Radiation, highly coherent, spectroscopy

Hepburn J W 1995 Generation of coherent vacuum ultraviolet radiation applications to high-resolution photoionization and photoelectron spectroscopy Laser Techniques in Chemistry vol 23, ed A B Myers and T R Rizzo (New York Wley) pp 149-83... 

This section has been devoted to the study of the surface excitons of the (001) face of the anthracene crystal, which behave as 2D perturbed excitons. They have been analyzed in reflectivity and transmission spectra, as well as in excitation spectra bf the first surface fluorescence. The theoretical study in Section III.A of a perfect isolated layer of dipoles explains one of the most important characteristics of the 2D surface excitons their abnormally strong radiative width of about 15 cm -1, corresponding to an emission power 10s to 106 times stronger than that of the isolated molecule. Also, the dominant excitonic coherence means that the intrinsic properties of the crystal can be used readily in the analysis of the spectroscopy of high-quality crystals any nonradiative phenomena of the crystal imperfections are residual or can be treated validly as perturbations. The main phenomena are accounted for by the excitons and phonons of the perfect crystal, their mutual interactions, and their coupling to the internal and external radiation induced by the crystal symmetry. No ad hoc parameters are necessary to account for the observed structures. 

If coherent radiation with a very high intensity is applied continuously or as pulse, non-linear effects can be observed which produce coherent Raman radiation. This is due to the quadratic and cubic terms of Eq. 2.4-14, which describe the dipole moment of a molecule induced by an electric field. Non-linear Raman spectroscopy and its application are described in separate chapters (Secs. 3.6 and 6.1), since this technique is quite different from that of the classical Raman effect and it differs considerably in its scope. 

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