Silver photocathode

Another alternative involves focusing the probe pulse onto the silver photocathode of an electron gun, providing a well-defined pulse of electrons through the photoelectric effect so that time-dependent electron diffraction data for shortlived intermediates may be obtained. 

Many different types of photocathodes are used in photomultipliers. With a selection of various cathodes, it is possible to cover the range of response from the soft x-ray region (approximately 5 to 500 A) to the near infrared (approximately 12,000 A). Materials and combinations used include cesium-oxygen-silver cesium-antimony cesium-antimony-bismuth sodium-potassium antimony sodium-potassinm-cesinm-antimony copper-iodine and cesium-iodine. The thermal emission at 25°C of copper iodide and cesium iodide tends to run less than the other materials. 

The equipment and the experimental procedures using the C02-methanol medium have already been described in previous papers. . For the photoelectrochemical experiments, a stainless steel pressure vessel was equipped with a 2-cm thick quartz window for illumination, p-type InP and GaAs wafers were cut into ca. 0.4 cm x 0.5 cm electrodes and were mounted using epoxy resin. Ohmic contact was made with successive vapor deposition of Zn (30 mn) and Au (100 nm), which was annealed afterward at 425 C in Ar. A silver wire (0.8 mm dia) was used as a quasi-reference electrode (Ag-QRE, ca. +80 mV vs. SCE). A Pt wire (0.8 mm dia) was used as the counter electrode. The photocathode was etched in hot aqua regia for ca. 5 s before each experiment. The electrolyte solution [3 cm, 0.3 mol dm" tetrabutylammonium perchlorate (TBAP) in CH3OH] was placed in a glass cell liner in the stainless steel vessel. Gases were introduced into the pressure vessel and were left to equilibrate for one hour at the desired pressure (1 to 40 atm). 

The operation of photocells and photomultipliers is based on the external photoelectric effect. Photons impinging on the surface of a photosensitive cathode (photocathode) knock out electrons which are then accelerated in the electrical field between the cathode and the anode and give rise to electric current in the outer circuit. The spectral sensitivity of a photocell depends on the material of the photocathode. The photocathode usually consists of three layers a conductive layer (made, e.g., of silver), a semiconductive layer (bimetallic or oxide layer) and a thin absorptive surface layer (a metal from the alkali metal group, usually Cs). A photocathode of the composition, Ag, Cs-Sb alloy, Cs (blue photocell), is photosensitive in the wavelength range above 650 nm for longer wavelengths the red photocell with Ag, Cs-O-Cs, Cs is used. The response time of the photocell (the time constant) is of the order of 10" s. 

Indeed, it has been demonstrated 88) that Cs suboxides are of essential importance in the famous infrared sensitive SI photocathode 89). These cathodes contain a thin layer of oxidized Cs on a silver substrate. It is evident from UPS measurements on such a cathode prepared in a PE spectrometer that the Cs—0 layer is essentially composed of Csj 1O3, or a higher oxidized, but still metallic suboxide, giving rise to the characteristic spectrum of the bulk material, although the layer has only the thickness of a few atoms. 88) The high yield of photoelectrons in the near infrared with the SI photocathode results from two effects CS11O3 is characterized by a sufficiently small work function (emit photoelectrons when irradiated with infrared light. Furthermore, the energy necessary to create surface plasmons... 

The photocathode material used in most commercial phototubes is a compound of cesium and antimony (Cs-Sb). The material used to coat the dynodes is either Cs-Sb or silver-magnesium (Ag-Mg). The secondary emission rate of the dynodes depends not only on the type of surface but also on the voltage applied. 

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