October 07, 2010

Polarized Laser Pulses could improve control of molecules enabling more efficient isotope separation and molecular nanofabrication

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Arxiv - Molecular Frisbee: Motion of Spinning Molecules in Inhomogeneous Fields

MIT Technology Review notes that this new technique for controlling the trajectory of spinning molecules with two polarized laser pulses could make isotope separation even easier and more efficient. This technique also could be used for improving molecular nanofabrication.

Several laser techniques have been suggested and demonstrated recently for preparing polarizable molecules in rapidly spinning states with a disc-like angular distribution. We consider motion of these spinning discs in inhomogeneous fields, and show that the molecular trajectories may be precisely controlled by the tilt of the plane of the laser-induced rotation. The feasibility of the scheme is illustrated by optical deflection of linear molecules twirled by two delayed cross-polarized laser pulses. These results open new ways for many applications involving molecular focusing, guiding and trapping, and may be suitable for separating molecular mixtures by optical and static fields.

In recent years, a number of techniques have emerged to set molecules in a gas spinning with their axes precisely aligned, like a three dimensional array of floating tops. These techniques all zap the molecules with a carefully prepared laser pulse to make them rotate in a certain way.

They fire the spinning molecules through an electric field produced by another laser. Provided the field has some intensity gradient, it will play a role analogous to air in frisbee flight. When that happens, the inclination of the spinning molecules will determine the trajectory they take.

This frisbee technique gives remarkable control over the path the molecules take. The trajectory depends on factors such as the strength of the field, the inclination of rotation and the mass of the molecule.

This has important implications for a number of emerging techniques, particularly in areas where ionisation cannot be used. For example, molecular nanofabrication in which tiny structures are built almost brick by brick must use neutral molecules because the build up of charge could distort the shape or even prevent construction entirely.

But perhaps the most important application, at least in the short term, will be isotope separation. Since the trajectory depends on the mass of the molecule, the technique will naturally separate molecules containing different isotopes.

Nuclear scientists will want to investigate this technique's potential for separating the more fissionable uranium 235 from uranium 238. In recent years, physicists have made great strides in separating these isotopes using lasers to selectively ionise one isotope while leaving the other neutral, which allows them to be separated using an electric field.

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