Shorter and Faster

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Like top athletes, leading scientists continually test the limits of human ability. While an athlete trains to break the 100-meter sprint record, for instance, a scientist might be found attempting to break the record for catching sight of the fastest movements known, such as those of the electrons moving inside molecules and atoms. For years scientists had referred to these movements as instantaneous. Are they truly instantaneous? Or has the available scientific equipment simply not been advanced enough to detect electrons in the act of getting from one place to another? By pushing the bounds of laser optic technology, physicists today are beginning to reveal electron movements taking place within unimaginably short timeframes.

 

Ultrafast pulses of laser light replace camera shutters when filming the very fastest activities; for instance, chemical reactions in which molecules break apart into smaller molecules. Like the fast-shutter cameras used to capture still shots of a basketball player in mid-dunk, the flashes of laser light serve to “freeze” molecular actions at various stages.

 

The faster the natural process, the shorter and faster the laser pulses must be to capture it on film. For a number of years, the fastest lasers available produced flashes down to the length of a femto-second – a millionth of a billionth of a second. This is rapid enough to observe many molecular processes. But, the movements of the electrons in a molecule are faster still and the technology was not up to capturing them.

 

“The limits on the capabilities of lasers stem from a basic, physical restriction,” says Dr. Nirit Dudovich, who joined the Weizmann Institute’s Physics of Complex Systems Department two years ago. “The single period of a light wave determines the lower limit for the duration of a laser pulse. A femtosecond pulse is the shortest that one can get from wavelengths in the visible range.”

 

Unwilling to stop pushing the physical boundaries, scientists have found a way to overcome this. An experiment with this method begins with powerful laser pulses lasting just a few femtoseconds aimed at a molecule, “tearing” out one of its electrons – a quantum phenomenon called tunneling. The electron, freed from its orbit around the molecule, takes a short “trip.” This escape is shortlived, and the electron soon returns to its home molecule. As the electron slips back into place, a photon (light particle) is emitted. The entire process evolves during less than one optical cycle of visible light; the duration of the emitted light is around an attosecond (a billionth of a billionth of a second).

 

This ability to capture the movements of electrons circling atoms or molecules in “freeze frames” has ushered in a new field of research. But Dudovich has taken that research a step further: “Why use lasers to trace other molecules, I thought, when so much could be learned about the process through the interaction of light with the molecules on which the laser itself is based?” In other words, she’s turned the camera back on itself to image the basic process in which photons are emitted and electrons change position.

 

Dudovich and her research team have been exploring ways to control the direction of the tunneling electrons so as to get them to reenter the molecule or atom from different angles. Like medical imaging that builds a three-dimensional picture from a series of “slices,” such an approach could give researchers an unprecedented view of molecules and molecular processes. In a paper that recently appeared in Nature Physics, the researchers demonstrated that by changing the polarization of the beam used to tear the electron from the atom, one can reroute the electron’s path and direct the angle of its reentry.

 

With this method, Dudovich and her team succeeded in characterizing the distribution of electrons in an atom. “In the future,” she says, “our goal is to measure the time, as well, and to integrate time with location. In other words, we’ll be able to produce a sort of movie that records electrons moving through various processes and chemical reactions as they take place.”