Absorption band inhomogeneous

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. 

One way to dissect fluorescence from a heterogeneous sample is fluorescence line narrowing, in which the temperature is lowered to freeze out fluctuations of the solvent. The underlying idea is the same as in holebuming absorption spectroscopy (Sect. 4.11). A tunable dye laser with a narrow spectral bandwidth is used to excite the sample selectively and thus to pick out a subpopulation of molecules that absorb at a particular wavelength. The emission spectrum for the subpopulation can be much narrower than the spectrum for the ensemble as a whole, and it often shifts as the excitation wavelength is tuned over the inhomogeneous absorption band [184]. 

Figure 5 shows a hole in the inhomogenous absorption band of protoporphyrin IX-substituted myoglobin. The insert shows the hole on an enlarged scale. 

Site-selection spectroscopy Maximum selectivity in frozen solutions or vapor-deposited matrices is achieved by using exciting light whose bandwidth (0.01-0.1 cm-1) is less than that of the inhomogeneously broadened absorption band. Lasers are optimal in this respect. The spectral bandwidths can then be minimized by selective excitation only of those fluorophores that are located in very similar matrix sites. The temperature should be very low (5 K or less). The techniques based on this principle are called in the literature site-selection spectroscopy, fluorescence line narrowing or energy-selection spectroscopy. The solvent (3-methylpentane, ethanol-methanol mixtures, EPA (mixture of ethanol, isopentane and diethyl ether)) should form a clear glass in order to avoid distortion of the spectrum by scatter from cracks. 

If microstates lead to the existence of a distribution of energies of interaction between aromatic groups and neighboring groups of atoms, then the individual spectra of these groups in different microstates shift differently, which results in an inhomogeneous contour of the absorption band. The application of selective photoexcitation permits specific effects of the distribution of microstates on spectral, temporal, and polarization fluorescence properties to be observed. 221 Such effects have been observed in studies of proteins, 1,8) and, as we show below, they may be used to obtain important information on dynamics. 

Molecules in crystals or dispersed in host lattices are often present in a range of environments, and this results in a broadening of the electronic absorption spectrum. Such an inhomogeneously broadened absorption band (envelope of transitions) may be considered as a superposition of several distinguishable sites. A narrow line laser can saturate one of the transitions under the envelope and the corresponding molecules will no longer take part in the absorption process. This phenomenon is referred to as hole... 

Figure 7.32 (a) Broadening of an electronic absorption band of a molecule due to an inhomogeneous environment (b) illustration of a laser-induced photochemical hole burned in the 0-0 A, line of free-base porphyrin in -octane at 2K (c) excitation spectrum of the 0-0 lines of the Sj-So transition of the free base in n-hexane, showing a frequency difference ( 100cm" ) between the two tautomeric forms (1) and (2) of the free base in a single type of site. Irradiation into the line A, transforms it into Aj and vice versa (d) hole burned in line A, at 4.2 K. (After Williams, 1983.)... 

These experimental data indicate that the optical absorption band of SG is inhomogeneously broadened due to the sites that differ in both spectral parameters and reactivity. 

The principle of the experiment is illustrated by Fig. 12. On the left-hand side the absorption band of an inhomogeneously broadened transition, e.g., the OH-stretching band in different H-bonded local environments, is depicted schematically. Structures with different OH-O bond angles and/or 0-0 bond lengths show up in the spectrum with different OH frequencies linear bonds and shorter bond length correspond to larger red shifts... 

The results for various alcohol solutions are compiled in Table 2. The data refer to room temperature except for methanol in C5CI6, which was measured at 333 K. A few spectroscopic parameters are shown in the first four lines. There is evidence for inhomogeneous broadening of the monomeric OH-stretching vibration for the various investigated alcohols with a width of the observed spectral holes as small as 50% of the conventional absorption band (DMEP). The anharmonic shift of the OH mode is indicated in the table to amount to 170 cm-1, independent of solute or solvent within experimental accuracy. The large anharmonicity of the monomeric alcohols is noteworthy. The population lifetime Ti of the mode... 

Investigation of methanol-pyridine complexes, on the other hand, produced a relatively narrow (75 cm 1 FWHM) bleached hole that is burned into the 260 cm 1 FWHM OH-stretch (v = 0 -> 1) absorption band. The methanofpyridinc complex OH-stretch absorption band was better fit by a Gaussian function than with a Lorentzian bandshape, indicating this system is inhomogeneously broadened on the >1 ps timescale. 

The solubilized molecules are in a dynamic equilibrium between complexed and uncomplexed forms. Therefore, a fraction of the molecules will react in the uncom-plexed form in solution during exposure to radiation. Some molecules are likely to interact with the hydroxyl residues or substituents on the outer surface of the cyclodextrin unit and thereby experience a partly hindered degree of freedom. The remaining fraction will experience a less polar environment, protection from oxygen, and a decrease of the intramolecular rotational freedom inside the cavity during excitation. The inhomogenity of cyclodextrin systems can be demonstrated by a typically broad lifetime distribution and a red shift of the emission maximum when excited in the red edge of the absorption band (Bortolus and Monti, 1996). 

Experimental study of the size-dependent oscillator strength, which determines the radiative rate, is discussed in Section I1I.A. There has been no systematic study of the size dependence of the absorption cross section of semiconductor nanoclusters. The enhancement in the exciton absorption cross section with decreasing cluster size has been observed for CdS qualitatively (e.g., Figure 1) [47,50]. Quantitative study is still not available because of problems such as size inhomogeneity and surface defects. One of the most puzzling observations is that many of the semiconductor clusters synthesized so far show no exciton absorption bands at all, in spite of the positive identification of their existence by X-ray diffraction [2, 11, 51]. While size inhomogeneity is usually conveniently invoked as the explana-... 

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