Theoretical Predictions of High-Energy-Density Physics Shed Light on the Universe

Extreme pressures and temperatures cause both the atoms and molecules to behave in a very different manner. While such extreme matter is present abundantly in the universe, particularly in the deep interiors of stars and planets, they do not occur naturally on the planet earth.

By interpreting the way atoms respond under extreme pressure conditions—a field called high-energy-density physics (HEDP)—researchers can gain useful insights into the fields of astrophysics, planetary science, national security, and fusion energy.

The field of HED science poses an important question, that is, how would matter under extreme pressure conditions would absorb or produce radiation in ways that are completely different from people’s conventional interpretation.

In a study published in the Nature Communications journal, researchers have successfully applied the theory and calculations to estimate the presence of a couple of novel phenomena—that is, the breakdown of dipole selection rule and the interspecies radiative transition (IRT)—in the transport of radiation in molecules and atoms placed under HED conditions.

This study was carried out by Suxing Hu, a distinguished researcher and also a group leader of the HEDP Theory Group at the Laboratory for Laser Energetics (LLE) of the University of Rochester, along with collaborators from the LLE and France. The study improves one’s interpretation of HED science and may lead to additional data about the evolution of astrophysical objects, including stars, in the universe.

What is Interspecies Radiative Transition (IRT)?

Radiative transition can be described as a physical phenomenon that occurs within the molecules and atoms, wherein their electron or electrons can “jump” from different levels of energy by either absorbing or emitting (radiating) a single photon.

The researchers discovered that for a matter that is involved in people’s day-to-day lives, such radiative transitions largely occur inside each individual molecule or atom; the single electron jumps between energy levels that belong to one molecule or atom, and the jumping process does not often take place between varying molecules and atoms.

According to the prediction made by Hu and his collaborators, when both molecules and atoms are placed under HED conditions and packed very tightly such that they become quite close to one another, the result is radiative transitions that involve adjacent molecules and atoms.

Namely, the electrons can now jump from one atom’s energy levels to those of other neighboring atoms.

Suxing Hu, Distinguished Scientist and Group Leader of HEDP Theory Group, Laboratory for Laser Energetics, University of Rochester

What is the Dipole Selection Rule?

Within a single atom, electrons exhibit particular symmetries. For instance, “s-wave electrons” are invariably spherically symmetric, which implies that they resemble a ball, with the nucleus situated in the center of the atom; on the other hand, “p-wave electrons,” resemble the dumbbells. More intricate shapes are exhibited by D-waves and other types of electron states.

Generally, radiative transitions will take place when the electron jumping follows the dipole selection rule. The jumping electron in this so-called dipole selection rule alters its shape from s-wave to p-wave, from p-wave to d-wave, and so on.

Under standard, non-extreme conditions, “one hardly sees electrons jumping among the same shapes, from s-wave to s-wave and from p-wave to p-wave, by emitting or absorbing photons,” added Hu.

But as Hu and his collaborators discovered that the dipole selection rule is usually broken down when materials are packed very tightly into the unusual HED state.

Under such extreme conditions found in the center of stars and classes of laboratory fusion experiments, non-dipole x-ray emissions and absorptions can occur, which was never imagined before.

Suxing Hu, Distinguished Scientist and Group Leader of HEDP Theory Group, Laboratory for Laser Energetics, University of Rochester

Using Supercomputers to Conduct Calculations

To perform their calculations, the scientists utilized supercomputers at the University of Rochester’s Center for Integrated Research Computing (CIRC) and also at the LLE.

Thanks to the tremendous advances in high-energy laser and pulsed-power technologies, ‘bringing stars to the earth’ has become reality for the past decade or two.

Suxing Hu, Distinguished Scientist and Group Leader of HEDP Theory Group, Laboratory for Laser Energetics, University of Rochester

To conduct their research, Hu and his collaborators used the density-functional theory (DFT) calculation that provides a quantum mechanical depiction of the bonds that exist between molecules and atoms in intricate systems.

Initially elucidated in the 1960s, the DFT technique was the subject of the Nobel Prize in Chemistry awarded in 1998, and since then, DFT calculations have been incrementally improved. One such enhancement to facilitate DFT calculations to involve the main electrons was made by Valentin Karasev, the study’s co-author and a researcher at the LLE.

The outcomes suggest the appearance of new absorption and emission lines in the X-ray spectra of these extreme matter systems. These systems represent the earlier unrecognized channels of IRT and the disintegration of the dipole selection rule.

Hu, and Philip Nilson, the study’s co-author and a senior researcher at the LLE, have now planned to perform more experiments in the future in which these latest theoretical predictions will be tested at LLE’s OMEGA laser facility. This facility allows users to produce unusual HED conditions in nanosecond timescales, thus enabling researchers to analyze the matter’s exclusive behaviors at high conditions.

“If proved to be true by experiments, these new discoveries will profoundly change how radiation transport is currently treated in exotic HED materials. These DFT-predicted new emission and absorption channels have never been considered so far in textbooks,” Hu concluded.

The study is based on work that was financially supported by the New York State Energy Research and Development Authority and the United States Department of Energy (DOE) National Nuclear Security Administration. The National Science Foundation partly supported the research.