Photoelectric current

Figure 6. Scanning electron micrograph (SEM) of n-GaAs surface electrochemically etched with a scanning electrochemical and tunneling microscope (SETM). Etching was accomplished in Aq. 5 mU NaOH, 1 mM EDTA. Photoelectric current - 0.7 /iA, Scan rate - 0.1 /tm/sec, bias voltage — 4 V. Tip was moved in an "L" pattern. Reproduced with permission of Ref. 89. Copyright 1987 The Electrochemical Society Inc.

If the generation process is characterized by the primary quantum efficiency rj, the transport properties by the mobility /1, and the recombination by the free-carrier liefetime t, the steady-state photoelectric current can in the case of mobile electrons and immobile holes generally be described by Eq. (13) 14> ... 

Fig. 22. Amount of hydrogen taken up by potassium (curve a) and photoelectric current of the layer (curve b) as a function of the time of exposure to hydrogen atoms 205).

The explanation of these phenomena is that at room temperature the mobility of the potassium atoms of the surface of the metal is high enough to cause a migration of these atoms onto and over every area of surface hydride which is formed by the take up of hydrogen atoms. Consequently the photoelectric current rises in direct proportion to the number of atoms adsorbed and thus with time (Fig. 22). The photoelectric cathode which is formed by these phenomena may be represented by the symbol 206) ... 

When oxygen is acting on cesium at room temperature the mobility of the cesium atoms is so high that a polyatomic layer of cesium atoms forms on top of any oxide layer that has been formed. We may also say that, apparently, the cesium oxide is absorbed by the metal, i.e., it is dissolved in the cesium. The appearance of the surface is unchanged, and so is its photoelectric behavior. Only after the cesium has been almost completely oxidized on continuous supply of oxygen, does a very thin adsorbed cesium layer appear on the cesium oxide and does the photoelectric current temporarily rise sharply until, on further supply of oxygen, also these last cesium atoms are converted into oxide. 

The mobility of potassium is smaller than that of cesium. The exposure of potassium metal to oxygen at room temperature leads to a complete oxidation of the metal, but, unlike cesium, the potassium atoms, during the oxidation, do not form a polyatomic layer on top of the oxide formed, but a layer of only one-atom thickness. Consequently the photoelectric current rises and continues to rise with the amount of oxide formed until a maximum is reached—when 4 X 10-4 g. of oxygen has been taken up/cm.2 of potassium surface 208)—after which it decreases... 

Photoelectric current is defined as a charge flux in the semiconductor channel... 

Then we substitute G ff,h) from Eq. (123) into the expression (113) for photoelectric current. Thus, the following expression for the current can be obtained ... 

Finally, the photoelectric current can be found from Eq. (124) where the QD Green s functions are now the solution of different Dyson equations (127) ... 

Figure 8. A copy of the 1887 publication by J. Moser on the amplification of photoelectric currents by optical sensitization of silver halides by erythrosin [7]. The system described in this paper was indeed the first dye-sensitized photovoltaic solar cell.

The most versatile and convenient method to overcome flame emission applicable to all elements is the modulation of the light emitted by the hollow cathode. This can be accomplished by running the lamp on alternating current or by interrupting the light beam mechanically with a rotating disk. The photoelectric current from the photomultiplier tube is then amplified by an a.c. amplifier. The d.c. component of the output of the photomultiplier tube, deriving from the continuous flame emission. 

Based on the oriented dipole and the interfacial proton transfer mechanism described above, an equivalent circuit was established which described the relaxation time course of a photoelectric current generated by a single chemical reaction step of charge separation and recombination regardless of whether the charge separation is confined within the membrane or takes place across a membrane-water interface [21]. This equivalent-circuit analysis is notable for the absence of any adjustable parameters each and every parameter used for the computation can be measured experimentally (figure 10.2). An example of the... 

In the example just cited, the photoelectric signal is capacitance coupled (AC coupled) to the metal electrode. The system possesses no interfacing problem. However, a number of investigators have observed that no DC photoelectric current can be observed from BR-coated metal electrode sensor systems. In... 

Helium gas at a pressure of a few microns inside a waveguide is subjected to electron bombardment. Of the small number of ions excited to the 22P and 22S% levels, those in the former uecay immediately. An excess accumulates in the 22S level because of its metastability. Transitions from 22 j. to 22P are induced by the radiofrequency fields in the waveguide, and are detected by observation of an increase in the 41 eV photoelectric current. As in the hydrogen experiment, the interaction region is situated in a magnetic field which can be varied through a resonance line, while the microw ave frequency remains constant. 

Experimentally, a sample of the compound is irradiated in the gas phase by an ultraviolet light source. The emitted electrons are sorted by means of electric fields according to their kinetic energy and counted. The photoelectron spectrum is then obtained by plotting this count rate (intensity of the photoelectric current) against the energy difference /iv-JS kin (e) of equation 3. This is depicted schematically in Figure 1. 

Meanwhile, the photoelectric effect was a focus of scientific interest, with a history synchronized with that of photography. The concept of dye enhancement of the photoeffect was carried over from photography to the sensitization of an electrode already in 1887 by Moser [5] (Fig. 2) using the dye ery-throsin on silver halide electrodes and confirmed by Rigollot in 1893 [6]. In the retrospectively quaint report, written substantially before the Einstein theory of the photoelectric effect, Moser records his observations on dye-induced enhancement as an increased photopotential [V] rather than the more fundamental current (A), despite the title - Strengthened Photoelectric Current Through Optical Sensitization. ... 

The total average fundamental-repetition-frequency two-quantum photoelectric current W, for different intensity ratios of the two constituent beams (11/I2), is presented in Fig. 7.2. The solid curve represents the equation -1-/2) which is simply the parabola + when the intensity ... 

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