In 1913 L. C. Martin discovered an ultra-violet band-system in the tube discharge through carbon disulphide vapour and in the carbon arc fed with sulphur, and ascribed it, on experimental evidence, to the CS molecule. The bands are degraded towards the red, and the majority of them are double headed. Martin arranged all but seven of the observed bands into seven groups (A, B, ... G) each formed by closely neighbouring bands. The present communication records the quantum analysis of the band-heads, and the substantial similarity of structure between this CS system, the ultra-violet systemof SiO and the “4th Positive” system of CO. A brief examination of the photographs reproduced by Martin and of the wave-numbers derived from his tabulated wave-lengths leads to a Deslandres scheme for the band-heads and to a plausible assignment of the initial and final vibrational quantum numbers,
n
',
n
". In this scheme each of Martin’s seven groups is a sequence of bands corresponding to a constant change of vibrational quantum number,
n
"—
n
', except that groups B and C together with one band of group A form only one sequence,
n
"—
n
' = — 2. It further appears that of the pair of heads (bracketed in Martin’s table of data) of each double headed band, the less refrangible head is formed by a Q branch, the corresponding P branch having, of course, no head, and therefore remaining undetected with the dispersion employed by Martin (about 7.8 A./mm. at λ 2420 to 12.7 A./mm. at λ 2850).
Time-resolved Spectroscopy of the violet luminescence of undoped AlN