Dark current, 236 scanning

Tabulate values of scattered intensity, corrected for the dark current, for each scan and 0 = 30,. .., 150°. Include values for the reference. Correct all readings for differences, if any, in amplifier gain or filters used. Tabulate as in Refs 15, 16, and 41... 

The Effect of Illumination. In an alkaline solution, an n-GaP electrode, (111) surface, under illumination shows an anodic photocurrent, accompanied by quantitative dissolution of the electrode. The current-potential curve shows considerable hysterisis as seen in Fig. 2 the anodic current, scanned backward, (toward less positive potential) begins to decrease at a potential much more positive than the onset potential of the anodic current for the forward scanning, the latter being slightly more positive than the Ug value in the dark, Us(dark). 

The slow-scan CCD, also called the scientific CCD, or in the spectroscopy literature simply CCD, is the detector of choice for most applications of Raman spectroscopy. A well-designed CCD has essentially zero dark current, very low readout noise, and high quantum efficiency (peak 45—70% near 700 nm) in the visible region of the spectrum. However, the response drops quickly above 800 nm and there is no photon response above 1.05 J m. For routine spectroscopy or process control, thermoelectrically cooled (to about —40° C) CCDs are adequate. Although these detectors are somewhat noisier than detectors operated at —100° C or lower, the former do not require liquid nitrogen cooling. The general properties and spectroscopic applications of the CCD have been reviewed (22). 

A spectrum in a specified ranalogue signals from eadi photodiode are digitised and transferred to a computer, where they e corrected for dark current response and transformed to absorbance. A number of digital techniques are available to increase sensitivity and to extend the use of rapid-scanning detectors to multicomponent analysis, reaction kinetics, tablet dissolution tests, process control, and detection in HPLC (A. F. Fell et al, Chrom-atographia, 1982, 16, 69-78). 

FIGURE 6.19. Dark current-potential curves of a p-type Si(lOO) RDE (a) and an n-type Si(lOO) RDE (b) in a lOM HE + 0.5M HBr solution with and without Br2 (10mM). The scan rate was 50mV/s and the rotation rate was 400rev/min. (Reprinted from Bressers et al. 1996, with permission from Elsevier Science.)... 

Diode array readout (scanning) noise and other electronic noises such as Johnson thermal noise In a readout resistor and photodetector dark current noise belong to this category. 

Since our main objective was to remove all the chlorine and hydrogen atoms from the polymer chain, C-PVC films were further exposed to the UV radiation of the medium pressure mercury-lamp. This led to a dark brown material w.hich was found to be unable to carry an electrical current, even after extended irradiation time. Therefore we turned to a powerful laser source, a 15 W argon ion laser tuned to its continuous emission at 488.1 nm. At that wavelength, the degraded polymer film absorbs about 30 % of the incident laser photons. The sample was placed on a X-Y stage and exposed to the laser beam at scanning rates in the range of 1 to 50 cm s, in the presence of air. 

Figure 8. STM image of the basal-plane surface of HOPG obtained in 1 M NaCl. Bias = 550 mV (tip +), it = 1 nA. Image was obtained in the fast scan mode. Dark spots, separated by 2.5 A, correspond to large tunneling currents.

The formation of pores during anodization of an initially flat silicon electrode in HF affects the I-V characteristics. While this effect is small for p-type and highly doped n-type samples, it becomes dramatic for moderate and low doped n-type substrates anodized in the dark. In the latter case a reproducible I-V curve in the common sense does not exist. If, for example, a constant potential is applied to the electrode the current density usually increases monotonically with anodization time (Thl, Th2]. Therefore the I-V characteristic, as shown in Fig. 8.9, is sensitive to scan speed. The reverse is true for application of a certain current density. In this case the potential jumps to values close to the breakdown bias for the flat electrode and decreases to much lower values for prolonged anodization. These transient effects are caused by formation of pores in the initially flat surface. The lowering of the breakdown bias at the pore tips leads to local breakdown either by tunneling or by avalanche multiplication. The prior case will be discussed in this section while the next section focuses on the latter. 

Figure 5. Current—voltage curves of a TiOs electrode in 1.0M KOH with 200 mV/s scan from 0 V (SCE). Curve 1, in dark Curve 2, with 350-run light on Curve 3, in dark after 30 s illumination with 350-nm light atO V (SCE) (28).

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. 

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