Photocathode materials

Wavelength dependence of the radiant sensitivity of various photocathode materials. S-1 = AgOCs,... 

The very low work functions of alkali metal suboxides suggest interesting chemical and physical applications. The reducing power of the suboxides should be even stronger than that of the pure metals Rb and Cs. But no chemical experiment has been performed so far to prove this assumption. From a physical point of view the low work fimctions make Cs suboxides very interesting photocathode materials. 

In the VUV. two possibilities exist. First MgF or LiF windows can be used with a variety of photocathodes, mostly of the solar-blind type. Second, the photocathode material can be directly deposited on the input of the MCP. For instance, Csl, KBr, and CsTe photocathodes are good to approximately 100 nm, whereas LiF, BaFj, MgF , and CaFj respond below 100 nm. Third, the channels themselves respond directly to photons of energies below approximately 100 nm. 

MicroChannel plates are becoming widely used in spectroscopy and two dimensionaj imaging at EUV (100 - 1000 A) and soft X-ray wavelengths (10 - 100 A) for astronomy and microscopy. Although bare microchan-nel plates have low sensitivity (5% - 10% quantum detection efficiency) in this spectral region, use of photocathodes can increase substantially (to 30%-40%) microchannel plate performance. This, combined with the high spatial resolution (<50/rm), fast time response (<300 ps), and large effective area (up to 100 mm diameter) achievable with microchannel plates make the latter a very attractive and versatile tool. We discuss the properties of microchannel plates, photocathode materials, and various microchannel plate detector readout schemes that have been used for soft X-ray and EUV detection. 

Regarding the nature of the active surface composition of the bare MCP s, detailed investigations have been made by Panitz et al. (13) and Siddiqui (20). These show that, although the MCP faces are coated with an electrode material (Ni, Cr, nichrome), there is a thin (100 A) top surface layer rich in potassium (20) that has been transported up from the underlying glass. The inner surfaces of the channels also have a surface layer which is rich in alkali (primarily K) metals (oxides), Si, and SiC>2 (13, 20). These surface layers, in addition to the composition of the bulk glass (PbO + Si02, predominantly), determine the QDE (19). Improvements in the QDE may, however, be obtained by the use of photocathode materials deposited on the MCP surface, which will be discussed later in this paper. 

Photocathodes may be used in two configurations, opaque (or reflection) photocathodes and transmission photocathodes. Detailed calculations of the performance of both types of photocathode may be found in the articles of Henke (43-45). Opaque photocathodes usually are deposited directly onto the input face of the MCP such that the photocathode material penetrates a short distance into the channels. Thus incident radiation which enters the channels strikes the photocathode material, resulting in enhanced detection efficiency. As in the case of a bare MCP, radiation striking the interchannel web is not normally detected, so electron deflection grids in front of the MCP are sometimes used to further the enhancement of detection efficiency. 

MgF2 has been used extensively as an opaque photocathode material for the EUV and X-ray regions Q, 48, 55-59). This is due to its stability in air (55-57) and its ability to increase the quantum efficiency of bare MCP s by up to a factor of four. 

Figure 63 Emission spectra of Nal(Tl), CsI(Tl), CsI(Na), and anthracene, compared to the spectral response of two photocathode materials. PMT, photomultiplier tube (from Harshaw Research Laboratory Report, Harshaw Chemical Company, 1978).

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. 

Photometer, Fig. 1 Photomultipliers are constructed from a glass vacuum tube which houses a photocathode, several dynodes, and an anode. Incident photons strike the photocathode material which is present as a thin deposit on the entry window of the device, with electrons being... 

The most critical aspect of the design of a photoelectrochemical device for water splitting is the choice of suitable photoanode and/or photocathode materials. Several of the requirements imposed on these materials appear to be in conflict, and certain trade-offs have to be made. In some cases, these trade-offs can be avoided by adopting smart architectures and materials combinations. The remainder of this chapter gives a few general considerations and approaches more detailed accounts of strategies to cope with these trade-offs are given in later chapters. 

Most of the requirements for suitable water-splitting photoanode and/or photocathode materials have already been alluded to in the previous sections of this chapter. They can be summarized as follows [81] ... 

Fig. 5.6. A possible model for a single microerystal of S-1 (AgCsO) photocathode material. The band structure for the (CsO) layer is as in Fig. 5.3, while that for a coated particle of Ag is similar to Cs in the same figure. A heterojunction barrier occurs between the two materials...

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