  The operation of photoemission electron sources with wavelengths near the
photoemission threshold has been shown to dramatically decrease the minimum
achievable photocathode emittance, but at the cost of significantly reduced
quantum efficiency (QE). In this work, we show that for femtosecond laser and
electron pulses, the increase in required laser intensities due to the low QE
drives the photocathode electronic distribution far from static equilibrium. We
adapt an existing dynamic model of the electron occupation under high intensity
laser illumination to predict the time-dependent effects of the nonequilibrium
electron distribution on the QE, mean transverse energy (MTE), and emission
brightness of metal phtocathodes. We find that multiphoton photoemission
dramatically alters the MTE as compared to thermal equilibrium models, causing
the MTE to no longer be a monotonic function of photon excess energy.
