Subtle quantum effects in the motion of molecules across a metal surface have been detected by Stephen Jenkins and postdoc Marco Sacchi, in collaboration with groups at the Cavendish and at Rutgers University in the US.
When pyrrole molecules are deposited onto a metal surface, they nestle into the hollows between the metal atoms and lie flat. The molecules hop from one site to another, with the rate determined by the amount of energy required to loosen the bond between the molecule and the surface enough to allow it to move sideways.
Experimental results from the Rutgers/Cavendish team had showed that when two coherent beams of helium atoms are fired at the surface with a delay of a few picoseconds, the frequency of the molecules hopping between sites could be determined from the interference between the beams, and therefore the height of the energy activation barrier.
However, much to their surprise, the density functional theory calculations carried out by Stephen and Marco didn’t match up with the experimental observations, as they had for earlier studies on cyclopentadienyl ions. This was something of a mystery, until Marco came up with the idea that perhaps zero-point energy was involved.
This esoteric quantum phenomenon is linked to the fact that we are unable to make definitive pronouncements about the state of an object, including whether or not it is standing still. In classical mechanics, an object can slowly lose energy until it stops moving, but in quantum mechanics this is impossible as it only loses energy in discrete packets, or quanta, and when the last has gone it will still retain some residual vibrational energy.
This is known as zero-point energy, and is so small it is often assumed not to have any effect on any measurable properties. While it can be observed when bonds are broken, the molecule–surface bonds are only weakened in this case, and the vibrations of the molecule relative to the surface do not change much. ‘We always knew the biggest contribution to the molecule’s zero-point energy would come from the residual motion of its hydrogen atoms,’ Marco says. ‘They are the lightest atoms, and thus most sensitive to quantum effects. But would that contribution be constant, or would it change as the molecule moved across the surface?’
It turns out that the bending of the molecule as it moves from one site to another causes the zero-point energy to change. Including this in the calculations gives a much better correlation with the experimental results. ‘This is unlikely to be limited to pyrrole,’ Stephen says. ‘It may happen whenever a molecule with significant zero-point energy moves across a surface with relatively low activation barriers. It may have important consequences for various applications, from catalysts to electronic and optical devices.’
B.A.J. Lechner, H. Hedgeland, J. Ellis, W. Allison, M. Sacchi, S.J. Jenkins and B.J. Hinch, Angew. Chem. 2013, article first published online: 9 Apr 2013, doi: 10.1002/anie.201302289