6.1 Figure 1

 

The coordinate system used in this work. The unit vectors $\hat p_1$and $\hat p_2$ are in the direction of the initial and final electron momenta, respectively. The unit vector $\hat k$ in the direction of the photon momentum defines the z axis. $\theta_1$ is the angle between $\vec k$ and the initial electron momentum $\vec p_1$, $\theta_2$ is the angle between $\vec k$ and the final electron momentum $\vec p_2$, and $\phi_2$ is the angle between the plane containing $\vec k$ and $\vec p_1$ and the plane containing $\vec k$and $\vec p_2$.