External photoeffect

Although measurements of the photoelectron emission from organic solids began about 1910, and the first measurements of the external photoeffect from dye films date from the 1930 s,39 precise measurements... 

The kinetic spectrum reveals more peculiar details than the usual spectral efficiency curve of the external photoeffect. For example, the kinetic spectra for pinakryptol and pinacyanol are quite different, whereas the efficiency curves are much alike.45... 

Afanas ev AM, Kohn VG (1978) External photoeffect in the diffraction of X-rays in a crystal with a perturbed layer. Sov Phys JETP 47 154-161... 

Totally absorbing material is heated by incident irradiation (bolometer). A relationship between intensity and temperature effects exists. Since temperature measurement are very sensitive, this effect is used to determine intensities. Semiconductor detectors either use the internal or external photoeffect [117,118]. In a photodiode, an incident photon causes a photocurrent by charge separation. It can be amplified and depends linearly on the number of incident photons. 

In photocathodes or photomultipliers the incident photons force electrons to leave the material. This external photoeffect can be calibrated and amplified by acceleration and multiplication of the electrons in multi-electrode arrangements (dynodes). These devices have very short response times and can be successfully used to control the stability of a light source. Therefore such devices are frequently included in commercially available set-ups. An example is given in Fig. 4.32 combining an irradiation source with a measurement set-up. This is commercially available [119] and allows simple control of a photoreaction. However, due to geometry and inhomogeneity of the sensitive layer of the photodiode, non-homogeneous irradiation can cause errors. 

Stark and Steubing [2] in 1908 were the first to investigate the relationship between fluorescence and the photoelectric eflect (i.e. the external photoeffect), which a few years before had been explained by A. Einstein using the quantum hypothesis of light, i. e. the concept of photons. They carried out their experiments on a large number of different organic solids. This included benzene, naphthalene, and anthracene, but also many other aromatics with N- or OH-substituents. The following quote from one of their publications is still valid today ... 

The third of the principal photon effects is the photoemissive one, also known as the external photoeffect. As the name implies, the action of the incident radiation is to cause the emission of an electron from the surface of the photocathode into the surrounding space, there to be collected by an anode. The spectral responses of selected photocathodes are illustrated in Fig. 27. 

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 photocurrent-voltage curve of a cell made with the I /I2 redox couple (Fig. 8) shows behavior typical of the standard DSSC. The substantial photovoltaic effect is expected from the fact that the dark current (Fig. 4) is negligible positive of about -0.5 V. On the other hand, a cell made with the FcCp2 70 redox couple shows no measurable photoeffect Its current under illumination (Fig. 8) is essentially equal to its dark current (Fig. 4). The photovoltaic effect is negligible because practically all photogenerated charge carriers recombine before they can be collected in the external circuit. In general, fast rates of reactions (4) and (5) tend to eliminate the photovoltaic effect in DSSCs. 

All photoeffects involve the absorption of photons to produce an excited state in the absorber or liberate electrons directly. With the direct release of electrons, photoemission may occur from the surface of solids. While the excited state may revert to the ground state, it may proceed further to a photochemical reaction to provide an electron-hole pair (exciton) as the primary photoproduct. The exciton may dissociate into at least one free carrier, the other generally remaining localized. In an externally applied electric field, photoconduction occurs. Photomagnetic effects arise in a magnetic field. Absorption of photons yield photoelectric action spectra which resemble optical absorption spectra. Photoeffects are involved in many biological systems in which charge transfer takes place (e.g., as observed in the chlorophylls and carotenoids) [14]. 

Photoeffects are also observed at metal electrodes, although the resulting photocurrents are much smaller. For example, the irradiation of a metal electrode can cause the photoejection of an electron into the solvent. If this electron is scavenged by some reactant in solution, a net cathodic photocurrent results (76, 77) (see Section 18.3 and Problem 8.10). These electron photoejection studies are of interest, because they can provide information about the nature of an electron at the instant of injection into a medium, as well as the energetics and kinetics of its relaxation to equilibrium solvation. Excitation of dyes adsorbed on metals can also lead to photocurrents, but they are usually much smaller than the photocurrents obtainable at semiconductor electrodes under comparable conditions (74). This low efficiency of net conversion of photons to external photocurrent is attributed to the ability of a metal to act as a quencher of excited states at or very near the surface by either electron or energy transfer (18, 19). 

As was true for that of photoeffects, the objective of this discussion of noise mechanisms is to acquaint the reader with the broad concepts of noise in detectors without deriving in great detail the appropriate equations. See Van Vliet [2.141] for a detailed treatment. Nevertheless, it will be necessary to present certain equations which describe the dependence of noise upon internal material parameters and external system parameters. The discussion will consider initially noise in semiconductor detectors, followed by noise in photoemissive devices. 

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