An Ar-Kt ion laser was used to excite the Raman spectrum of powdercd anthracene with two laser lines of 4880 and 6471A. There are significant differences in the intensity distribution in these two Raman spectra, These digerences can be best evplaiced by considering that Raman intensities are contributed by two alIomed excited states of anthracene, and their contributions depend upon the laser frequency. From these Ramaa spectra, i t is also possibIe to infer the vibrational structures in :he higher energy, structureless excited state of anthracene.The dependence of Raman intensities on the frequency of the exciting light is well known. The Raman spectrum of a compound was observed to become more and more intense as the frequency of exciting light approached the lowest excited state from below.'' When the exciting light frequency coincides with the absorption band, the scattering process is called resonance Raman scattering*) and its intensity might be loo times more intense than the ordinary Raman scattering when the frequency of exciting light is well below the absorption b a d . Kondilenko and ~t h e r s~)~" had also observed that the overtone vibatiocs also became intense when the frequency of exciting light was increased. In the above investigations, however, it was not mentioned that the relative intensities of various Raman lines showed dependence on the frequency of exciting light.In our previous theoretical considerations,')P) it was expected that all the allowed excited states in a molecule should contribute to the Raman scattering, and their contributions should be additive in terms of the derived polarizability. The contribution by an excited state to a particular Raman line depends upon the foIlowing factors :1. The integrated intensity of the transition between the ground state and the 2. The Franck-Condon integral of the normal vibration in the above transition; and 3. The difference between the transition energy and the energy of incident photons. excited state;
The corrosion of printed circuit boards 'can be effectively prevented with a polyurethane conformal coating. In this study, the protective performance of PU coating was investigated systematically through electrochemical measurements and simulated exposure/immersion tests. Some fungi tests were also included. All experimental results show that a polyurethane coating effectively inhibits corrosion on printed circuit boards. The optimal thickness of the coating film should be about 60 pm. Long-term exposure tests .showed that this coating material could preserve the reliability of the investigated printed circuit boards for more than 400 days in a 90% relative humidity environment at 25°C.
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