Free-free Absorption Coefficient Research Articles

THEORY OF GYRORESONANCE AND FREE-FREE EMISSIONS FROM NON-MAXWELLIAN QUASI-STEADY-STATE ELECTRON DISTRIBUTIONS

Currently there is a concern about ability of the classical thermal (Maxwellian) distribution to describe quasi-steady-state plasma in solar atmosphere including active regions. In particular, other distributions have been proposed to better fit observations, for example, kappa- and $n$-distributions. If present, these distributions will generate radio emissions with different observable properties compared with the classical gyroresonance (GR) or free-free emission, which implies a way of remote detecting these non-Maxwellian distributions in the radio observations. Here we present analytically derived GR and free-free emissivities and absorption coefficients for the kappa- and $n$-distributions and discuss their properties, which are in fact remarkably different from each other and from the classical Maxwellian plasma. In particular, the radio brightness temperature from a gyrolayer increases with the optical depth $\tau$ for kappa-distribution, but decreases with $\tau$ for $n$-distribution. This property has a remarkable consequence allowing a straightforward observational test: the gyroresonance radio emission from the non-Maxwellian distributions is supposed to be noticeably polarized even in the optically thick case, where the emission would have strictly zero polarization in the case of Maxwellian plasma. This offers a way of remote probing the plasma distribution in astrophysical sources including solar active regions as a vivid example.

The influence of the Ramsauer-Townsend effect on free-free absorption coefficients of the negative argon ion at far infrared wavelengths

free-free absorption coefficients in the far-IR are calculated using relativistic continuum wavefunctions from solutions of the Dirac equation. Absorption coefficients are shown to exhibit a Ramsauer-Townsend minimum near 1200 K. We compare these results with coefficients derived from non-relativistic and experimental atomic data.

The continuous free–free absorption coefficient of the negative lithium ion

A multichannel theory, based upon the R-matrix method for electron-atom collisions, is employed in the present work to evaluate the continuous free-free absorption coefficient for the negative ion of lithium. The calculation is performed over a wide range of radiation wavelengths λ = 9113–151 890 Å and temperatures T = 1400–10 080 (K). Significant differences are found to occur when the present results are compared with a previous theoretical evaluation.

The reliability of absorption coefficients for free-free transitions of the negative helium ion

The reliability of current estimates of free-free absorption coefficients of the negative helium ion are assessed. Results by sophisticated versions of the multiconfiguration-self-consistent field, close-coupling acid polarized orbital methods are shown to have converged to within a few per cent of each other for wavelengths greater than 1 mu m over the temperature range 1400 K to 25200 K. A table of coefficients giving the best current estimates of He- continuous absorption are presented.

The free-free absorption coefficients of the negative ion of molecular hydrogen in the far-IR. spectrum

The free-free absorption coefficients of the negative ion of molecular hydrogen in the far-IR. spectrum

Continuous absorption by He2(+) and H2(+) in cool white dwarfs

Photodissociation cross sections and absorption coefficents of the He2(+) molecular ions for transitions from the Chi (2)Epsilon(+) ground state to the vibrational continuum of the A (2)Epsilon(+) excited state have been determined for photon wavelengths lambda between 50 nm and 20 microns and for temperatures T between 4200 and 50,400 K and between 3150 and 25,200 K, respctivley. The cross sections are obtained through a fully quantum-mechanical method, and it is shown that inclusion of the high-lying rotational states is important for determination of the correct long-wavelengthh cross section amplitude and the peak wavelength. Free-free absorption coefficients are also obtained using the semiclassical method of Bates. The detection possibility of He2(+) and H2(+) in cool white dwarfs is dicussed and shown to be insignificant. An effectively temperature independent cutoff in the He2(+) photodissociation cross section in the EUV at approximately 60 nm appears to be the only possible observable feature. It is suggested that He2(+) may be responsible for an observed UV flux deficiency in some cool DB stars as well as in the DZ stars WD 0552 - 041 and WD 1334 + 039. We further suggest that the star WD 0752 - 676 currently assumed to be helium-rich may actually have a hydrogen-dominated atmosphere.

Free-free transitions of Formula in the soft-photon limit

He − free-free absorption coefficients in the far-IR are calculated using accurate continuum wavefunctions that include polarization and target-atom correlation. The coefficients are expected to have an accuracy of 1 per cent or better

A method for calculating H- free-free absorption coefficients from scattering phaseshifts

The author presents an empirical procedure for determining free-free absorption coefficients of the negative hydrogen ion from phaseshifts for electron-hydrogen scattering, that seems to give results to an accuracy of a few per cent for wavelengths greater than 0.5 mu m. Applications of the method to cases not previously considered yield data on the convergence and on the reliability of free-free coefficients. These data support the case that the total absorption coefficient of H- is now known to within an accuracy of one or two per cent or better.

The free-free absorption coefficient of the negative nitrogen ion

A multichannel theory of free-free transitions of an electron in the presence of a general atomic system is employed to evaluate the continuous absorption coefficient for the negative nitrogen ion. The calculation is evaluated over a wide range of radiation wavelengths lambda (AA)=3037-151 890 and temperatures T(K)=1800-10 080. Comparison with previous calculations shows significant differences.

Elastic scattering of electrons by krypton and the free-free absorption coefficient of Kr-

Elastic scattering of electrons by krypton atoms is considered using the R-matrix method. Phaseshifts, differential, integral and momentum transfer cross sections are reported for electron impact energies in the range 0.01-16 eV. The free-free absorption coefficient of the negative ion of krypton is also calculated for wavelengths longer than 1500 AA and for a wide range of temperatures.

The free-free absorption coefficient of the negative carbon ion

The free-free absorption coefficient for the negative ion of carbon is calculated at a wide range of radiation wavelengths and temperatures. The calculation employs a multichannel theory based upon the R-matrix method for electron-atom collisions. Significant differences are found when the present data are compared with previous calculations.

The free-free absorption coefficient of the negative argon ion

The free-free absorption coefficient for Ar- is calculated at a wide range of radiation wavelengths and temperatures. The calculation employs a multichannel theory based upon the R-matrix method for electron-atom collisions. Comparison is made with other theoretical methods and with experiment. Highly satisfactory agreement exists with the bremsstrahlung emissivity experimental data of Ranson et al. (1977).

Free-free absorption coefficient of the negative hydrogen ion

The free-free absorption coefficient of the negative ion of hydrogen is calculated within the framework of the R-matrix method. The 1s, 2s and 2p states of hydrogen together with three pseudostates have been employed in the eigenfunction expansion. The results have been found to be in good agreement with other calculations and are believed to be currently the most accurate available.

Embedding an iron atom in a plasma: Influence of the host plasma on the photoeffect.

The density-functional theory of plasmas is used to investigate the changes in the electronic structure of an iron atom immersed in plasmas of various atomic numbers, but all having the same average electron density and temperature. We have calculated the radial distribution functions for ions and electrons around the iron atom and also the Kohn-Sham eigenfunctions, phase shifts, etc., for the electrons. The increase of positive charge around the Fe atom when the plasma ions become lighter induces more bound states and increases electronic binding. The resulting changes in photoeffect cross sections, free-free absorption coefficient, and average opacities are calculated in the case of H, He, Be, C, Ne, and Fe plasmas.

The free-free absorption coefficient of the negative helium ion

The free-free absorption coefficient of the negative ion of atomic helium is evaluated using a multichannel theory based upon the R-matrix method for electron-atom collisions. The results are expected to be accurate to at least 10%.

The free-free absorption coefficient of the negative pion of molecular hydrogen

The free-free absorption coefficient of the negative ions of molecular hydrogen is calculated in the dipole-length formulation using wavefunctions determined by the two-centre close-coupling expansion method and including both exchange and polarisation effects. The results are in fair agreement with the most recent calculations for this process, and are the most sophisticated calculations yet performed.

Free-free absorption by N - and O -

A multichannel theory is used to investigate free-free absorption by electrons moving in the field of neutral nitrogen and oxygen atoms. Tables of the free-free absorption coefficient are given for a range of temperatures and frequencies, and include details of contributions from various transitions involving ground state terms.

Resonance in the infra-red spectrum of N −

The free-free continuous absorption coefficient of N - is calculated using multichannel theory and Hartree-Fock target functions. The 3P e shape resonance due to e-N interactions gives rise to a resonance feature in the absorption coefficient.

Free-free radiative absorption coefficient for the negative argon ion

The free-free absorption coefficient for ${\mathrm{Ar}}^{\ensuremath{-}}$ is calculated at radiation wavelengths of 2000 and 20000 \AA{} for various temperatures. Electron correlation corrections to the Hartree-Fock approximation are obtained by using many-body perturbation theory. Comparison is made with other theoretical methods and experiment.

Temporal evolution of the electron density in high-pressure electron-beam-excited xenon plasmas

Measurements of the free-free absorption coefficient in high-pressure (50–400 psia) e-beam-excited rare-gas plasmas have been used to deduce the temporal evolution of electron density in these systems. The measurements were carried out using a focused CO2 laser beam passing through the excited region of the plasma. Comparison of theory and experiment in the early afterglow, where Penning ionization of excimers dominates the electron production, yields a room-temperature rate coefficient of 8×10−11 cm3/sec for this process in xenon.