Photochemical hole burning is a special type of saturation spectroscopy in the optical domain having many analogies with NMR methods. The holes, which are burnt with laser irradiation, appear as small indentations in the absorption spectra of dye molecules which are doped into a polymer or glass in minute concentrations. Based on their narrow line width, photochemical holes can be regarded as highly sensitive spectroscopic probes. They can be used to detect small perturbations of the system by external parameters, giving rise to line-shifts and broadenings. Besides the many well documented, spectroscopic applications of hole burning, it may offer interesting future developments for the spectroscopy of biomolecules and for high-density data storage. IntroductionThe optical properties of many crystalline and polymeric organic solids are frequently dominated by guest molecules. Guest molecules can either be impurities or doping materials. It has, for example, been known from the early days of low temperature spectroscopy of crystalline organic materials that impurities in low concentrations (lop4 mol/mol) can dominate the emission spectra of crystalline host matrices ['**].This article deals with hole burning spectroscopy of organic host-guest systems. This spectroscopic method was discovered in 1974 by two Soviet research groupsf3-']. Photochemical hole burning (PHB) spectroscopy is a special kind of saturation spectroscopy: With narrow-band excitation very narrow and stable photochemical holes can be "burned" into the absorption bands of guest molecules. From the line width and line shape of these holes one can obtain information about both the host and guest systems (examples of photochemical holes are shown in Figs. 14,19,30,and 31). Most attractive is the very high optical resolution of the hole burning method. In this article we shall mainly focus on the spectroscopy of glasses and polymers, since the high resolution aspects of the new method are especially useful in this field.In this context we mention that organic host-guest systems are also interesting from the viewpoint of applications. Examples are sunlight-collectors which use organic molecules in polymer or glass matrices to scatter diffuse sunlight into a small solid Other, more complex host-guest systems, which are still not completely understood, are polymeric p h o t o c~n d u c t o r s [~~~~, photographic layers["], and biological light-harvesting pigments ["]. Energy Bands, Localized Excitations, and PhononsThe optical properties of organic solids which are doped with dye molecules are determined both by the absorption spectrum of the guest and that of the host. The host states are characterized by bands of bandwith B (Fig. 1). The origin of the band states is the close packing of the host molecules leading to a strong molecular interaction. Band states have dispersive character, i.e. the excitation energy is, in most cases, delocalized over many molecule^^'^^'^^ (excitonic state). In contrast to the band states, the excited states of the guest ...

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Reversible and irreversible states of pressure-dissociated casein micelles were studied by in situ light scattering techniques and ex situ atomic force microscopy. AFM experiments performed at ambient pressure reveal heterogeneities across the micelle, suggesting a sub-structure on a 20 nm scale. At pressures between 50 and 250 MPa, the native micelles disintegrate into small fragments on the scale of the observed sub-structure. At pressures above 300 MPa the micelles fully decompose into their monomeric constituents. After pressure release two discrete populations of casein aggregates are observed, depending on the applied initial pressure: Between 160 and 240 MPa stable micelles with diameters near 100 nm without detectable sub-structures are formed. Casein micelles exposed to pressures above 280 MPa re-associate at ambient pressure yielding mini-micelles with diameters near 25 nm. The implications concerning structural models are discussed.

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Low-temperature UV-vis absorption and Stark-effect hole-burning spectra of Zn substituted cytochrome c are studied experimentally and theoretically using quantum mechanical and Poisson-Boltzmann electrostatics models. Both the Q and Soret bands show resolved splitting at temperatures below ∼180 K. The trend observed in the splittings when comparing cytochromes from different species is found to be the same as that observed for the Q(0,0) band of ferrous cytochrome c. The relative magnitudes of the Q and Soret splittings are found to be consistent with predictions based on Gouterman's four orbital model. For horse heart and yeast cytochrome c, which show the greatest difference in the UV-visible band splittings, Stark effect measurements on persistent spectral holes in the Q(0,0) band indicate that the protein-induced polarization is distinctly different for these two species. Incorporation of the protein electrostatic field as virtual point charges into quantum mechanical calculations utilizing the INDO/s semiempirical Hamiltonian is used to demonstrate that the effects of the protein on the heme electronic structure can be considerably different for the two proteins, consistent with the experimental observations.