Focus: Shooting an X-ray Movie
Atoms and molecules don’t waste a lot of time getting together, so to watch a chemical reaction you have to be fast. Really fast. An experiment appearing in the 9 July PRL uses some of the world’s shortest pulses of x-ray light to watch an oxygen molecule flip on a platinum surface. The result is the first super-slow-motion picture of a chemical reaction on a surface, and it should allow theorists to determine the precise mechanism of the reaction. Surface-based reactions are important in industry, and similar techniques may someday allow precise control over chemical reactions in everything from refining to pharmaceuticals.
Until recently, chemists have had a “before” and “after” picture of chemical reactions: Reactants come before; products come after. But the precise way in which reactants create products is still poorly understood because chemical reactions happen in a fraction of a picosecond (a trillionth of a second). Only in recent years, with the advent of ultrashort-pulsed lasers, have physicists and chemists observed chemical reactions as they unfold.
Now a team at the University of Colorado in Boulder and the University of Michigan in Ann Arbor reports for the first time watching a complete chemical reaction unfold on a surface. Initially, an oxygen molecule lies flat on a platinum surface, bound by a single electron. When an infrared laser pulse heats the platinum, a second electron jumps to the oxygen and causes the molecule to rotate horizontally.
The team was able to watch the flip using a series of 10-femtosecond x-ray pulses, which ejected electrons from a layer of oxygen molecules on the surface. A peak in the energy spectra of these electrons grew with time as the molecules rotated to their final state. The researchers believe the height of the peak corresponds to the precise position of the oxygen molecule during the transition.
But William Gadzuk of the National Institute of Standards and Technology in Gaithersburg, MD, remains unconvinced by the team’s results. “Their story [of flipping Oxygen] is consistent with the data, but the data are far from proving the story,” he says. Still, Gadzuk believes that the team’s two-pulse technique is “revolutionary.” Margaret Murnane of Colorado, one of the team leaders, agrees. “I think this will be the first experiment in an emerging field where soft x-rays will be used to probe reactions,” she says.