Molecules get More Classical at High Pressures

A new study of molecules being squeezed in a diamond anvil cell shows that as the pressure goes up, the force between atoms in a two-atom molecule behaves more and more like the classic Hooke's law, according to which the force between two objects connected by an elastic spring is proportional to the contraction or extension of the spring.

Two scientists, Alexander Goncharov at the Carnegie Institution of Washington, and Jonathan Crowhurst at Lawrence Livermore National Laboratory, have loaded several species of molecule, such as H₂, D₂, and N₂, into their cell and then observed what happened at high temperature and high pressure. By varying these two parameters, the molecular sample can often be transformed from a fluid into a crystal or back again, or the molecules themselves might even be broken apart.

The researchers (contact Goncharov at goncharov@gl.ciw.edu) first heated the samples using a near-infrared laser and then probed the various excited vibrational quantum states using the technique of Raman spectroscopy. By carefully noting the frequency and linewidths of these stretching modes, they could deduce the energetics of the binding between the atoms even as the molecule was being subject to the extreme conditions.

The findings, such as the realization that the binding becomes more like a classical harmonic oscillator at high pressure, should aid in such pursuits as the quest to observe metallic hydrogen.

Goncharov and Crowhurst, Physical Review Letters, 10 February 2006
Contact Alexander Goncharov at goncharov@gl.ciw.edu