Homogeneous absorption bands

If molecules are embedded in solid hosts (e.g., in glassy or crystalline states) and temperatures are sufficiently low, sharp homogeneous absorption bands begin to show up within the broad inhomogeneous absorption band of the dopant [61]. A solute molecule and its noncovalently bonded solvent shell can be classified as a supermolecule [62]. Each solute molecule will thus create a unique supermolecule with its own arrangement of the solvent shell. Each supermolecule exists in a local energy well with a potential energy barrier which caimot be overcome at low temperatures. These supermolecules can show different physical properties— specifically, a spread in their absorption. In essence every supermolecule is different and can be addressed individually. 

Another intermediate of the photolysis of TiO was observed in experiments with platinized particles (in the absence of polyvinyl alcohol). The spectrum shown in Fig. 22 is prraent immediately after the laser flash. The signal decays as shown by the inset in the figure. The rate of decay is not influenced by oxygen but is increased by oxidizable compounds such as Br ions in the solution. The broad absorption band in Fig. 22 with a maximum at 430 nm was attributed to trapped positive holes. Chemically, a trapped hole is an 0 radical anion. In homogeneous aqueous solution, 0 ... 

An important point concerning the spectra in Figure 3.71 is that at intermediate doping levels three principal absorption bands can be seen, at c. l.OeV, 2.7eV and 3.6eV, that are not simply the superposition of the as-grown and neutral polymer absorptions. The authors interpreted this observation in terms of the homogeneous doping of the film throughout its bulk, not just the oxidation of the surface layer or the layer next to the electrode. 

It is useful to view optical absorption and emission processes in such a system in terms of transitions between distinct vibrational levels of the ground and excited electronic states of a metal atom-rare gas complex or quasi-molecule. Since the vibrational motions of the complex are coupled with the bulk lattice vibrations, a complicated pattern of closely spaced vibrational levels is involved and this results in the appearance of a smooth, structureless absorption profile (25). Thus the homogeneous width of the absorption band arises from a coupling between the electronic states of the metal atom and the host lattice vibrations, which is induced by the differences between the guest-host... 

The radical anion Cw, can also be easily obtained by photoinduced electron transfer from various strong electron donors such as tertiary amines, fer-rocenes, tetrathiafulvalenes, thiophenes, etc. In homogeneous systems back-electron transfer to the reactant pair plays a dominant role resulting in a extremely short lifetime of Qo. In these cases no net formation of Qo is observed. These problems were circumvented by Fukuzumi et al. by using NADH analogues as electron donors [154,155], In these cases selective one-electron reduction of C6o to Qo takes place by the irradiation of C6o with a Xe lamp (X > 540 nm) in a deaerated benzonitrile solution upon the addition of 1-benzyl-1,4-dihydronicoti-namide (BNAH) or the corresponding dimer [(BNA)2] (Scheme 15) [154], The formation of C60 is confirmed by the observation of the absorption band at 1080 nm in the near infrared (NIR) spectrum assigned to the fullerene radical cation. 

To extend the comparison between homogeneous and supported systems, we have carried out kinetic measurements on the reactions of polymer-supported [Rh(CO)2I2] complexes with Mel. In order to monitor the reactions in situ, polymer films were inserted between the windows of a conventional infrared liquid cell, fitted with a thermostatted jacket. The cell was then filled with neat Mel or a solution of Mel in CH2C12, and transmission spectra recorded directly through the polymer film immersed in the Mel solution. An example of a series of spectra recorded in this way is shown in Figure 5. The absorption bands of [Rh(CO)2I2] are replaced cleanly by those of the product acetyl complex, [Rh(CO)(COMe)I3], with the high frequency band of the reactant almost coinciding with the terminal v(CO) band of the product. 

This homogeneous chain is now equated to a string of length l for which we have v cs 1// or v = k/n X = kn. In these dyes the position of the first absorption band is in fact proportional to the length of the polyene chain. 

The CO2 laser-induced multiphoton decomposition of silanes, known to be a really homogeneous reaction, was utilized for the chemical vapour deposition of fluorine containing SiC films from the parent (fluoromethyl)silanes [25, 26]. In contrast to work with H3CS1H3, irradiation of (fluoromethyl)silanes with a single unfocused CO2 laser pulse at fluence of S 0.9 Jcm" tuned to absorption bands of either the SiH bending or the CF stretching vibrations results in an explosive reaction. This is accompanied by an intense chemoluminescence when the sample pressure exceeds a certain limit in the range of 0.1-6.7 kPa (Fig. 1). 

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