Directional light absorption

Hirota used doped crystals to observe weak Ti-<- So absorption spectra by phosphorescence excitation spectroscopy. Triplet excitons of the host are formed by direct light absorption. The guest molecules, chosen to have lower triplet energy, act as traps and emit guest phosphorescence. 

Until now we had been talking of gas reactions. Many substances undergo photochemical reactions in liquid state. Again the reaction in initiated by Stark-Einstein law by direct light absorption on the part of reactants. However, it maybe anticipated that quantum efficiency of these reactions will be less than for the same reaction in the gas phase. The reason for this is that in the liquid state an active molecule may readily be deactivated by frequent collisions with other molecules. Furthermore, because of the very short mean free path in the liquid phase free radicals or atoms when formed photochemically will tend to recombine before they have a chance to get very far from each other. The net effect of these processes will be to keep the quantum yield relatively low. In fact, only those reactions may be expected to proceed to any extent for which the primary products of the photochemical act are relatively stable particles. Otherwise the active intermediates will tend to recombine with the solvent and thereby keep the yield low. 

Types of Halocarbon Photoreactions. In the following sections, we discuss several types of halocarbon photoreactions that provide sinks in aquatic environments. Emphasis in this chapter is placed on photoreactions that result in dehalogenation, in which halide ions (X ) are produced from halocarbons (RX). Here the term direct photoreactions indicates reactions that involve direct light absorption by the halocarbon itself (eq 4). 

Even if direct light absorption as above does not occur, polymerization can still be initiated if photosensitizers are present that produce free radicals when they absorb ultraviolet or visible light. The same substances that are used for thermal initiation are often used for photosensitization. For example, azo compounds and peroxides are photosensitizers, and the photoinitiation reaction is the same as is the thermal initiation process, described earlier in this chapter. However, the photoinitiation can take place at much lower temperatures than in the thermal initiation by the same initiators. Moreover, many initiators can be used as photosensitizers even though they do not dissociate thermally at convenient rates or temperatures to be useful as thermal initiators. For example, azoisopropane does not dissociate sufficiently rapidly below 180°C to be useful thermal initiator. However, it photodissociates even at low temperatures when irradiated with near-ultraviolet light ... 

Even if direct light absorption by nionomer as described above does not occur, polymerization can still be initiated by the use of photosensitizers. Being highly photosensitive, a photosensitizer (S) gets readily activated on exposure to light to... 

The figure also illustrates that a thexi state can be obtained in several ways (a) spectroscopically by direct light absorption, (b) by thermal activation or the reverse of radiationless deactivation, (c) by chemical reaction (as must be true in chemiluminescent processes), and (d) by photochemical reaction (as in chemical laser systems). Viewed in this light, both photochemical and photophysical processes (other than emission and light absorption) leave their usual spectroscopic milieu to join the broad field of reaction kinetics. 

A comparison of the spectral qualities of the test compounds and those of aqueous CRM-1 Indicates that the Indoles underwent sensitized photolysis In the aqueous CRM-1 matrix. The matrix absorbed significantly at wavelengths below 320 nm, which Is the Important region for direct photolysis of the Indoles studied. Since direct light absorption was significantly diminished by the matrix while observed photolysis rates were enhanced, the Indoles probably underwent sensitized photolysis Initiated by other components In the matrix. Such a process could account for the Increased photolysis rates for Indoles In the matrix. 

If one wants to use p-type semiconductors where the water reduction can be obtained by illumination, one suffers from the small band gaps and has to apply an auxiliary external voltage. Such systems could be used for a photoassisted electrolysis where only part of the power needed comes from the direct light absorption in the semiconductor electrode. The additional voltage could be taken from a photovoltaic cell coupled in series to the electrolysis cell. A p-type tungsten selenide electrode appears suitable for this purpose because its conduction band is located somewhat above the hydrogen potential in acidic solution. 

Reactive MMCT states cannot only be generated by direct light absorption into MMCT bands but also by other processes [60]. The absorption spectrum of (NH3)5Co -pyz-Fe (CN)5 (pyz = pyrazine) does not display a MMCT bad which seems to be obscured by an intense Fe(II) pyz MLCT absorption. Nevertheless, MLCT excitation leads to the same products which should be obtained by direct MMCT excitation. 

The observation of excited states that cannot be easily produced by direct light absorption ... 

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