Please consider downloading the latest version of Internet Explorer
to experience this site as intended.

Research provides new insights into the evolution of stars

April 24, 2020
glow over earth's horizonScientists at the Laboratory for Laser Energetics studied how matter under high-pressure conditions—such as the conditions in the deep interiors of planets and stars—might emit or absorb radiation. The research enhances an understanding of high-energy-density science and could lead to more information about how stars evolve. (NASA photo)

Atoms and molecules behave very differently at extreme temperatures and pressures. Although such extreme matter doesn’t exist naturally on the earth, it exists in abundance in the universe, especially in the deep interiors of planets and stars. Understanding how atoms react under high-pressure conditions—a field known as high-energy-density physics (HEDP)—gives scientists valuable insights into the fields of planetary science, astrophysics, fusion energy, and national security.

One important question in the field of HED science is how matter under high-pressure conditions might emit or absorb radiation in ways that are different from our traditional understanding.

In a paper published in Nature Communications, Suxing Hu, a distinguished scientist and group leader of the HEDP Theory Group at the University of Rochester’s Laboratory for Laser Energetics (LLE), together with colleagues from the LLE and France, has applied theory and calculations to predict the presence of two new phenomena—interspecies radiative transition (IRT) and the breakdown of dipole selection rule—in the transport of radiation in atoms and molecules under HED conditions. The research enhances an understanding of HED science and could lead to more information about how stars and other astrophysical objects evolve in the universe.

Suxing Hu standing in front of graphic

Suxing Hu is group leader of the High-Energy-Density Physics Theory Group at the Laboratory for Laser Energetics, (University of Rochester photo / Eugene Kowaluk)

What is interspecies radiative transition (IRT)?

Radiative transition is a physical process happening inside atoms and molecules, in which their electron or electrons can “jump” from different energy levels by either radiating (emitting) or absorbing a photon. Scientists find that, for matter in our everyday life, such radiative transitions mostly happen within each individual atom or molecule; the electron does its jumping between energy levels belonging to the single atom or molecule, and the jumping does not typically occur between different atoms and molecules. However, Hu and his colleagues predict that when atoms and molecules are placed under HED conditions, and are squeezed so tightly that they become very close to each other, radiative transitions can involve neighboring atoms and molecules.

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

What is the dipole selection rule?

Electrons inside an atom have specific symmetries. For example, “s-wave electrons” are always spherically symmetric, meaning they look like a ball, with the nucleus located in the atomic center; “p-wave electrons,” on the other hand, look like dumbbells. D-waves  and other electron states have more complicated shapes. Radiative transitions will mostly occur when the electron jumping follows the so-called dipole selection rule, in which the jumping electron changes its shape from s-wave to p-wave, from p-wave to d-wave, and so forth.

Under normal, non-extreme conditions, Hu says, “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.”

However, as Hu and his colleagues found, when materials are squeezed so tightly into the exotic HED state, the dipole selection rule is often broken down.

“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,” Hu says.

Using supercomputers to conduct calculations

The researchers used supercomputers at both the University of Rochester’s Center for Integrated Research Computing (CIRC) and at the LLE to conduct their calculations.

“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,” Hu says.

Hu and his colleagues performed their research using the density-functional theory (DFT) calculation, which offers a quantum mechanical description of the bonds between atoms and molecules in complex systems. The DFT method was first described in the 1960s, and was the subject of the 1998 Nobel Prize in Chemistry. DFT calculations have been continually improved since. One such improvement to enable DFT calculations to involve core electrons was made by Valentin Karasev, a scientist at the LLE and a co-author of the paper.

The results indicate there are new emission/absorption lines appearing in the x-ray spectra of these extreme matter systems, which are from the previously unknown channels of IRT and the breakdown of dipole selection rule.

Hu and Philip Nilson, a senior scientist at the LLE and coauthor of the paper, are currently planning future experiments that will involve testing these new theoretical predictions at the OMEGA laser facility at the LLE. The facility lets users create exotic HED conditions in nanosecond timescales, allowing scientists to probe the unique behaviors of matter at extreme conditions.

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

This research is based upon work supported by the United States Department of Energy (DOE) National Nuclear Security Administration and the New York State Energy Research and Development Authority. The work is partially supported by the National Science Foundation.

The LLE was established at the University in 1970 and is the largest DOE university-based research program in the nation. As a nationally funded facility, supported by the National Nuclear Security Administration as part of its Stockpile Stewardship Program, the LLE conducts implosion and other experiments to explore fusion as a future source of energy, to develop new laser and materials technologies, and to conduct research and develop technology related to HED phenomena.

Read more

person in protective gear inside laser labRochester recognized as leader in high-energy-density physics
Three of eight national research grants recently awarded by the Department of Energy were given to researchers at the University of Rochester, which is home to the largest university-based DOE research program in the nation.
group of stars against a dark skyRochester leads multi-institutional effort to study ‘extreme matter’
Institutions including Cornell, Michigan, Princeton, and Stanford join Rochester in developing an instrument to produce and study matter that exists under pressures far higher than either on or inside Earth.
woman overlooking laser lab siteLaser lab ‘truly inspiring’ to federal government visitors
National Nuclear Security Administration Administrator Lisa Gordon-Hagerty said the University’s Laboratory for Laser Energetics plays a crucial role in advancing research vital to maintaining the safety of America’s nuclear security enterprise.


Tags: , , , , , ,

Category: Science & Technology