# Interpretation of Feynman diagrams

I'm currently taking the second part of a two semester course in quantum mechanics and while discussing the interaction of matter and radiation some Feynman diagrams came up. The context is "lightscattering", meaning that we look at processes where a photon described by $$\vec k, \lambda$$ gets scattered into a photon $$\vec k', \lambda'$$, where $$\vec k$$ is the wave vector and $$\lambda$$ the polarization. The states $$a$$ and $$b$$ are one-particle states.

In the end we are interested in figuring out transition-amplitudes of the form $$\langle b; \vec k',\lambda'\vert e^{-iH(t-t_0)/\hbar}\vert a;\vec k,\lambda\rangle,$$ where the Hamiltonian is given by $$H=\frac{1}{2 m} \sum_{i}\left[\vec{p}_{i}-\frac{q_{i}}{c} \vec{A}\left(\vec{r}_{i}, t\right)\right]^{2}+\frac{1}{2} \sum_{i \neq j} \frac{q_{i} q_{j}}{\left|\vec{r}_{i}-\vec{r}_{j}\right|}+\int d^{3} r \frac{E_{\perp}^{2}+B^{2}}{8 \pi}$$

Now to the actual question: Can someone explain to me what is going on in (C)? I think I have an idea what is happening in (A) and (B), but when it comes to (C) I'm completely lost...

B and C simply differ in whether the charged particle absorbs the photon with momentum $$k$$ before or after it emits the photon with momentum $$k’$$.
• In both B and C you have to deal with the polarizations of the charged particle (unless it has spin $0$) and the polarizations of the photons. Jun 19 '19 at 19:51