What is the simplest Feynman diagram for photon absorption by an atom?
Is it described by an incoming photon and bound atomic electron interacting at one vertex with an outgoing virtual electron which then, at a second vertex, emits a virtual photon and a real bound excited electron. The virtual photon is finally absorbed by the nucleus of the absorbing atom at the 3rd vertex.
What are the dimensionless factors involved in the cross-section for the process. If my 3 vertex diagram is correct then I guess the fine structure constant cubed will be in the mix but are there any other numeric constants?
Some background to the question: I'm trying to determine the cross-sections for photon absorption of Hydrogen Balmer lines, specifically I'm trying to link these cross-sections to the Einstein absorption/emission rate coefficients. I've published this work, which is still being developed, on my website. If I assume a 3 vertex diagram and hence a $4\pi \alpha^3$ dimensionless constant then I obtain a good estimate for the thickness of the Sun's photosphere.