Kinetics of alkali-based photocathode degradation

Vitaly Pavlenko,Fangze Liu,Nathan A Moody,Mark A Hoffbauer,Enrique R Batista

doi:10.1063/1.4967349

Abstract

We report on a kinetic model that describes the degradation of the quantum efficiency (QE) of Cs3Sb and negative electron affinity (NEA) GaAs photocathodes under UHV conditions. In addition to the generally accepted irreversible chemical change of a photocathode’s surface due to reactions with residual gases, such as O2, CO2, and H2O, the model incorporates an intermediate reversible physisorption step, similar to Langmuir adsorption. This intermediate step is needed to satisfactorily describe the strongly non-exponential QE degradation curves for two distinctly different classes of photocathodes –surface-activated and “bulk,” indicating that in both systems the QE degradation results from surface damage. The recovery of the QE upon improvement of vacuum conditions is also accurately predicted by this model with three parameters (rates of gas adsorption, desorption, and irreversible chemical reaction with the surface) comprising metrics to better characterize the lifetime of the cathodes, instead of time-pressure exposure expressed in Langmuir units.

Highlights

The pace of high brightness electron source development has increased in response to design requirements for future x-ray free-electron lasers (XFEL) and x-ray energy-recovery linacs (XERL).[1]
While alkali-based photocathodes with high quantum efficiency (QE) in the visible range[5] are attractive candidates as sources of high brightness electron beams,[6,7,8] they are known to suffer from fast degradation under typical UHV conditions.[7,9,10,11]
Several distinct processes lead to their QE degradation, such as surface “poisoning,” deviation from stoichiometry, ion back bombardment, electrolysis, and thermally activated decomposition.[5]

Summary

Introduction

The pace of high brightness electron source development has increased in response to design requirements for future x-ray free-electron lasers (XFEL) and x-ray energy-recovery linacs (XERL).[1]. This type of behavior cannot be accurately represented by the Langmuir adsorption model, but suggests a two-step mechanism wherein the gas molecules can adsorb and desorb, or irreversibly react with the surface as described by the following process: kads

Results

Conclusion