Intrinsic Photoemission of Alkali Halides

Abstract

The photoemission from evaporated films of 12 alkali halides has been measured in the 10-to 21-eV energy region. In addition, some preliminary optical-density values were obtained by measuring the transmission of evaporated layers of these compounds deposited on 450A-thick, self-supporting, AI2O3 films. The photoemission yield curves show broad structures correlated with the absorption coefficients. In general, the photoemission is a maximum for photons of energies of about 5 eV greater than the forbidden-gap energy E e , with additional peaks beyond 2 E g . The correlation of the yield with the absorption coefficient is most pronounced in the alkali fluorides, which have relatively low yields, but is less apparent in the other halides, which have peak values of as much as 0.8 electron/photon. A random-walk model of electron scattering is used to estimate the magnitude of the yields and the dependence on the absorption coefficient, with good qualitative agreement with the experimental results, but other processes cannot be ruled out.

Absolute Photon Counting in the Ultraviolet

“…5.1 that the photoelectric yield of a high workfunction metal or semiconducting cathode decreases gradually with de creasing wavelength and approaches a yield of about 5% near 300 Á. Measurements of photoelectric yields exceeding 50% have been reported for the alkali halides and the alkali-earth halides for wavelengths below about 1000 Á (Lukirskii et al, 1960a,b;Duckett and Metzger, 1965;. Except for MgF 2 , these materials are not satisfactory for use as cathode materials in windowless photomultipliers, because they are not stable when exposed to air.…”

“…Coating the cathode with MgF 2 and il luminating the cathodes at grazing angles can increase the yields signifi cantly, but this is not always convenient, and care must be taken to prevent contamination of the coated cathodes. On the other hand, thin-window, gas-flow GM counters and proportional counters, because of their high efficiencies, are particularly well suited for absolute measurements of low-level steady-state fluxes at wavelengths shorter than -300 Á. Α. GEIGER COUNTERS Ederer and Tomboulian (1964) have established that GM counters can be used as absolute photon counters at wavelengths below -300 Á. When rare gases are used as counter gases at wavelengths shorter than their ionization thresholds, the absorption of one photon within the counter produces at least one electron either by photoionization or Auger pro cesses.…”

“…The outer wall of the counter is machined to accept a window support to position the window close to the inner cylindrical wall of the counter. This feature was introduced in the GM counter developed by Ederer and Tomboulian (1964) and is necessary to minimize the dead space between the counter window and the active volume of the counter, where electrons can be lost. This reentrant window design should be incorporated in any GM counter used as an absolute photon counter between about 30 and 300 Á.…”

“…A convenient gas han dling system is illustrated in Fig. 5.20 and is similar in operation to that described by Ederer and Tomboulian (1964) system external to the vacuum chamber are panel mounted for ease of operation. Copper or stainless steel tubing is used for the control system.…”

Internal-grating Electronographic Spectrographs for the Far-ultraviolet and X-Ray Wavelength Ranges

“…Angular dependence of quantum yield of a tungsten photocathode at four wavelengths in the far ultraviolet 6 . 6. Quantum yields of CsI photocathodes in the far ultraviolet (500-1000 A) 7 ' 8 and in the soft X-ray region (15-30Q A)''.…”