Alkali photocathodes

The application of NIR luminescent materials depends on the availabiUty of robust, cost-effective excitation sources and light detectors for such materials. For NIR luminescent lanthanide complexes, it is especially the relatively limited choice of accessible detectors that has slowed their investigation and further development. Most standard spectrofluorimetric and microscopic equipment are equipped with detectors that are mainly sensible in the visible. An extension of the spectroscopic sensitivity of this equipment farther into the red usually does not go to wavelengths longer than 850 nm (the limit of typical multi-alkali photocathodes). We will briefly discuss some recent developments in excitation sources and detectors that are applicable in particular to NIR luminescent lanthanide complexes. 

Hamamatsu Photonics K.K. Alkali photocathode, http //www.hamamatsu.com/jp/ja/ technology/innovation/index.html (2013). Accessed 18 Sept 2014. 

A vapor generator for the successive deposition of Na, K and Cs on photocathodes without emission of solid particles may be used to prepare alkali-metal-gold compounds. ... 

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. 

XPS (ESCA) and UPS are powerful tools to determine electronic energy levels by irradiating a sample with monochromatic X-rays and UV light, respectively, and measuring the energy distribution of the emitted electrons. With alkali metal suboxides UPS provides a quantitative proof of the bond model. Furthermore, the UPS results offer an explanation for the low energy photoemission process with oxidized Cs and thus (unexpectedly) open up a field of applied research with infrared sensitive photocathodes. The discussion of the results will be focussed on the chemical bonding. 

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. 

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. 

The spectral response of a photomultiplier tube varies with the coating materials used on the photocathode. Spectral responses of various photomultiplier tubes are given in Table 6-3. Chapter 6 also includes a general discussion of photomultiplier phototubes. The 1P28 tube (S-5 response) is sensitive from 2000 to 6500 A and is frequently used for atomic absorption spectroscopy. The Hamamatsu R106 also has an S-5 response but uses a silica window to lower the usable short wavelength response to about 1700 A. The S-20 response of the RCA 4459 permits measurements to 8500 A and is very useful for most of the alkali metals. 

The exit radiation is measured in a secondary electron multiplier (SEM) used as a detector as the photons hit the photocathode. The latter usually consists of alkali-metal alloys, and is of varying sensitivity, depending... 

Thin composite layers of CS2O and Cs on Ag play an important role in the IR sensitive SI photocathodes which have been manufactured for approximately sixty years. [245, 246] The characteristic spectral response of such cathodes can be explained by allowing for the presence of alkali metal suboxides, CsnOj or CS3O , since these have the appropriate electronic properties. Thus, these materials have a low work function of approximately 1 eV (compared to 2 eV for elemental Cs) and low energies of the surface plasmons (1.5 eV for (IIS11O3). The enhancement of the photoelectric yield is due to surface plasmon decay. [247]... 

Photoemissive detectors, such as the photocell or the photomuliplier, are based on the external photoeffect. The photocathode of such a detector is covered with one or several layers of materials with a low work function 0 (e.g., alkali metal compounds or semiconductor compounds). Under illumination with monochromatic light of wavelength X = cfv, the emitted photoelectrons leave the photocathode with a kinetic energy given by the Einstein relation... 

The most commonly used photocathodes are metallic or alkaline (alkali halides, alkali antimonide or alkali telluride) cathodes. The quantum efficiency rj = is defined as the ratio of the rate of photoelectrons to the rate of incident photons... 

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