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DOE OSTI · 1772045

Free-free matrix-elements for two-photon opacity

More, R. M.·Pain, J.-C.·Hansen, Stephanie B.·Nagayama, Taisuke·Bailey, James E.
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Abstract

Recent opacity measurements have inspired a close study of the two-photon contributions to the opacity of hot plasmas. The absorption and emission of radiation is controlled by dipole matrix-elements of electrons in an atom or ion. This paper describes two independent methods to calculate matrix-elements needed for the two-photon opacity and tests the results by the f-sum rule. The usual f-sum rule is extended to a matrix f-sum that offers a rigorous test for bound-bound, bound-free and free-free transitions. An additional higher-order sum-rule for the two-photon transition amplitudes is described. In this work, we obtain a simple parametric representation of a key plasma density effect on the matrix-elements. The perturbation theory calculation of two-photon cross-sections is compared to an independent method based on the solution of the time-dependent Schroedinger equation for an atom or ion in a high-frequency electromagnetic field. This is described as a high frequency Stark effect or AC Stark effect. Two-photon cross sections calculated with the AC Stark code agree with perturbation theory to within about 5%. In addition to this cross check, the AC Stark code is well suited to evaluating important questions such as the variation of two-photon opacity for different elements.

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More, R. M., Pain, J.-C., Hansen, Stephanie B., Nagayama, Taisuke, Bailey, James E.. 2019-10-04. Free-free matrix-elements for two-photon opacity. https://doi.org/10.1016/j.hedp.2019.100717

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