Absorption spectra, diffusiveness

Figure 4 UV-vis absorption spectra (diffuse reflectance) of the original unimplanted pure TiOo (a) and the Cr ion-implanted TiO (b-d). and the solar spectrum which reaches the earth (amounts of Cr ion implanted in 10 mol/g b 2.2 6.6 d 13.0).

Figure 6 UV-vis absorption spectra (diffuse reflectance) of the original undoped pure TiO2 (a) and TiO, chemically doped with Cr ions (b -e ). Cr ions chemically doped in 10" mol/g (a) undoped original pure TiOi (P-25), (b ) 16, (c ) 200, (d i 1000, (e ) 2000. The TiO2 photocatalysts chemically doped with Cr ions did not exhibit any photocatalytic reactivity.

As a contradistinction to the relatively simple case of AI2O3 Cr(III) where the color is due to a metal-centred electronic transition, we mention now on one hand the fact that the Cr(III) ion colors many transition-metal oxides brown (e.g. rutile Ti02 or the perovskite SrTi03 [15]), and on the other hand the fact that the color of blue sapphire (AI2O3 Fe, Ti [16]) is not simply due to a metal-centred transition. By way of illustration Fig. 1 shows the diffuse reflection spectrum of SrTiOj and SrTi03 Cr(III) [17], and Fig. 2 the absorption spectrum of Al203 Ti(III) and Al203 Ti(III), Fe(III) [18]. It has been shown that these colors are due to MMCT transitions and cannot simply be described by metal-centred transitions [19],... 

For a comparison of experimental Mossbauer isomer shifts, the values have to be referenced to a common standard. According to (4.23), the results of a measurement depend on the type of source material, for example, Co diffused into rhodium, palladium, platinum, or other metals. For Fe Mossbauer spectroscopy, the spectrometer is usually calibrated by using the known absorption spectrum of metallic iron (a-phase). Therefore, Fe isomer shifts are commonly reported relative to the centroid of the magnetically split spectrum of a-iron (Sect. 3.1.3). Conversion factors for sodium nitroprusside dihydrate, Na2[Fe(CN)5N0]-2H20, or sodium ferrocyanide, Na4[Fe(CN)]6, which have also been used as reference materials, are found in Table 3.1. Reference materials for other isotopes are given in Table 1.3 of [18] in Chap. 1. 

The first subnanosecond experiments on the eh yield were performed at Toronto (Hunt et al., 1973 Wolff et al., 1973). These were followed by the subnanosecond work of Jonah et al. (1976) and the subpicosecond works of Migus et al. (1987) and of Lu et al. (1989). Summarizing, we may note the following (1) the initial (-100 ps) yield of the hydrated electron is 4.6 0.2, which, together with the yield of 0.8 for dry neutralization, gives the total ionization yield in liquid water as 5.4 (2) there is -17% decay of the eh yield at 3 ns, of which about half occurs at 700 ps and (3) there is a relatively fast decay of the yield between 1 and 10 ns. Of these, items (1) and (3) are consistent with the Schwarz form of the diffusion model, but item (2) is not. In the time scale of 0.1-10 ns, the experimental yield is consistently greater than the calculated value. The subpicosecond experiments corroborated this finding and determined the evolution of the absorption spectrum of the trapped electron as well. 

In the diffuse reflectance mode, samples can be measured as loose powders, with the advantages that not only is the tedious preparation of wafers unnecessary but also diffusion limitations associated with tightly pressed samples are avoided. Diffuse reflectance is also the indicated technique for strongly scattering or absorbing particles. The often-used acronyms DRIFT or DRIFTS stand for diffuse reflectance infrared Fourier transform spectroscopy. The diffusely scattered radiation is collected by an ellipsoidal mirror and focussed on the detector. The infrared absorption spectrum is described the Kubelka-Munk function ... 

A yellow solution is formed when nitrous acid is added to thiosulphate ion in water84. This is believed to be due to the formation of nitrosyl thiosulphate [O3SSNOI, although this has not been isolated and even in solution decomposition is fairly rapid. The equilibrium constant for its formation Wxno is 1.66 x 107 dm6 mol 2 at 25 °C and the UV-visible absorption spectrum is very similar to that of other S -nitroso compounds85. The rate constant for its formation is very large and is believed to represent a diffusion controlled process. Thiosulphate ion does appear to catalyse nitrosation but, over the range studied... 

Figure 8.5. Palm of the hand. Fluorescence spectrum (Fb-mode, full line) and diffuse reflectance absorption spectrum (dashed line, normalized to X = 700 nm, reference filter paper) measured with a sensitized diode array spectrometer.

Suppose we have a pH indicator like Phenol Red whose absorption spectrum is pH-sensitivewith pKa = 7.6 (Figure 10.12). Phenol Red displays two distinct absorption spectra for protonated form (pH 2.5) and for unprotonated form (pH 10.4). One of the possible donors is an Eosin which displays an emission spectrum that overlaps with the absorption spectra of the protonated and unprotonated forms (acceptors) of Phenol Red (Figure 10.12). The critical distances for energy transfer (R0),(32) calculated from spectral properties of Eosin and Phenol Red, are 28.3 and 52.5 A for protonated and unprotonated forms of Phenol Red, respectively. For randomly distributed acceptors in three dimensions with no diffusion, the donor decay is... 

Electron transfer [Eq. (1)] would occur at a rate near the diffusion limit if it were exothermic. However, a close estimate of the energetics including solvation effects has not been made yet. Recent support of the intermediacy of a charge transfer complex such as [Ph—NOf, CP] comes from the observation of a transient (Amax f 440 nm, t =2.7 0.5 ms) upon flashing (80 J, 40 ps pulse) a degassed solution (50% 2-propanol in water, 4 X 10 4 M in nitrobenzene, 6 moles 1 HCl) 15). The absorption spectrum of the transient is in satisfactory agreement with that of Ph—NO2H, which in turn arises from rapid protonation of Ph—NOf under the reaction conditions ... 

Dissociation occurring by tunneling from a bound to an unbound rovibronic state (i.e., a state corresponding to a particular rotational energy level of a vibrational level of an electronic state). 2. The appearance of a diffuse band region within a series of sharp bands of an absorption spectrum. 

In a typical spectroelectrochemical measurement, an optically transparent electrode (OTE) is used and the UV/vis absorption spectrum (or absorbance) of the substance participating in the reaction is measured. Various types of OTE exist, for example (i) a plate (glass, quartz or plastic) coated either with an optically transparent vapor-deposited metal (Pt or Au) film or with an optically transparent conductive tin oxide film (Fig. 5.26), and (ii) a fine micromesh (40-800 wires/cm) of electrically conductive material (Pt or Au). The electrochemical cell may be either a thin-layer cell with a solution-layer thickness of less than 0.2 mm (Fig. 9.2(a)) or a cell with a solution layer of conventional thickness ( 1 cm, Fig. 9.2(b)). The advantage of the thin-layer cell is that the electrolysis is complete within a short time ( 30 s). On the other hand, the cell with conventional solution thickness has the advantage that mass transport in the solution near the electrode surface can be treated mathematically by the theory of semi-infinite linear diffusion. 

---