Absorption by diatomic gases

Ever since the classical paper by Franck21 in 1926 on the relationship between spectroscopic phenomena and the photochemical primary process in diatomic molecules and the equally important papers in 1924 by Henri and Teves22 who clearly pointed out that diffuse bands in the spectrum of the sulfur molecule S2, as shown in Fig. 2b, meant that the molecule must dissociate upon absorption, the study of photochemical reactions initiated by absorption by diatomic molecules has been on a sound basis. 

The spectroscopy of diatomic molecules has been treated so extensively that it will not be necessary either to describe the phenomena in great detail or to summarize the rules governing the complex symbols for designating energy states of such molecules. Readers are referred to any of the excellent treatises on this subject23. 

however, necessary for us to summarize a few of the fundamentals of the photochemical primary processes for diatomic molecules. 

The symbol for the normal hydrogen molecule is and means the following (/) there is no net orbital angular momentum around the axis of the molecule (/ /) the two electron spins are paired (as they have to be if the Pauli Exclusion Principle is not to be violated) (iii) the sum of the f s is an even number (zero in this case) and leads to the subscript g, which means that the wave function does not change sign by inversion through the center of symmetry (otherwise the symbol u would be used). 

Most simple diatomic molecules with like atoms have the symbol Eg+ for their ground states but there are exceptions. Thus the ground state of the oxygen molecule is 3E . 

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