Why does Photoionization cross section reduce with frequency?

In summary, the photoionization cross section decreases with increasing frequency due to the energy dependence of the ionization process. It is inversely proportional to the ionization energy of an atom or molecule, and the atomic or molecular structure can greatly affect it. There is a limit known as the photoionization threshold, and the cross section can vary greatly between different types of atoms or molecules.
  • #1
bushnoh
1
0
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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  • #2
I vaguely recall that it is a resonance effect.
 

Related to Why does Photoionization cross section reduce with frequency?

1. Why does the photoionization cross section decrease with increasing frequency?

The photoionization cross section, or the likelihood of an atom or molecule being ionized by a photon, decreases with increasing frequency due to the energy dependence of the ionization process. As the frequency of the photon increases, so does its energy. This higher energy photon is more likely to pass through the atom or molecule without causing ionization, resulting in a decrease in the overall cross section.

2. What is the relationship between the photoionization cross section and the ionization energy of an atom or molecule?

The photoionization cross section is inversely proportional to the ionization energy of an atom or molecule. This means that as the ionization energy increases, the cross section decreases. This is because a higher ionization energy requires a higher energy photon to cause ionization, resulting in a decrease in the probability of ionization occurring.

3. How does the atomic or molecular structure affect the photoionization cross section?

The atomic or molecular structure can greatly affect the photoionization cross section. For example, atoms with more electrons have a higher cross section due to the increased number of possible ionization pathways. Additionally, the arrangement of electrons and their energy levels within an atom or molecule can also impact the cross section, as certain energy levels may be more easily ionized than others.

4. Is there a limit to how low the photoionization cross section can go with increasing frequency?

Yes, there is a limit known as the photoionization threshold. This is the minimum frequency required for a photon to have enough energy to ionize an atom or molecule. As the frequency increases beyond this threshold, the cross section may continue to decrease, but it will never reach zero.

5. How does the photoionization cross section change with different types of atoms or molecules?

The photoionization cross section can vary greatly between different types of atoms or molecules. This is because the structure, number of electrons, and ionization energies of different atoms and molecules can all impact the cross section. Additionally, the presence of different elements or functional groups within a molecule can also affect its photoionization cross section.

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