Determining Internal Motions of Molecules

Following the interpretation of atomic spectra, one of the principal services of the quantum theory has been to give a complete explanation of the highly complex molecular spectra. While the relatively simple line spectra of the atoms have given impetus to the development of the quantum theory, molecular spectra have only lately been taken in hand and could only be satisfactorily employed in conjunction with wave mechanics for determining the internal structure and the force fields of the molecule. 

The frequencies of an atomic spectrum may all be represented as differences in energy states of the atom, so that each of these states is characterized by a definite arrangement of the electrons in the atom, i.e. by a definite distribution of the negative charge density. A line is emitted if the atom passes from one state to another its frequency is cal- 

In the molecule, the energy of a given state is determined not only by the distribution of the electrons but also by the particular state of the atomic nuclei building up the molecule. In addition to the energy of electron transition, there are energies involved in changing the states of oscillation and of rotation. This explains why the spectrum of a molecule consists of very many more lines than that of an atom. A mere glance shows that this is so, if one compares an atomic spectrum with a molecular spectrum the former can properly be described as a line spectrum, the latter as a band spectrum. 

Experimental (examination of band spectra has made extraordinary strides in recent years and a mass of experimental material has accumulated, which can be utilized profitably in regard to molecular structure flnly by the enlightening ideas of quantum mechanics. 

The energy changes in the electron transitions are of the order of magnitude of erg per atom, the corresponding frequencies (v 10 sec ) 

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