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Electrons Receive an Optical Kick

Photonics Spectra
Nov 2001
Hank Hogan

Precisely timed bursts of light may help to explain the basics of chemistry. A research team at the University of California in Los Angeles has demonstrated that femtosecond laser pulses can temporarily, and in a controlled way, free electrons. Examining how the electrons recombine with nearby atoms will lead to a better understanding of electron transfer from donor to acceptor atoms, a fundamental aspect of liquid chemical reactions.

Benjamin J. Schwartz, a professor of chemistry at the university and a member of the team, said the researchers hope that a refinement of the femtosecond technique will allow them to duplicate in practice what theorists do in simulations. "We are trying to use the control we have over this system to place the electron at a controlled series of precise distances," he said.

The researchers used a Spectra-Physics Ti:sapphire laser to generate light of three wavelengths, with pulse widths of approximately 120 fs. A beam at 780 nm ionized negatively charged sodium atoms dissolved in tetrahydrofuran. When freed from the atom, Schwartz explained, the electrons have two distinct final resting spots: One is in the same solvent cavity as the sodium atom, and the other is in a more distant, but still nearby, solvent cavity.

In the first case, the electron will recombine with the sodium atom within 2 ps unless something stops it. To do that, the researchers used a second femtosecond pulse centered at 2000 nm. This applied a kick to the electrons and enabled them to wander. The researchers observed this by monitoring the atom-electron recombination rate, applying a third pulse at 490 nm to probe for negatively charged sodium atoms.

Controlling recombination

The results indicate that the technique produces electrons and moderates their recombination. By applying the 2000-nm pulse to the more widely separated electron-atom contact pairs, the researchers accelerated recombination, while they hindered it by applying the pulse to the immediate contact pairs.

Further research is under way, using different wavelengths for the probe, in an attempt to better monitor electron-atom spacing. "Our hope is that this will uncover a lot of the details as to how solvent molecules control electron-transfer reactions," Schwartz said.


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