At what atomic level does the photoelectric interaction occur?

The photoelectric interaction occurs primarily at the atomic level involving the K shell, which is the closest electron shell to the nucleus. When a photon with sufficient energy strikes an atom, it can transfer energy to an electron in the K shell, leading to the ejection of that electron from the atom. The K shell electrons typically require higher energy to be removed compared to those in outer shells, making them more significant in the context of the photoelectric effect.

It's important to note that while outer shells and valence shells contain electrons that can also be involved in processes requiring energy absorption, they generally do not exhibit the same level of interaction efficiency with high-energy photons as those found in the K shell. On the other hand, the nucleus itself is primarily composed of protons and neutrons and does not participate in the photoelectric effect, as this phenomenon specifically involves the interactions of photons with electrons, rather than nuclear processes.