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Ultraviolet photoelectron spectra of Cs evaporated onto Ag and oxidized under ultrahigh-vacuum conditions have been measured. Time variation of the spectra indicates the formation of Cs snboxides (e.g. , Cs,O, Ca&~03). The high infrared photoelectric yield of Ag/0/Cs cathodes can be explained as due to the excitattion of surface plasmons in these compounds.The infrared sensitivity of Ag/0/Cs photocathodes has been interpreted in terms of different mechanisms, such as defects in the lattice of Cs,O formed during the production process of Ag/0/Cs cathodes, " as well as photoemission from silver. "~A ll of these interpretations generally link the photoemission in the infrared region to the low work function of one of the cesium oxides. ' Cathodes with an excess of cesium (compaI'ed to CssO) show 'the highest infrared yield. Composition and thickness of the Ag/0/Cs layers as well as the electron emission process are still a matter of dispute. Bulk samples of metalrich cesium oxides (suboxides, e.g. , Cs,O, Cs»0, ) have recently been investigated by ultraviolet photoelectron spectroscopy (UPS). ' lt has been shown that the spectra reflect compositions and crystal structures of the bulk compounds. 'We report here UPS measurements of thin films of cesium deposited on a silver substrate and oxidized under ultrahigh-vacuum (UHV) conditions, simulating the a,ctive layers in Ag/0/Cs (S-l) photocathodes. The spectral response of films prepared in this way is similar to S-1 cathodes according to Heimann et al. ' The UPS data of these films are almost identical to those of the bulk cesium suboxides. ' A comparison with bulk data yields information about the composition of the films, the layer thickness, and the low-energy photoelectron emission process.The measurements were performed at room temperature in a Leybold-Heraeus LHS 10 ESCA system equipped with a preparation chamber and a diff erentially pumped cold-cathode gas-discharge lamp using the Hei resonance line (It v =21.2 eV). The residual gas pressure in the analyzer chamber was 2x 10 " Torr. Evaporated silver served as a substrate and was etched by argon-ion bombardment, The photoelectron energy distribution curve of this sample is shown in Fig. 1(a). The energy scale is relative to the Fermi edge; the resolution is 0.1 eV. The photoelectron spectrum of Cs evaporated for 3 min at 390 K onto the Ag substrate at room temperature is shown in Fig. 1(b). Oxygen was leaked into the analyzer chamber (basic pressure p = 2x 10 " Torr) at a pa, rtial pressure of 5&& 10 ' Torr for ft hy=21. 2 eV \ I t -120rnin I~x 1 CL BINDlNG ENERGY f.eV] FIG. 1. Photoelectron energy distribution curves for silver, a cesium layer on silver, and reaction products vrith oxygen.

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The change in the surface composition of an AI/Ni alloy (SO0/o by weight Ni) on heating in O2 and treatment in diluted NaOH was investigated by quasi-simultaneous XPS-ISS measurements, using a hemispherical condenser analyser both for ESCA and ISS. The oxidizing experiments were carried out in a valveless, integrated, high pressure, high temperature preparation lock.

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Geiger-counters with low filling pressure can be used for the registration of ions. The probability of registration of a counter filled with iodine vapor is studied for rare gas-ions and alkali-ions in the energy-range of 0.3 to 3.0 keV. All ions used are found to be registrated at least with a probability of 90%, if they got an energy of 1.8 keV.