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Infrared Light Probes Hydrogen Under Extreme Pressure

The two-atom hydrogen molecule is perhaps the simplest molecule in existence, but, surprisingly, scientists still don’t know some basic information about it, such as how it responds to very high pressures. This is particularly true for solid hydrogen, where, for varying densities and at very low temperatures, scientists have not been able to conduct experiments that would verify theory.

a hydrogen sample while being squeezed in the diamond anvil cell

A photo taken of a hydrogen sample while being squeezed in the diamond anvil cell. The sample was at a pressure of 360 GPa and room temperature.

At Brookhaven National Laboratory’s National Synchrotron Light Source (NSLS), researchers from the Geophysical Laboratory at the Carnegie Institution of Washington recently filled in some of these gaps using mainly infrared light, gathering new information about solid hydrogen’s stability and electronic properties when subjected to high pressures and a range of temperatures. As described in a paper appearing in the April 6 online edition of Physical Review Letters, they show that hydrogen molecules remain intact to remarkably high pressures at room temperature and below. The results also show that hydrogen does not become a metal under those conditions as reported in a paper last year.

“It’s very exciting to reach more extreme static pressures and temperatures on hydrogen and accurately probe the material under these conditions,” said Russell Hemley, director of the Geophysical Laboratory and a co-author on the paper. “The synchrotron radiation at NSLS provides the kind of infrared source that is essential for measuring small samples at very high pressures.”

The group applied pressure using diamond anvil cells, which squeeze a small sample between the cut faces of two diamonds. They were interested in what happens at pressures above 200 gigapascals (GPa), or about two million times atmospheric pressure, but particularly the 300-400 GPa range. They subjected their sample to temperatures ranging widely from 12 degrees Kelvin (K) to close to room temperature.

The hydrogen molecules remained intact to pressures well above 300 GPa and over the entire temperature range. The group learned this by measuring the “vibron,” the stretching vibration between the two atoms of the molecule, which is a key indicator of the bonding state and the material’s phase transformations (transitions to new structural and electronic states). At the highest pressures, the hydrogen absorbed the infrared light strongly at the frequency of the vibron, indicating that the molecule remained stable.

These measurements also yielded information about the hydrogen’s electronic behavior under the highest pressures, where it enters a state known as “phase III.” (Beginning at about 100 GPa, hydrogen progresses through different molecular phases. The phase III transition starts at about 150 GPa). The researchers used that data to calculate the maximum possible carrier density – the electrons that would contribute to conductivity – and found that it was well below that of a metal. However, phase III could be semimetallic.

This study is also important in that it utilizes improved high-pressure experimental techniques, allowing the researchers to take measurements that had not been possible before because they were technically very difficult. For example, hydrogen can react with components in the sample chamber, decreasing the sample size, and at high pressures can “attack” the diamond, requiring coatings that can compromise measurements. And very high pressures stress the diamond anvil, which, when coupled with exposure to the lasers typically used to probe the sample, can cause the diamonds to fail.

Hemley and his group largely avoided these issues by using very low-energy optical lasers and, at the highest pressures only, infrared light at NSLS beamline U2A to probe the material. They performed 20 experiments in total.

Laura Mgrdichian

2012-3072  INT/EXT  |  Media & Communications Office

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