Benzene lower excited state

The chromophores of the amino acids studied here are substituted benzene molecules. In order to get a better qualitative idea of how the lower excited states originate, we compare the excited states of benzene to the other chromophores and discuss the effects of the substituents. A methyl group has been added to each chromophore in the... 

Fluorescent and triplet state yields for benzene add to between 0.9 and 1.0 for exciting wavelengths greater than 2530 A. There is, however, some effect of wavelength and as the pressure is lowered it appears that vibrational relaxation is more rapid for some modes of vibration than for others. These data are based on the progression for which in the excited state there is one unit of the frequency 521 cm"1 (an asymmetric mode of vibration) and 0, 1, 2, etc., units of the frequency 923 cm-1 (symmetrical stretching). 

Modified Notation.—The Platt notation is applied mainly to aromatic molecules and based on the conceptually simple perimeter model description of electronic excitations (7). Ground states are labeled A, the excited states involved in certain very high intensity transitions are labeled B and the excited states produced in partially forbidden transitions (i.e., those in which selection rules are violated) are labeled L and C. The notation is derived from selection rules appropriate for imaginary monocyclic aromatic systems. States to which transitions are forbidden because of a large change in angular momentum are L states. Transitions to C states are parity forbidden that is, they violate the g g, u u selection rule. In common aromatics other than benzene these selection rules break down and transitions to L and C states occur but at lower intensities relative to B states. 

The excitations of lower triplet states of benzene in these two approximations have been calculated in Ref. 133. A similar study of excitation cross sections for triplet states of a nitrogen molecule has been done in Ref. 134. 

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