Atoms and molecules behave very differently at extreme temperatures and pressures. Although such extreme matter doesn鈥檛 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鈥攁 field known as聽high-energy-density physics (HEDP)鈥攇ives scientists valuable insights into the fields of聽planetary science, astrophysics, fusion energy,听补苍诲听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 , , a distinguished scientist and at the 人妻少妇专区鈥檚 (LLE), together with colleagues from the LLE and France, has applied theory and calculations to predict the presence of two new phenomena鈥攊nterspecies radiative transition (IRT) and the breakdown of dipole selection rule鈥攊n 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.
What is interspecies radiative transition (IRT)?
Radiative transition is a physical process happening inside atoms and molecules, in which their electron or electrons can 鈥渏ump鈥� 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.
鈥淣amely, the electrons can now jump from one atom鈥檚 energy levels to those of other neighboring atoms,鈥� Hu says.
What is the dipole selection rule?
Electrons inside an atom have specific symmetries. For example, 鈥渟-wave electrons鈥� are always spherically symmetric, meaning they look like a ball, with the nucleus located in the atomic center; 鈥減-wave electrons,鈥� on the other hand, look like dumbbells. D-waves聽 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, 鈥渙ne 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.鈥�
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.
鈥淯nder 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,鈥� Hu says.
Using supercomputers to conduct calculations
The researchers used supercomputers at both the 人妻少妇专区鈥檚 and at the LLE to conduct their calculations.
鈥淭hanks to the tremendous advances in high-energy laser and pulsed-power technologies, 鈥榖ringing stars to the earth鈥� has become reality for the past decade or two,鈥� 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 . 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.
鈥淚f proved to be true by experiments, these new discoveries will profoundly change how radiation transport is currently treated in exotic HED materials,鈥� Hu says. 鈥淭hese DFT-predicted new emission and absorption channels have never been considered so far in textbooks.鈥�
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.
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