  Aspects of the preparation process and performance degradation are two major
problems of photocathodes. The lack of a means for dynamic quantum efficiency
measurements results in the inability to observe the inhomogeneity of the
cathode surface by fine structural analysis and in real time.Here we present a
simple and scalable technique for in situ real-time quantum efficiency
diagnosis. An incoherent light source provides uniform illumination on the
cathode surface, and solenoid magnets are used as lens for focusing and imaging
the emitted electron beam on a downstream scintillator screen, which converts
the quantum efficiency information into fluorescence intensity distribution.
The microscopic discontinuity and the dynamic changes of the quantum efficiency
of a gallium arsenide photocathode are observed at a resolution of a few
microns. An unexpected uneven decrease of the quantum efficiency is also
recorded. The work demonstrates a new observation method for photoemission
materials research.
