Laser Yields Real-Time Glimpse of Water Molecules

Daniel Woolls

Using a homemade ultrafast laser, researchers have tinkered with liquid water molecules to achieve real-time observation of atoms flirting. Teams from the Centre National de la Recherche Scientifique -- one at the Université Pierre et Marie Curie and the other at the École Polytechnique in Palaiseau -- say their feat is unprecedented.

Until now, the life and death of molecules could be observed only in their gas phase. But the French scientists say they've pulled it off in a liquid, so researchers can view the variations of molecular geometry during a chemical reaction.

The technique involves the laser equivalent of a one-two punch on a femtosecond scale. Water molecules at room temperature are first hit with a pump pulse to excite their bonds, then zapped with a weaker probe pulse to monitor the bonds as they ease back toward equilibrium. Both pulses are in the infrared range, from 2.5 to 4.5 µm.

A key factor in all of this is a force called hydrogen bonding, a fleeting interaction between an electropositive OH group of one water molecule and the electronegative oxygen atom of a neighboring one.

Molecular movies

The stronger this bond, the weaker the internal bond of the OH group and the lower its rate of vibration. It is the latter bond that is targeted with the first laser pulse, which causes the bond to vibrate at a selected frequency. This alters the bond length between that group and the oxygen atom of a neighboring molecule. The second laser pulse follows this bond as it settles back to equilibrium.

By changing the vibration of the OH group, the scientists manipulate the hydrogen bonding and monitor the rate of change in real time. "This method permits us to 'see' the variations of the OH-O bond lengths directly, like on a movie screen," said Savo Bratos, one of the researchers.

The teams designed and constructed a laser to accommodate the rapid chemical processes, Bratos said, because lasers capable of firing 10-fs pulses were available in the visible range but not in infrared. "So we built our sources ourselves," he said. "It took us three years to do it. We had contacts with outside engineers, but only occasionally."

On the downside, he said it is risky to extrapolate molecular geometry from time-dependent laser spectra. Time-resolved x-ray diffraction reveals molecular structures, but not yet for intervals faster than 1 ns.