V. The structure and origin of the Swan band spectrum of carbon

Abstract

Since its discovery by W. Swan almost exactly seventy years ago, speculations as to the origin of this familiar band spectrum have been prolific in spectroscopic literature. A good summary of these opinions and the experimental data upon which they were founded is given by Watts (1), writing in 1914. With the exception of one or two writers, the two opposing schools have favoured a Carbon molecule and a Hydrocarbon molecule, respectively. The latter school have for the most part specified in particular an acetylene molecule. Thus among a vast number of experimenters Van der Willigen (1859), Attfield (1862-1875), Dibbits (1864), Morren (1865), Plucker and Hittorf (1865), Huggins (1868), Wullner (1872), Salet (1873), Secchi (1873), Ciamician (1880), Deslandres (1888), and Eder (1890), favoured a carbon molecule (presumably Ca). On the other hand Swan (1856), Angstrom and Thalen (1875), Liveing and Dewar (1880), and many others favoured a hydrocarbon. The latter writers conducted a great deal of careful research (2) on the flames of Carbon compounds, the vacuum tube spectra of Carbon gases, and the Carbon arc in various gases, and they affirm that the emitter is an acetylene molecule, since this gas can be withdrawn from the flames of burning hydrocarbons which, show the bands well. In the light of our present knowledge one or two comments are suggested as we review all this work. It is clear that in the vast quantity of experimental work of the former school the extraordinary difficulty of ensuring the complete absence of Hydrogen—either occluded in the carbons or present as water vapour—was not appreciated. We have, for example, a similar and comparatively recent controversy (3) with reference to the origin of the Nitrogen afterglow, in which the degree of freedom from Oxygen of a sample of Nitrogen was the point at issue. Another very natural idea which prevailed until recently was that the emitters of band spectra were necessarily molecules known to chemistry, the most stable ones being those most likely to radiate under discharge conditions. Our present views are almost the reverse of these, and it seems probable that polar molecules cannot radiate band spectra at all. An excellent paper representative of these views has recently been published by Mulliken (4)—“On a class of one-valence electron emitters.” The old ideas, therefore, which led to the choice of an acetylene molecule as the radiator of the Swan bands were certainly ill-founded, but by a coincidence it happens that a C 2 H 2 molecule must now be quite definitely accepted as their emitter. The present paper contains the evidence for this statement. Hitherto measurements of the fine structure of only three heads have been available, and these from photographs take under arc conditions. It has been found desirable to make a complete re-measurement of the fine structure of these and other heads under new low temperature conditions of production. Some 2,000 lines are tabulated. A considerable number of these have been given series assignment, and the modern quantum theory of band spectra in its various aspects has been applied to the data. The scope of the resulting analysis may be gauged from the table of contents.