ELECTRONIC EXCITED STATES OF SIMPLE MOLECULES^ 



G. WiLSE Robinson 



Gates and Crellin Laboratories of Chemistry' 



California Institute of Technology 



Pasadena, California 



L Introduction 



In an approximate way the electronic structure of a large molecule 

 is a superposition of the electronic structures of the small entities 

 of which it is comprised. If the large molecule is made up of parts 

 which do not interact among themselves, and if the large molecules 

 are themselves randomly oriented so that dichroic effects are absent, 

 the spectrum corresponding to the absorption or radiation of energy 

 in the form of light by such a molecule will be precisely that given 

 by a superposition of the randomly oriented parts. This means that 

 the intensity of absorption or emission is merely summed as a function 

 of wavelength over the various parts of the large molecule. 



In reality, interactions between parts of the large molecule do occur.' 

 If certain interactions are very large, because of conjugation, for ex- 

 ample, these parts should be considered together as the basic molecu- 

 lar entity in terms of which the large molecule is described. Weaker 

 but non-zero interactions affect the energy levels in a less spectacular 

 manner. The problem is analogous to that of electrons in a set of 

 potential wells of various sizes and shapes. If the potential barriers 

 betAveen the wells are infinitely high, electrons cannot delocalize into 

 the neighboring wells and the energy levels are those of the isolated 

 parts. Ag the barriers are lowered to allow interaction between parts, 

 electrons can delocalize by an amount depending upon the energy 

 of the electrons and the width of the barriers ("distance" between 



' Supported in part by Office of Ordnance Research and the National Science 

 Foundation. 



-Contribution No. 2569. 



''This has been discussed in some detail for double molecules (refs. 2 and 19). 

 References are gi\en here lo other pertinent papers on this subject. See ref. 16a 

 and literature cited there for the more general case. 



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