Weak photon localization

In the case of naphthalene, transitions to the two lowest excited states (again, often indicated with Lb and La) are two-photon forbidden, as in benzene. However, due to vibronic coupling, the Lb band is visible in the 2PA spectrum of naphthalene in the 575-650 nm region (see Fig. 5), while La gains intensity in the IPA spectrum and peaks around 275 nm [44-46], but is basically absent from the 2PA spectrum this is again in line with predictions based on the pseudoparity of the states. Polarization ratio data were used to aid the band assignment. A weak 0-0 peak of the Lb band can actually be seen in the 2PA spectrum (at 630.5 nm for naphthalene in cyclohexane [45] and at 631.8 nm in carbon tetrachloride [47]), probably because of local perturbation of the symmetry due to the solvent environment or other effects [44,45]. The 2PA... 

Here the authors consider the possibility of inferring such statistical characteristics from the spectral features of probe photons or particles that are scattered by the density fluctuations of trapped atoms, notably in optical lattices, in two hitherto unexplored scenarios, (a) The probe is weakly (perturbatively) scattered by the local atomic density corresponding to the random occupancy of different lattice sites, (b) The probe is multiply scattered by an arbitrary (possibly unknown a priori) multi-atom distribution in the lattice. The highlight of the analysis, which is based on this random matrix approach, is the prediction of a semicircular spectral lineshape of the probe scattering in the large-fluctuation limit of trapped atomic ensembles. Thus far, the only known case of quasi-semicircular lineshapes in optical scattering has been predicted [Akulin 1993] and experimentally verified [Ngo 1994] in dielectric microspheres with randomly distributed internal scatterers. 

The spectrum of the excitations is shown in Fig. 10.5 for 2 A = 80 meV. The dashed lines show the uncoupled molecular excitons and photons, and the solid lines show the coherent part of the spectrum with well-defined wavevector. The crosses show the end-points of the spectrum of excitations for which q is a good quantum number. The spectrum of incoherent (weakly coupled to light) states is shown by a broadened line centered at the energy Eq. It follows from the expression for the dielectric tensor that this spectrum is the same as the spectrum of out-of-cavity organics. The spectrum of absorption as well as the dielectric tensor depend on temperature. This means that in the calculation of the temperature dependence of the polariton spectrum we have to use the temperature dependence of the resonance frequency Eo as well as the temperature dependence of 7 determining the width of the absorption maximum. However, the spectrum of emission of local states which pump polariton states can be different from the spectrum of absorption. The Stokes shift in many cases... 

As mentioned in the introduction to Parts A and B, new experimental methods have enriched and advanced the field of atomic spectroscopy to such a degree that it serves not only as a source of atomic structure data but also as a test ground for fundamental atomic theories based upon the framework of quantum mechanics and quantum electrodynamics. However, modem laser and photon correlation techniques have also been applied successfully to probe beyond the traditional quantum mechanical and quantum electrodynamical theories into nuclear stracture theories, electro-weak theories, and the growing field of local realistic theories versus quantum theories. 

Raman Spectroscopy Historically, Raman spectroscopy was never considered a sensitive technique because only 1 in 10 photons emitted from a molecule is collected. However, Raman systems have improved tremendously in the last several years. It is no longer deemed an insensitive, irreproducible, fluorescence-dominated technique. Raman is a versatile technique capable of providing information on several parameters simultaneously, such as monomer concentration and particle size. Raman is especially amenable for monomer detection in water-soluble polymers because symmetric vinylic monomer structures are good Raman scatterers and water has a weak signal. To that end, Raman is a complementary technique to FTIR and can be used to monitor monomer concentration and conversion. By employing a near-IR laser (785 nm) which removes most of the fluorescence, along with sharp monomer and polymer peaks that are often separated, monomer concentrations may be determined with univariate calibration. Additionally, since Raman is sensitive to the local molecular environment, it may be used to provide particle size information. 

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