Why does Photoionization cross section reduce with frequency?

[bushnoh]

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

 

[mathman]

I vaguely recall that it is a resonance effect.