Normal-incidence grating spectrograph with large acceptance for inverse photoemission

A spectrometer for inverse photoemission in the vacuum ultraviolet range is described. A spherical grating with an acceptance of f/4 is used in normal incidence. Two position-sensitive detectors allow the registration of spectra covering the whole range of photon energies from 8 to 28 eV in parallel. The optical resolution is 18 Å for the Lyman-α line of hydrogen. A space-charge-limited electron gun with an energy spread of 0.25 eV is used to excite inverse photoemission spectra. The large acceptance angle of the grating allows one to measure spectra with high efficiency and low background level.