Vacuum ultraviolet photon detector with improved resolution for inverse photoemission spectroscopy

Abstract

We have significantly improved the energy resolution of a vacuum ultraviolet isochromat spectrometer for inverse photoemission spectroscopy. The detector is based on a Geiger–Müller counting tube with acetone as filling gas and a CaF2 entrance window working as an energy selective optical bandpass at a mean energy of 9.84eV. Under this operating conditions the detector achieves an optical energy resolution of 320meV (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 9.72eV. The energy resolution improves to about 115meV (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.

Momentum resolution in inverse photoemission

“…Various examples of IPE spectra with normal electron incidence on Cu(111) with intensity ratios between SS and IS ranging from 0.32 (small ∆θ) to 1.8 can be found in the literature. 29,30,37,39,40,43 The different values primarily attest to different beam divergences. One should keep in mind, however, that besides the energy resolution and the surface quality, the photon-detection angle also plays an important role for the peak-to-background ratio of SS and IS due their z-dipole-like emission characteristics.…”

Inverse photoemission with energy resolution better than 200meV

“…[19][20][21][22] Acetone vapour in combination with a CaF 2 window provides an optical resolution of about 400 meV (FWHM) at almost the same sensitivity as the iodine-filled counter. 19 Compared with iodine, acetone turned out to be much easier to handle with respect to corrosion of the stainless steel tube leading to regular leakage problems at the sealing surfaces in the case of iodine.…”

“…Figure 4 presents the optical bandpass of our counter (black dots), measured with a VUV light source and a monochromator built by R. Stiepel. 22 The solid line shows the best fit to the data by a Gaussian function with 330 meV FWHM. In addition, Fig.…”

Note: Characterization of CaF2/acetone bandpass photon detector with Kr filter gas

“…3,4 The acetone filled detector has received more attention because of its non-corrosive nature and low dead time. [5][6][7][8][9][10] Generally, improvement of the detector optical resolution inherently decreases the detector sensitivity and hence the count rate. 10 So, the provision of tunable resolution would be useful for a trade-off between the count rate and resolution depending on a particular measurement.…”

“…8 Technically, to use the Kr gas, the detector needs a filter gas chamber in front of the acetone tube. Stiepel et al 8 used one more entrance window in the single tube detector to build the filter gas chamber. However, the complete information about the detector design is not available in their work.…”