76 BELL SYSTEM TECHNICAL JOURNAL 



than the energy at the disposal of corpuscles of visible or ultra-violet 

 light. Modern atomic theory makes this vanishing of the excited 

 states seem very plausible; for the said excited states correspond to 

 particular arrangements of the electrons at the surface of the atom, 

 which are completely disorganized when atoms are crowded close 

 together. But, apart from theory, it is an experimental fact. 



One can therefore scarcely hope to find scattered light with a spec- 

 trum composed of discrete separate lines, unless one can find atoms or 

 complexes of atoms of which the low-energy excited states remain 

 discrete, separate and accessible when the substance is liquefied or 

 solidified. With atoms, as I have said, this appears to be impossible. 

 The high-energy excited states, in which one or another of the deep- 

 lying electrons is absent from the atomic system, are indeed the same 

 whether the atoms are free or are crowded together into a solid or a 

 liquid ; but corpuscles of light of the visible or the ultra-violet spectrum 

 have not energy enough to excite them, and therefore for the time 

 being they fall out of our purview. Molecules, however, do possess 

 excited states, into which they may be transferred from the normal 

 state by ofi"ering them one or two or three, or even a fraction of one 

 equivalent volt of energy; and these they possess, even when jammed 

 together in the liquid state. 



These excited states of molecules correspond, according to modern 

 theory, to various amplitudes of vibration of the atom-nuclei relatively 

 to one another within the molecule. The simplest case, of course, is 

 that of the diatomic molecule, of which there are so many examples — 

 oxygen and hydrogen and nitrogen, for instance. The nuclei of the 

 two atoms, being positively charged, repel each other; but the elec- 

 trons, aided possibly by additional magnetic fields, exert upon each 

 nucleus a force which tends to push them together; and there is a 

 certain internuclear distance of equilibrium, for which the two op- 

 posing forces balance. If the nuclei are displaced slightly from their 

 points of equilibrium, they vibrate. Vibration is a quantized form of 

 motion; only certain amplitudes and certain energy- values are per- 

 mitted. The low-energy excited states of the molecules correspond to 

 the permitted amplitudes and the permitted vibration-energies; when 

 a quantum excites one of these and rebounds with the remainder of 

 its energy, the energy which it gives up is spent in augmenting the 

 vibrations of the nuclei. 



These low-energy excited states are responsible for Raman's dis- 

 covery; partly because, as I have stated, they survive when the 

 molecules are jammed together into a liquid — a fact which evidently 

 means that the electrons, which produce the force upon the nuclei 



