Fabrication of an inverse photoemission spectrometer to study unoccupied electronic states

Abstract

We discuss the performance of an inverse photoemission spectrometer that has been recently fabricated in our laboratory. The photon detector is of band-pass type with acetone gas filling and CaF₂ window (acetone/ CaF₂). We determine the optimal operating conditions of the detector to be 4 mbar acetone pressure and 745 ± 20 V anode voltage. At these operating conditions, the count rate is improved by a factor of three to what has been reported earlier by Funnemann and Merz. We show that unlike other gas-filled detectors, acetone/CaF₂ detector works in the proportional region. Its dead time is negligible and addition of multiplier gas like argon worsens its performance. The performance of this detector has been compared with an ethanol/MgF₂ detector. High count rate, reasonable resolution, negligible dead time, no requirement of multiplier or quench gas, satisfactory stability, and ease of handling make the acetone/CaF₂ detector an attractive candidate for use in inverse photoemission spectroscopy (IPES). A low-energy electron gun of Stoffel-Johnson type has been fabricated and the beam current profile has been determined as a function of electron kinetic energy. The IPES spectra of polycrystalline Ag and Ta are shown and compared with data in the literature. By fitting the IPES Fermi edge of Ag, we find the overall resolution of the spectrometer to be 0.55 eV and the band-pass of acetone/CaF₂ detector to be 0.48 eV FWHM.