Empirical tests are accumulating by which the electronic states of conjugated organic molecules may be identified as to spin, parity, electron-excitation-type, and momentum or angular-momentum.
The identifications are made as for atomic spectra, by finding Rydberg series in some cases, but more generally by examining the selection rules and the effects of perturbations. The observed intensities and transition times vary over a factor of 1010, so that a superposition of selection rules giving several different degrees of forbiddenness must be recognized. The perturbations used are not the classical electric or magnetic fields. They are instead the strong local disturbances produced by polar solvent interactions; the spectral changes on chemical substitution and the changes produced in going to the next molecule of a homologous chemical series may also be treated in some degree as perturbations.
The theory behind these methods of making assignments, just as in atomic spectra, is based only on general quantum-mechanical principles. Consequently the identifications of electronic states obtained by these methods are regarded as more certain than identifications based on detailed theoretical predictions of the energies or intensities which must assume the accuracy of particular atomic or molecular approximations.
A list is given of the identifying characteristics of several different types of transitions, and examples of some types are shown. In a few molecules, complete assignments for six or more of the lower excited states are available.
© 1953 Optical Society of AmericaFull Article | PDF Article
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