Why does Photoionization cross section reduce with frequency?

AI Thread Summary
The discussion centers on the confusion regarding the reduction of the photoionization cross section with increasing frequency in the context of Hydrogen and Helium IGM clouds. It highlights that while ionization requires a certain energy threshold, the rate of ionization decreases beyond this threshold. Participants suggest that this phenomenon may be related to resonance effects, which influence how photons interact with atoms. The original poster expresses frustration at the lack of clear information available online. Overall, the conversation seeks to clarify the underlying physics of photoionization in astrophysical contexts.
bushnoh
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I'm currently starting a large comp-astro project, and trying to understand a bit of the background about heading of Hydrogen and Helium IGM clouds I am a bit confused about the the actual heating process.

From the standard photoelectric effect, frequency has to pass a certain frequency (or energy) threshold for ionization to occur but beyond this threshold why does the ionization cross section reduce (rate of ionization)?

I've scoured the internet for several days and not come up with any answers...

Please help, thanks :D
 
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I vaguely recall that it is a resonance effect.
 
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