Dissociation energies and excitation levels of alkaline-earth oxides

Abstract

AMBIGUITY about the dissociation energy of alkaline-earth oxides, as derived from flame photometric measurements, originates from the uncertainty with regard to the emitter of the band spectra, the designation and energy of the ground and excited state levels, respectively, the a’-parameter of the atomic resonance lines involved, and the possible side-reactions of these elements in the flame. Possible side-reactions may lead to the formation of hydroxides, which may also give visible band emissions. We have tried to eliminate the uncertainty about the emitter by using special “dry” CO-air flames (containing virtually no hydrogen (-compounds)), comparing the results with those of similar (“moist”) flames into which a small amount of water vapour was introduced. Alkalineearth salt was fed into the “dry” flame by dry evaporation of the salt in a heated pot placed in the Na conduit to the flame. Besides, the height of the upper excitation levels of the bands observed could be deduced from the measured temperature-dependence of the band intensities, while correcting for the (slight) dependence on temperature of the metal oxide concentration and flame depth. Plotting the observed intensities semilogarithmically against reciprocal temperature directly yields the excitation energy. The excitation energies found are listed in Table 1 and compared with data from the literature. It appears that the lower state of the transitions involved is by O-6 to 1.0 eV higher than the ground state for all alkaline-earth oxides investigated (cf. last column in Table 1).