The n-electron energy band

Frame (c) shows the next most complex organic molecule containing a polar atom, an atom with at least one unpaired electron. This unpaired electron is in the ground or neutral state. Its energy level can be precisely defined as in (a) and is shown as the narrow n-band in (c). Excitation of this electron is difficult in dilute solution because of its low absorption cross section relative to the other bands. 

68Pauli, W. (1925) — Hamburg, Germany Institute of Theoretical Physics 

It should also be noted that the sharing caused by conjugated carbon bonds is at its maximum in the all-trans configuration. For cis configurations, the sharing is reduced and the subsequent absorption cross section is generally lower with a peak at a shorter wavelength. 

If there is more than one polar atom in the molecule, conjugate carbon chemistry and Pauli s rules force the unexcited electrons associated with these atoms into different individual energy bands. These bands remain difficult to detect because of their low absorption cross section relative to the other bands. 

This is the mechanism proposed in the model for the transduction of visual spectrum photons by the eye. It has led to a very accurate equation for the perceived spectral response of all eyes in the animal world, including the ultraviolet sensitivity of many animals. This is particularly true for the well-documented sensitivity of (many) insects. 

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