Vacuum ultraviolet photon detector with improved resolution for inverse photoemission spectroscopy

We have significantly improved the energy resolution of a vacuum ultraviolet isochromat spectrometer for inverse photoemission spectroscopy. The detector is based on a Geiger-Mueller counting tube with acetone as filling gas and a CaF2 entrance window working as an energy selective optical bandpass at a mean energy of 9.84 eV. Under this operating conditions the detector achieves an optical energy resolution of 320 meV (FWHM). By adding a gas chamber with two CaF2 windows at the entrance of the counting tube we are able to use absorption lines of oxygen and krypton to further enhance the optical resolution of the detector system. In the case of krypton the mean energy shifts to a somewhat lower value of 9.72 eV. The energy resolution improves to about 115 meV (FWHM), while the integrated sensitivity decreases to approximately 30%. Due to the ability to easily switch between high sensitivity and high resolution this new counting tube differs from other resolution-enhancing suggestions. Measurements on the prominent (n=1) image potential state on the Cu(111) surface indicate that the photon detector is no longer the major resolution limiting component in the spectrometer