Intensity distributions in molecular spectra: The swan system (C 2 )

The problem of obtaining accurate measurements of the relative intensities of spectral lines or bands when these cover a considerable wave-length range is one of the most difficult in experimental spectroscopy. These difficulties have been enumerated by many writers,e. g ., in a recent paper by Read and L. W. Johnson, and it is not proposed to enter into a detailed discussion here. The problem is much simpler when a small spectral range only is being investigated, such as, for example, the fine structure of an individual band or the intensity within a line multiplet, and a large amount of experimental work has been done within these limits. The desirability of quantitative data on intensity distributions within band systems is apparent from the theoretical work of Condon published in 1926, explaining the general characteristics of such distributions. The cases to which Condon’s theory has been applied by way of illustration have been almost all naked-eye estimates of band intensities on an arbitrary scale (0-10). An exception to this is the α-system of BO on which Elliot has made accurate measurements. In the present paper we have made preliminary observations on the well-known “Swan” system of the C2 molecule. This system has the advantage, from an experimental standpoint, of lying wholly within the visible region of the spectrum, and, further, that its intensity distribution is characteristic of a large class of band systems, and that it can be produced under a variety of experimental conditions.