Brightness of nonequilibrium photoemission in metallic photocathodes near threshold

TL;DR

This paper investigates how high-intensity femtosecond laser illumination causes nonequilibrium electron distributions in metallic photocathodes, significantly affecting their quantum efficiency and brightness near the photoemission threshold.

Contribution

It adapts a dynamic model to predict time-dependent effects of nonequilibrium electron distributions on photocathode performance under high laser intensities.

Findings

Multiphoton photoemission alters the mean transverse energy (MTE).

MTE is no longer a monotonic function of photon excess energy.

Nonequilibrium effects are significant at high laser intensities.

Abstract

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.