Vectorial photoelectric effect and accelerator emittance According the the vectorial photoelectric effect, directing a laser at a photocathode with some special angle will maximize the quantum efficiency of that material. Let's say incident light hits a Cu photocathode at an angle of 72 degrees (approximately brewster's angle) where the quantum efficiency is maximized. The emitted electrons are then accelerated immediately after they escape in the direction normal to the Cu plate.
From my understanding of the photoelectric effect, the electrons will be emitted at the angle of reflection. Also from my understanding, accelerators try to minimize beam emittance to make the luminosity as high as possible. 
So my question is: 
How does the incidence angle influence emittance, and if it causes more beam emittance, does the higher quantum efficiency really outweigh the greater beam emittance?
One possible answer is that the Cu plate is simply tilted at the appropriate angle to make electrons escape in the desired direction, but literature searches do not indicate that this is an existing practice. 
 A: It doesn't really matter. While it is true that if your laser impinges at an angle you:


*

*illuminate a larger spot on the photocathode

*electrons do not emerge orthogonally to the surface


both the aspects are quite pointless.
The momentum that the electrons carry from the photoelectric effect is irrelevant compared to the one that is quickly gain from the electric field: we are talking about few eV/c to some MeV/c in less than a meter (with RF guns). 
In addition, more often than not, the emittance is the result of space charge and field quality immediately after the source, more than the image of the source itself. This means that reducing the laser spot size does not help in getting a smaller operational emittance into the linac and preserving it all the way along.
More crucial aspects in the optimisation of the photo-cathodes are durability and reliability, especially in high current regimes. Of course better quantum efficiency is always welcomed as it allows to reduce the laser power and therefore the stress on the materials.
