Localization, optical centers, interaction

Although no quantum confinement should occur in the electronic energy level structure of lanthanides in nanoparticles because of the localized 4f electronic states, the optical spectrum and luminescence dynamics of an impurity ion in dielectric nanoparticles can be significantly modified through electron-phonon interaction. Confinement effects on electron-phonon interaction are primarily due to the effect that the phonon density of states (PDOS) in a nanocrystal is discrete and therefore the low-energy acoustic phonon modes are cut off. As a consequence of the PDOS modification, luminescence dynamics of optical centers in nanoparticles, particularly, the nonradiative relaxation of ions from the electronically excited states, are expected to behave differently from that in bulk materials. 

On the basis of a definite analogy between e tx and F-centers we may expect the appearance under certain conditions of (e tr)2- particles of the type of F -centers. From the polaron model (57) it follows that the bipolaron (two electrons localized in a common polarization well) can not exist. In accordance with the work of Vinetskii and Giterman (63), in some cases the formation of the bipolarons becomes energetically possible in the result of interaction of the polarization wells of two separate polarons. However, the saving in energy for such bipolaron states is not large and hence they will not be stable in liquids under room temperature. Actually, up to the present time a series of attempts have been made to detect (e aq)2 in the irradiated liquid water but these attempts were not successful. The polaron theory (57) predicts that F -centers (two electrons in the anionic vacancy) may be stable. For this it is necessary that the ratio e/n2 (e and n2 are the static and optical dielectric constants, n—refraction index) should be more than 1.5. Evidently, in the glassy systems under consideration this requirement is fulfilled. 

Typical dimensions of the flow passage are width W = 0.5mm and depth D = 1mm. The laser beam is focused to approximately 40 im in the center of the measurement volume. Since the local intensity of a focused laser beam does change along the beam with the highest intensity in the focal plane, it is important to adapt the beam shaping optics not only for uniformity of illumination across the beam, but also along the depth of the flow passage. For a detailed analysis of the interaction of a focused laser beam with a sample flow, see [15]. 

The fonnation of quinone anions in bacterial reaction centers produces distinctive optical absorption shifts of the bacteiiopheophytin and bacteiiochlorophyll chromophores . The absence of a direct molecular contact between the quinones and these chromophores " suggests that the electrochromic shifts may be due to an electrostatic interaction between the optical transition dipoles of the chromophores and the electric field associated with the quinone anions. In this case, the electrochromism induced by the quinone anions potentially provides an opportunity to examine the propagation of electric fields, and hence the local dielectric, within the reaction center protein. 

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