Spectral photosensitivity

Spectral photoconductivity. Each image device has a desirable spectral photosensitivity relevant to its purpose. For color television cameras, in particular, this range of photosensitivity should cover the whole visible spectrum and extend very little into the infrared region. 

Fig. 8. Spectral photosensitivity of the sputtering-produced a-Si H target. The target structure is Si02 (15 nm)/a-Si H (2 /rni)/Sb2S3 (60 nm).

Spectral photosensitivity is high and favorable for color imaging. Sensitivity in the green region is more than twice as large as that for conventional vidicon tubes. 

Figure 1. Spectral photosensitivity of the SegoGe2o inorganic resist. The optical absorption coefficient is also shown (2).

The detection of spectral sensitizing action often depends on amplification methods such as photographic or electrophotographic development or, alternatively, on chemical or biochemical detection of reaction products. Separation of the photosensitization reaction from the detection step or the chemical reaction allows selection of the most effective spectral sensitizers. Prime considerations for spectral sensitizing dyes include the range of wavelengths needed for sensitization and the absolute efficiency of the spectrally sensitized process. Because both sensitization wavelength and efficiency are important, optimum sensitizers vary considerably in their stmctures and properties. 

In SXAPS the X-ray photons emitted by the sample are detected, normally by letting them strike a photosensitive surface from which photoelectrons are collected, but also - with the advent of X-ray detectors of increased sensitivity - by direct detection. Above the X-ray emission threshold from a particular core level the excitation probability is a function of the densities of unoccupied electronic states. Because two electrons are involved, incident and the excited, the shape of the spectral structure is proportional to the self convolution of the unoccupied state densities. 

Studies in the photoinitiation of polymerization by transition metal chelates probably stem from the original observations of Bamford and Ferrar [33]. These workers have shown that Mn(III) tris-(acety]acetonate) (Mn(a-cac)3) and Mn (III) tris-(l,l,l-trifluoroacetyl acetonate) (Mn(facac)3) can photosensitize the free radical polymerization of MMA and styrene (in bulk and in solution) when irradiated with light of A = 365 at 25°C and also abstract hydrogen atom from hydrocarbon solvents in the absence of monomer. The initiation of polymerization is not dependant on the nature of the monomer and the rate of photodecomposition of Mn(acac)3 exceeds the rate of initiation and the initiation species is the acac radical. The mechanism shown in Scheme (14) is proposed according to the kinetics and spectral observations ... 

The synthesis of oxo-squaraines and related compounds, including their spectral properties and applications as biomedical probes, photoconducting materials, and photosensitizers are provided in a recent review [56]. 

CCD detector consists of 224 linear photodetector arrays on a silicon chip with a surface area of 13 x 18 mm (Fig. 4.16). The array segments detect three or four analytical lines of high analytical sensitivity and large dynamic range and which are free from spectral interferences. Each subarray is comprised of pixels. The pixels are photosensitive areas of silicon and are positioned on the detector atx -y locations that correspond to the locations of the desired emission lines generated by an echelle spectrometer. The emission lines are detected by means of their location on the chip and more than one line may be measured simultaneously. The detector can then be electronically wiped clean and the next sample analysed. The advantages of such detectors are that they make available as many as ten lines per element, so lines which suffer from interferences can be identified and eliminated from the analysis. Compared with many PMTs, a CCD detector offers an improvement in quantum efficiency and a lower dark current. 

There are many studies on photoconductivity in these films, many of them early ones and focused on annealed films (since air annealing is necessary for optimal photoconductivity). The use of a chemical oxidant (which never seems to be specified) gives much higher photosensitivity for as-deposited films than for films deposited without oxidant, although even here annealing is used to obtain maximum performance. Some studies on photoconductivity in as-deposited PbSe films have shown shifts in photoconductivity spectral response, with onsets shifted to 2.2 (xm instead of the ca. 4.5 xm more typical of annealed films. As with optical absorption studies, these shifts can be attributed to size quantization. 

Films were deposited from solntions of lead and tin salts (the salts used were not specified) with ammoninm acetate, ethylenediamine, and selenonrea at a pH > 9 (probably at least 11) [34]. To obtain thicker films, deposition was repeated a nnmber of times and the films were annealed therefore it is not known if solid so-Intion formation occnrred in as-deposited films. In annealed films, Pbi -j Snj Se solid solntions with x np to 0.11 were verified by XRD. The spectral response of the photoconductivity of the (annealed—as-deposited films were not photosensitive) films shifted from a peak at ca. 4 p,m (pure PbSe) to ca. 7.5 p,m (11% Sn), supporting solid solution formation of the annealed films. The room-temperature, dark resistance of the (probably annealed, but not certain) films varied from 1 to 300 kO, depending on deposition conditions. 

A large proportion of spectral data is acquired by dispersive spectrophotometry. The discussion that follows is restricted to instruments that use a diffraction grating as the principal dispersive element. The sense of the following also applies to systems that use a prism. In general, we treat systems using photosensitive detectors and fixed-position slits. Scanning is achieved by rotation of the diffraction grating. 

Thus, photosensitizers are useful mainly by being capable of extending the spectral sensitivity of certain photoinitiators under specific conditions. 

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