Abstract
We combine accurate measurements at low energies and theoretical calculations at high energies to construct sets of photoionization cross sections of He and H2 that should be reliable at all energies. We control the accuracy of the recommended cross section data by ensuring that they satisfy a range of oscillator strength sum rules. We provide accurate analytic formulae for the photoionization cross sections that have the proper asymptotic form. At high frequencies the cross sections for H2 are 2.8 times those for H.
Highlights
ImTpohretapnhtoptoairoanmizeatetirosnincrthoessdseetcetrimoninsaotfioHn,oHf t2h, eainodniHzaetiaorne structure of cosmic gas subjected to ultraviolet and X-ray radiation
Hth2eofrreotmicaal calculations, and we show that their accuracy can be controlled by requiring that they be consistent with oscillator strength moment sum rules and that they have the correct physical form at high energies
DeÐne the oscillator strength moment S(k), P S(k) v)k df dv where I is the ionization potential, v is the energy of the ejected electron, and df/dv is the di†erential oscillator strength for absorption into the ionization continuum
Summary
ImTpohretapnhtoptoairoanmizeatetirosnincrthoessdseetcetrimoninsaotfioHn,oHf t2h, eainodniHzaetiaorne structure of cosmic gas subjected to ultraviolet and X-ray radiation. The cross section for H is known analytically as a function of photon energy The. Kang 1987 ; Cen 1992 ; Haiman, Thoul, & Loeb 1996) yield cross sections that are probably adequate at ultraviolet and extreme-ultraviolet wavelengths, but with the exception of the expressions given by Band et al (1990) and Verner et al. (1993), they do not have the correct form at X-ray wavelengths. We construct here cross sections for He combination of experimental measurements and and. Hth2eofrreotmicaal calculations, and we show that their accuracy can be controlled by requiring that they be consistent with oscillator strength moment sum rules and that they have the correct physical form at high energies
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