Possible changes of intensity of cadmium arc lines, emitted from different upper energy states, have been calculated on the basis of thermal equilibrium for two definite temperatures of the arc due to variation of the arc current. The arc temperatures were measured with optical pyrometers. Large discrepancies were noted between the calculated and observed values. The results call for a considerable amount of collisional excitation.
A. K. DUTTA and U. GANTAYET, "Excitation Mechanism of Arc Spectra Lines from a Study of Cadmium Arc Spectra," J. Opt. Soc. Am. 46, 1046-1046 (1956)