For Frisbee players, finessing the tilt of the spinning disc is crucial for directing it into a pair of waiting hands. Prof. Ilya Averbukh
and research student Erez Gershnabel of the Chemical Physics Department in the Faculty of Chemistry applied a similar principle to rotating molecules moving in electric, magnetic and laser fields, and their findings
could open up a number of applications in nanotechnology, optics, chemistry and scientific research.
In the normal state of affairs, a game involving atoms or molecules placed in spatially uniform fields would be pretty boring: neutral particles don’t feel any force when a homogeneous electric, magnetic or laser field is applied to them. Whereas a charged particle, such as an electron, will be accelerated by the fields, an atom or a molecule, which has a net charge of zero, will stay at rest or maintain a constant velocity. It’s not that the neutral particles are completely insensitive to the surrounding field; it’s that they turn polarized and their electric charges become separated – positive charges move to one side of the particle, negative to the other. As a result, the forces acting on the charges cancel each other, and the atoms and molecules feel no thrust.
Scientists had made some progress in bringing neutral atoms into motion by creating an inhomogeneous field. In such a field, the force is stronger on one side of the polarized atom than on the other, and the atoms move because the stronger force takes over. Most atoms resemble a round soccer ball – they can be equally polarized in any direction. But even the simplest molecules such as hydrogen are not spherical – if anything, they’re shaped more like dumbbells. Polarized molecules separate their charges at either end of the “dumbbell,” and a molecule positioned at right angles to the field would be affected differently from one that’s parallel. Averbukh and Gershnabel realized that, as in a game of Frisbee, orientation and spinning must both come into play when trying to move molecules.
The researchers showed how it should be possible to get the molecules spinning around any desired axis by giving them precise “kicks” with very short laser pulses. In an inhomogeneous field, these molecules will feel a force that depends on the orientation of the rotation axis.
“The spinning, polarizable molecules behave like tiny gyroscopes,” says Averbukh. “Once we control the orientation of their rotation axes, we can direct them to exactly where we want them to go. Our study considered lasers for the source of the field, but the same principle can apply to static electric and magnetic fields as well.”
The possibilities for applications are numerous, and various scientific groups have already shown interest in the method, including a group in Canada that has begun planning a related experiment. Molecular optics, for instance, designed along the general lines of electron microscopy and atom optics, could open the door to new kinds of imaging. Researchers working in various nanotechnology fields might use this method to focus beams of spinning molecules on specific targets or to gain precise control over molecules that are being deposited on a surface. Others might use it to separate specific molecules out of a mixture, or to help trap selected molecular species.
Prof. Ilya Averbukh is the incumbent of the Patricia Elman Bildner Professorial Chair of Solid State Chemistry.