Photocathode system,

Bokris and Uosaki (1) have studied transient photo-assisted electrolysis current for systems including a p-type semiconductor photocathode and dark Pt anode. A set of current vs. time scans taken with a ZnTe photocathode system is shown in Figure 6. 

Because tenfold-lower emittance beams can be generated from photocathodes compared with those from conventional thermionic guns, photocathode systems have significant advantages in efficient transportation of the charge from the gun to the radiolysis target and in magnetic pulse compression elfectiveness. 

Photoelectrochemical studies with ternary chalcogenide systems containing zinc as one of the components have been published however, such investigations on bulk or thin film binary ZnS and ZnTe electrodes are practically absent from the literature or may be found fragmentary in electrosynthesis-oriented works. ZnTe has been studied as a possible candidate for a photocathode in the photoelectrochemical production of hydrogen. Related information will be given in the relevant section. 

Figure 44. Schematic of electron beam image projection systems (a) image formed by electrons emitted from a photocathode, and (b) image formed by an absorbing mask pattern on an electron transparent substrate.

The 5 ns pulses of about 10 electrons released at the anode by a photon absorbed by the photocathode of a PM tube can be used to count photons. In such instruments the intensity of light is displayed as a count per second which varies between about 15 (dark count) and 105. A photon-counting detector system is of course much more complex than the simple PM/ampli-fier used in conventional spectrofluorimeters. Figure 7.27(a) is a block diagram of such a photon counter (b) gives a simple illustration of the important process of pulse selection through a discriminator. The output of... 

The photocurrent onset potential is often taken as the flatband potential, since the measurement of the flatband potential is typically only good to 100 mV and the onset of photocurrent is often observed with less than 100 mV of band bending. This practice is dangerous, however, since the onset potential is actually the potential at which the dark cathodic current and the photoanodic current are equal. Even though in the case of the p-GaP illustration, the observation of an anodic current and a photocathodic current are separated by several hundred millivolts, in many systems these two currents overlap. In those cases, the relationship between the flatband potential and the onset potential becomes unclear. 

Takahashi and co-workers (69,70,71) reported both cathodic and anodic photocurrents in addition to corresponding positive and negative photovoltages at solvent-evaporated films of a Chl-oxidant mixture and a Chl-reductant mixture, respectively, on platinum electrodes. Various redox species were examined, respectively, as a donor or acceptor added in an aqueous electrolyte (69). In a typical experiment (71), NAD and Fe(CN)g, each dissolved in a neutral electrolyte solution, were employed as an acceptor for a photocathode and a donor for a photoanode, respectively, and the photoreduction of NAD at a Chl-naphthoquinone-coated cathode and the photooxidation of Fe(CN)J at a Chl-anthrahydroquinone-coated anode were performed under either short circuit conditions or potentiostatic conditions. The reduction of NAD at the photocathode was demonstrated as a model for the photosynthetic system I. In their studies, the photoactive species was attributed to the composite of Chl-oxidant or -reductant (70). A p-type semiconductor model was proposed as the mechanism for photocurrent generation at the Chi photocathode (71). 

Taking a general view of the above studies, we note that Chl-coated metal (platinum) electrodes commonly function as photocathodes in acidic solutions, although the photocurrent effcien-cies tend to be lower compared to systems employing semiconductors. This cathodic photoresponse may arise from a p-type photoconduc-tive nature of a solid Chi layer and/or formation of a contact barrier at the metal-Chl interface which contributes to light-induced carrier separation and leads to photocurrent generation. 

A review of photo-assisted electrolysis studies performed with p-type semiconductor photocathode/dark Pt anode systems suggests that a complementary phenomena arising from the presence of OH ions produced during the reduction half-cell reaction,... 

These data show that for the image dissector there is excellent agreement between experimental and theoretical resolution and that the resolution of the image dissector system is about twice as good as that of the vidicon system. The deviation of the resolution of the image dissector from theoretical at 7024.05 A may be attributed to curvature of field in the reduced image of the spectral focal plane because the line is near the edge of the photocathode. 

An important test of a transient spectrometer s accuracy, stability, and noise level is provided by a baseline spectrum. This is an averaged spectrum obtained in exactly the same way as actual data, except that the ultraviolet excitation beam is kept blocked when it would otherwise be open. The lowest trace shown in Figure 2 is a typical 5-cycle baseline spectrum for our system. Systematic deviation from zero is less than 0. 01 absorbance units throughout, and the r. m. s. noise level varies from 0. 03 near the edges to 0. 007 near the center of the spectrum. These noise variations are inversely related to the detected single beam intensity spectrum, which drops on the blue side because of the continuum distribution and the transmissive properties of our beam combiner, and on the red side because of the photocathode response of the SIT detector head. A high degree of intensity linearity in the OMA is necessary for our... 

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