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2817mined experimentally, but the excess oscillator strength may well be contributed by a vibronic mechanism.22 I n a vibronically allowed band, one might expect to see vibrational fine structure, and indeed the lowest energy band does show such structure. The calculated intensity of the second transition agrees well with experimental results. The third transition is expected to be very weak and would not be seen under the nearby strong bands. Finally, the calculations predict much stronger bands at higher energy, and this agrees with the in-crease of absorption observed at high frequency, though not with our possible maximum at about 5 eV.As the geometry is changed away from D8h symmetry, calculations predict an enormous increase in the intensity of the lowest energy transition. This is a clear contradiction to the experimental results and would seem to rule out all but a very small distortion of the cyclooctatetraenyl anion from D g h symmetry.By correlating changes in absorption spectra on irradiation with the changes in the intensity of fluorescence emitted by solutions containing a pair of reversible photochemical isomers, one of which is fluorescent, it is possible to calculate the extinction coefficients of the nonfluorescent isomer as a function of wavelength, even where the absorption spectra of the two isomers show considerable overlap. By appropriate calculations, close approximations to the extinction coefficients of the fluorescent isomer can also be obtained over much of the range of its absorption. The procedure has been applied to thioindigo and N,N'-diacetyl indigo. The results indicate that the extinction coefficients of the nonfluorescent cis isomers can be determined to an accuracy of about 375, while those of the trans isomers a t their absorption maxima can be estimated to within approximately 4%.Certain indigoid dyes are known to show the property of light-induced cis-trans isomerization. 2-6 We have been studying such systems and wished to evaluate the quantum yields of the photochromic process. To obtain this information requires an accurate knowledge of the extinction coefficients of each of the two isomers at the wavelengths of interest or, in other words, the absorption spectra a t known concentrations. Other workers have previously attempted to obtain absorption spectra of pure cis-and trans-indigoids by chromatographic separation of mixtures of the isomers,216 but the difficulties inherent in this technique (necessity to exclude light, low solubility of the compounds, and the inevitable thermal equilibration of the isomers) have led to the development of methodsbased on mathematical analysis of absorption spectra of m i~t u r e s .~*~J These earlier methods have in common an assumption which is not independently testable, namely that there are one or more regions in the spectrum where only one of the two isomers has any appreciable absorption. Another assumption that has been made7 is that a thermally equilibrated system, kept in the dark, con-sists of essentially 100% of the trans i...

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Investigation of the asymmetric misfit dislocation morphology in epitaxial layers with the zincblende structure