Optical centrifuge puts molecules in a spin

17 Jun 2002

Combining four femtosecond laser beams, scientists at Canada's National Research Council (NRC) have developed an "optical centrifuge" that spins molecules of chlorine gas around so fast that their molecular bonds snap.

In the unprecedented experiment, David Villeneuve and colleagues split a 5 mJ, 10 Hz Ti:sapphire beam at 800 nm into four pathways. Two of these beams were polarized in opposite directions, and then combined such that the overall polarization axis rotates. Chlorine molecules in the combined beam follow this rotating axis, and spin about the centre of their molecular bond.

Accelerated from 0 to 6 THz in 50 ps, the molecules reach rotational quantum levels of 400 or more, distorting the electronic energy within the bond and forcing the two atoms apart. Two diagnostic beams are fired into the chlorine gas cloud as well: a 400 nm pulse, delayed to arrive 200 ps after the centrifuge pulse, cleaves the weakened chlorine bond, and a 50 fs, 800 nm pulse delayed by 500 ps ionizes any chlorine atoms or molecules so that they can be detected.

When either of the two beams that generated the centrifuge was blocked, or when these beams were linearly rather than circularly polarized, no atomic fragments were seen.

Villeneuve told Opto and Laser Europe magazine that the technique could be used in bond-selective chemistry: "This technique could selectively break a strong bond in the middle of a molecule, even when surrounded by weaker bonds."

Other potential applications include using the spinning molecules as optical modulators. According to Villeneuve, the "front" and "rear" of a linearly-polarized probe laser pulse travelling along a column of coherently spinning molecules would "see" a different refractive index. The back-end of the pulse could then "catch-up" with the front-end, presenting a method for compressing pulses to a shorter duration time.

Another possibility is a novel excitation source. Spinning a molecule with a dipole moment - where each end has an opposing electrical charge - could provide an intense source of tunable multi-THz radiation.

The team is now looking for a molecule with a strong bond surrounded by weak ones to spin around and break up selectively, and to modify the scheme to generate very short laser pulses.

Story courtesy of Opto and Laser Europe magazine