Beam brightness from Cs-Te near the photoemission threshold

We describe measurements of the mean transverse energy (MTE) of Cs-Te photocathodes near the photoemission threshold. The MTE displays an unexpected non-monotonic behavior as the drive laser's wavelength is tuned to threshold and changes significantly as the photocathode is cooled to cryogenic temperatures. We show that a simple analytical model of photoemission from multiple compounds with a work function below that of pure Cs2Te may describe this behavior. We identify the additional compounds as Cs5Te3 and metallic Cs, and by calculating the MTE numerically within the three step model, we reproduce both the wavelength and temperature dependence of the observed MTE. In our model, the MTE changes with temperature arise from realistically small changes in the workfunctions of both compounds and Cs5Te3's bandgap energy. These results suggest the existence of an illumination wavelength that is optimal for beam brightness and show that even trace impurities can dominate the MTE for near-threshold photoemission