On collisional free-free photon absorption in warm dense matter

Abstract

The rate of photon absorption in warm dense matter (WDM) induced by free-free electron-ion collisions is derived from Sommerfeld's cross-section for non-relativistic bremsstrahlung emission, making use of detailed balance relations. Warm dense matter is treated as a metal-like state in the approximation of a uniform degenerate electron gas and a uniform ion background. Total absorption rates are averaged over the electron Fermi distribution. A closed expression is obtained for the absorption rate, depending on temperature, density, and photon energy, that scales with ion charge Z. It is evaluated numerically for the full parameter space, which requires different representations of the hypergeometric functions involved. The results are valid for photon frequencies larger than the plasma frequency of the medium. They are compared with approximate formulas in various asymptotic regions.

Platform for probing radiation transport properties of hydrogen at conditions found in the deep interiors of red dwarfs

“…Additional corrections due to quantum and correlation effects are accounted for in a frequency-dependent correction factor g f f (ν, T ), the so-called Gaunt factor. [23] For a weakly coupled ideal plasma, the Gaunt factor can be interpreted as the logarithm of the ratio of maximum impact parameter b max and minimum impact parameter b min in the corresponding electron-ion collision (the so-called Coulomb logarithm [24]):…”

“…[30,31] Other authors perform similar calculations but integrate over the Fermi distribution of a degenerate electron gas. [24] In fact, for dense plasma conditions comparable to the interiors of main sequence stars, even advanced calculations of the Gaunt factor vary by more than 50 % for frequencies larger than the plasma frequency. [27] In particular, the specific treatment of dynamic screening, strong collisions, and re-normalization due to higher moments can make a significant difference in comparison to widely-used Born approximation treatments of the Gaunt factor.…”

Few-Femtosecond Dynamics of Free-Free Opacity in Optically Heated Metals

“…Photon absorption by free electrons in metals, warm dense matter, and plasmas is a longstanding fundamental problem that plays an essential role in laser-matter interactions [1,2]. Despite our excellent understanding of photoabsorption mechanisms on the attosecond timescale by bound electrons in dielectrics [3,4], semiconductors [5], and metals [6], the role of free electrons is still actively debated [7][8][9].…”