The Nature of Electronic Excitations

In aU compounds other than alkanes, the electrons may undergo several possible transitions of different energies. Some of the most important transitions are  

Not all of the transitions that at first sight appear possible are observed. Certain restrictions, called selection rules, must be considered. One important selection rule states that transitions that involve a change in the spin quantum number of an electron during the transition are not allowed to take place they are called forbidden transitions. Other selection rules deal with the numbers of electrons that may be excited at one time, with symmetry properties of the molecule and of the electronic states, and with other factors that need not be discussed here. Transitions that are formally forbidden by the selection rules are often not observed. However, theoretical treatments are rather approximate, and in certain cases forbidden transitions are observed, although the intensity of the absorption tends to be much lower than for transitions that are allowed by the selection rules. The n n transition is the most common type of forbidden transition. 

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The Nature of Electronic Excitations in Matter (Polymer Blends) 1793... 

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