A Novel Imaging Spectrometer for Energy-Distribution Measurements of Photoelectrons from GaAs Cathodes

The investigation of the photoelectron-escape mechanism from GaAs cathodes with negative electron affinity requires the detection of very low energy electrons. We have built a novel, UHV-compatible, spectrometer where the photoelectrons are imaged by a homogeneous electric field onto a position-sensitive detector. The time-of-flight of each single emitted electron and its position on the detector is measured. From these informations energy-distribution curves are extracted. The spectrometer has run successfully and preliminary energy-distribution curves have been measured. The system is now under improvement. With the optimized spectrometer, an excellent energy resolution (a few meV) can be achieved. INTRODUCTION To investigate the photoelectron-escape mechanism from a photocathode with negative electron affinity, a spectrometer is required that allows the simultaneous measurement of the energy and angular distributions of very low energy electrons. In the past, measurements of the longitudinal energy distributions have been performed by several groups. Quite recently, complete energy distributions have been studied as a ftinction of longitudinal and transverse energies. The employed method is based on selecting photoelectrons of a fixed longitudinal energy using a retarding field analyzer, and subsequently measuring the associated differential transverse energy distribution by applying an adiabatic magnetic compression technique [1]. Recently we have built up a novel imaging spectrometer which allows the simultaneous measurement of the longitudinal and transverse momenta for each individual photoelectron emitted from a GaAs surface [2]. Our spectrometer has run successfully and preliminary energy-distribution curves (EDC's) have been extracted from this information. However, the first tests have been performed under non-ideal conditions with high extraction fields and, hence, the thus far measured EDC's have lower resolution than what is ultimately achievable. Design changes have been made to be able to measure EDC's of photoelectrons with an improved energy resolution in the near future.