Quantum Break Up...

On the left is experimental data showing the distribution of strontium atoms after dissociation has occurred. The centre panel shows the quantum-chemistry simulation of the distribution, and the panel on the right is the quasi-classical model. (Courtesy: M McDonald et al./Nature)

Topics: Chemistry, Heisenberg Uncertainty Principle, Quantum Mechanics

A new study of how light causes diatomic molecules to break apart has revealed significant flaws in the traditional theory describing the photodissociation process. The work has been carried out by physicists and chemists in the US and Poland, and suggests that the dissociation of molecules prepared in pure quantum states is best described by a recently developed quantum-chemistry model. As well as providing further insights into the quantum nature of molecules, the experimental technique could form the basis of a new source of entangled atoms for matter-wave experiments.

Photodissociation occurs when a molecule is blown apart by absorption of a photon, and it has long been used to study the physics and chemistry of molecules. The process usually involves the electric-dipole moment of the molecule coupling to the oscillating electromagnetic field of the photon – although symmetry considerations forbid this interaction in some situations.

The process is usually studied by creating an ultracold, supersonic molecular beam that is irradiated with light from a pulsed dye laser. However, the minimum achievable temperature of such a molecular beam is too high to allow molecular ensembles to be prepared in pure quantum states before dissociation. Instead, what is observed is the average of the dissociation patterns of multiple quantum states. These observations are described very well by the quasi-classical model for electric-dipole dissociation that was developed in the 1960s by Richard Zare and Dudley Hershbach of the University of California, Berkeley, in 1963. Hershbach shared the 1986 Nobel Prize for Chemistry for his work on molecular beams.

Physics World: Molecules break up under quantum control, Tim Wogan