimpses of chemistry are not only incredibly fast, but may allow the more precise design of new materials and new drugs."

The Egyptian-born Zewail's "femtosecond spectroscopy" involves two pulses of high-speed laser light. (A femtosecond is one quadrillionth of a second.) The first excites the molecules that are the ingredients of a reaction; a second, weaker, pulse shows how the molecules have altered since the first pulse struck. By changing the spacing between the pulses, chemists can "see" what takes place at each increment of time.

In the 1980s, Zewail and colleagues began working with lasers that had much longer pulses, in the range of a billionth or trillionth of a second. "I was really fundamentally interested in understanding how atoms and molecules behave as they meet each other and divorce each other," he said from Caltech yesterday. But reaction events take place on the same time scale as the heat vibrations of the molecules in the reacting compounds--about a quadrillionth of a second. "The breakthrough in 1987 came when [laser pulses] reached a time less than the vibrational period of molecule," Zewail said.

't Hooft, 53, and Veltman, 68, won the physics prize for tackling one of the most baffling problems in modern physics: how to make sensible and mathematically accurate predictions from an insight that won a Nobel 20 years ago.

Their eventual solution was a major milestone in the centuries-long effort of physicists to unify and simplify the seemingly random and disparate forces of the universe. In the 17th century, Isaac Newton showed that the force that caused an apple to fall from a tree was the same force--gravity--that kept planets in orbit around the sun. In the 1860s, James Clerk Maxwell demonstrated that electricity and magnetism were not different phenomena, but complementary manifestations of the same thing, now known as electromagnetism. Albert Einstein's theories of relativity further unified other ideas early in the 1900s.

By the mid-20th century, science recognized four fundamental forces of nature: gravity; electromagnetism; the "strong" force that binds the nuclei of atoms together; and the "weak" force that governs, among other things, how nuclei break down in radioactive substances.

In the 1960s, three physicists--Sheldon L. Glashow, Abdus Salam and Steven Weinberg--proposed yet another unification, arguing that electromagnetism and the weak force were two aspects of the same "electroweak" interaction.

The trio shared the 1979 Nobel Prize in Physics for their theory. But there was a problem: Nobody knew how to use their ideas to make concrete calculations or accurate predictions. When physicists plugged in numbers, the equations kept coughing up absurd results such as infinite quantities.

"Abdus Salam and I both offered the opinion, when we did our own work, that the infinities in the theory could be dealt with, and that the theory would turn out to make finite predictions," Weinberg said yesterday from his home near the University of Texas. "But we were not able to do that."

Shortly thereafter, Veltman and 't Hooft, then his student, attacked that depressingly intractable problem. Using a variety of mathematical techniques akin to rethinking how to rotate objects in space, 't Hooft published a paper in 1971 indicating how electroweak theory could yield specific predictions.

Even when the methods were still comparatively crude, they correctly predicted some new electroweak effects. By the 1980s, when Veltman had moved to the University of Michigan and 't Hooft remained at the University of Utrecht, their prediction of the masses of two particles carrying the electroweak force was confirmed by particle physicists. Soon the predictions were precise enough to predict the mass of the elusive "top" quark--the only still-unobserved specimen of the six kinds of quarks that make up all heavy matter such as protons and neutrons--at somewhere around 190 times the mass of the proton. That gave experimenters an idea of what to look for.

Finally, in 1995, physicists at Fermilab outside Chicago observed the "top" for the first time. Its mass was 175 times that of the proton. Within a decade, physicists will test an even more critical prediction when they search for a particle called the Higgs boson--a hypothetical entity that pervades the field that gives all particles their masses.

"Outside elementary particle physics itself," Weinberg said, "the biggest impact will be in cosmology. During the first fraction of a second after the Big Bang, processes that were going on involved just these kinds of forces." Electroweak theory may ultimately help explain why there is more matter than antimatter in the universe, among other fundamental questions. So although the new laureates' methods "have no practical significance, they do have cosmic significance," he said.

The `Femtosecond' Technique

Ahmed Zewail was awarded the Nobel Prize for using ultra-fast lasers to measure chemical reactions as they occur. His lasers measure the reactions in time units called femtoseconds.

1. A laser fires a burst of light.

2. Part of it becomes the "pump" pulse that excites the chemicals in the container to begin a reaction. The other part, delayed a few quadrillionths of a second by diverting it through a set of mirrors, is the "probe" pulse.

3. When the probe pulse strikes the reacting chemicals, they emit a flash of light that shows exactly what the atoms are doing at that instant.

SOURCE: Scientific American

CAPTION: Martinus J.G. Veltman talks to reporters at his home in the Netherlands after winning Nobel Prize

CAPTION: Gerardus 't Hooft, who shared physics prize with his former teacher, at a blackboard at Bologna University.

CAPTION: Ahmed H. Zewail, of the California Institute of Technology in Pasadena, gestures to a colleague after learning he had won the Nobel Prize in Chemistry.