Green and orange band spectra of CaOH, CaOD and calcium oxide

Abstract

The green and orange flame bands have also been obtained in a ‘vacuum’ arc in water vapour and heavy-water vapour. An isotope shift has been observed, supporting James & Sugden’s assignment of the flame bands to CaOH. Absence of additional bands from an arc in mixed H 2 O and D 2 O eliminates Ca(OH) 2 . The fine and gross structure of the CaOH and CaOD bands is described and discussed; the molecule is non-linear; the excitation process is believed to be thermal rather than chemiluminescent. These flame bands are not the same as the green and orange bands obtained with calcium salts in an arc in air. These latter are probably due to an oxide, but their structure is too complex for them to be due, as has been suggested, to transitions between singlet electronic states of CaO. It is possible that they may be due to transitions between triplet states of CaO, but under large dispersion the structure is so very complex that a polyatomic emitter appears more likely, and the dimer Ca 2 O 2 is favoured. The relevance of these results to determinations of the dissociation energies of alkaline earth oxides, to the use of the bands in spectro-chemical analysis and to the absence of CaO bands from stellar spectra is discussed.