Short question: Spontaneous emission from Feynman diagrams?

[blue2script]

Hey all,
I am just wondering if one can directly calculate the Einstein coefficient in spontaneous emission of, say, two-level atoms through feynman diagrams? I searched for sources in google but could not find anything.

Thanks a lot for an answer!

Wit best regards,
blue2script

 

[blue2script]

Maybe I can render the question more precisely: Given a proton in an outer magnetic field I get two energy levels depending on the direction of spin. I can plug the outer field into the Lagrangian. After determing the feynman rules I should be prepared to calculate the decay rate of a proton in the upper energy state.

Am I right?