I mean the photoelectric effect of the hydrogen atom.
It is weird. By the Fermi golden rule, the transition or absorption rate is proportional to the density of the final states. At threshold, the electron has zero momentum and thus zero density of state. Therefore, the absorption coefficient...
After some search, I found a reference using this idea for fermions:journals.aps.org/pra/abstract/10.1103/PhysRevA.89.012504. Their idea is to use the Slater wave function to approximate a given fermionic wave function. They mentioned that this will provide a geometric measure of entanglement...
For a system consisting of multiple components, say, a spin chain consisting ofN≥3spins, people sometimes use the so-called geometric measure of entanglement. It is related to the inner product between the wave function and a simple tensor product wave function. But it seems that none used this...