Breaking the strongest chemical bonds with laser shock compression

Scientists at Lawrence Livermore National Laboratory (LLNL) recently obtained highly accurate thermodynamic data on warm, dense nitrogen under extreme conditions that could lead to a better understanding of the interiors of celestial bodies such as white dwarfs and exoplanets.

The team, made up of researchers from the University of California, Berkeley and the University of Rochester, used an advanced technique that combines pre-compression in a diamond anvil cell and laser-guided shock compression at the Omega Laser Facility at the University of Rochester.

molecules of nitrogen- (N₂) make up 78% of the air we breathe. They are unique because the two nitrogen atoms Cafe₂ are bonded with a triple covalent bond, the strongest of all simple diatomic molecules. Nitrogen is also an important constituent of celestial bodies in the outer solar system and beyond. For example, ammonia (NH₃Storms are believed to occur on giant planets such as Jupiter, while the dwarf planet Pluto, Saturn’s icy moon Titan, and Neptune’s icy moon Triton have₂-rich atmospheres.

Previous studies using this powerful technique revealed experimental evidence for superionic water ice and helium rain in gaseousgiant planets† In the new study, the team performed shock experiments on precompressed molecular nitrogen fluid up to 800 GPa (~8 million atmospheres) of pressure.

They observed clear signatures for the completion of molecular dissociation near 70-100 GPa and 5-10 kK (thousands of Kelvin) and the onset of ionization for the outermost electrons above 400 GPa and 50 kK.

“It’s very exciting that we can use shock waves to break these molecules and understand how pressure and density cause changes in chemical bonding,” said LLNL physicist Yong-Jae Kim, lead author of a paper appearing in Physical Assessment Letters† “Studying how to break nitrogen molecules and release electrons is a great test for the most advanced computer simulations and theoretical modeling.”

The team also theorized that studying nitrogen could help unravel some of the mysteries surrounding the behavior of hydrogen molecules in the early stage of inertial confinement fusion implosions at the National Ignition Facility.

“While nitrogen and hydrogen are both light diatomic molecules, hydrogen atoms are so small that reproducing their behavior under extreme pressure and temperature with computer simulations is very complex,” Kim said.

The team made the comparison between the experimental data in the new study and the associated simulated pressure-density curves assuming different initial densities. The equation gave greater confidence in the ability of computer simulations using the molecular dynamics technique of density functional theory (DFT) to accurately capture the subtle quantum physics changes in material properties under these previously undocumented conditions. In particular, the new data resolved a puzzling discrepancy between previous experiments with warm, dense nitrogen and predictions based on the results of the DFT simulations.

“We have shown that density functional theory works very well to describe our experiments. This is a very rigorous and useful test,” Kim said.

The research is part of a Laboratory Directed Research and Development (LDRD) project to develop new laser-guided dynamic compression experimental techniques with Diamond Anvil Cell (DAC) targets. These techniques could unravel new physical and chemical phenomena in low atomic number mixtures, such as those rich in water, over a wide range of unprecedented pressure-temperature-density conditions. The research has implications for planet formation and evolution and provides insight into the properties of matter under extreme conditions†

In particular, Kim is now leading experiments to develop the use of DAC targets at the National Ignition Facility. This could help to further study nitrogen and unravel new exotic phenomena at much lower temperatures, coupled with the observation of shock-induced cooling in the 1980s and the 2010 prediction of a first-order transition between molecular and polymeric nitrogen fluids under the 2000K.

“There are many more things we can learn from these types of laser dynamic compression experiments,” said Marius Millot, an LLNL principal investigator on the LDRD project and the paper’s senior author. “This is a very exciting field with multiple possibilities to develop innovative measurements and unravel the response of matter to extreme conditions. This is essential to interpret astronomical observations and better understand the formation and evolution of celestial bodies such as white dwarfs and exoplanets. ”