Kinetics of alkali-based photocathode degradation

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.

Photoemission and degradation of semiconductor photocathode

“…The static model discussed above does not include the surface degradation from the residual gases in vacuum. The continuous reduction of QE caused by the gas adsorption has been confirmed in experiments [23,44], while the impact on intrinsic emittance is not fully understood [25]. In the following, a dynamic model is established to describe the evolution of QE and intrinsic emittance during surface degradation process without ion back bombardment.…”

“…Examples are taken on Cs 3 Sb with the incident photon energy of 1.8 eV under the electric field of 4 MV=m and 50 MV=m, Torr and photon energy of 3.06 eV, is the same as in Ref. [23]. The inset shows the model fitting to Pavlenko's paper [23], which gives a L d of 6.1 nm, shown in typical values for the DC gun and rf gun, as presented in Fig.…”

“…An oxygen controlled contamination experiment has been done on Cs 3 Sb with incident photon energy 3.06 eV (405 nm) by a group from LANL [23]. The oxygen dominates the vacuum environment and the pressure is roughly 10 −9 Torr.…”

“…However, the dynamics of the degradation process was not discussed. Exponential fitting is a standard method used to evaluate the degradation of the QE [21,22], while clear deviation of the entire degradation process from the exponential trend was observed [23,24]. Pavlenko and coworkers suggested to include an intermediate and reversible physisorption step to account for the discrepancy.…”

“…Pavlenko and coworkers suggested to include an intermediate and reversible physisorption step to account for the discrepancy. Their model provided new insights into the physical picture of the degradation of QE for various materials, but the behavior of the intrinsic emittance was not discussed [23]. It has been confirmed experimentally that as the QE decays continuously with the accumulated oxygen dose, the intrinsic emittance would experience a reduction for GaAsP [25].…”