The Band Spectra of Carbon Monoxide (1926)

All of the numerous band spectra formerly attributed to carbon are due probably to carbon compounds. A majority of those analyzed are now definitely known to be due to carbon monoxide, as a result of the intercorrelation of the various systems and the correlation of one of these systems, the fourth positive group of carbon, with the known absorption spectrum of CO.The known band spectra of neutral carbon monoxide comprise the fourth positive group of carbon, the \AA{}ngstr\"om CO bands, the Cameron bands, and the third positive group of carbon,---this latter group having been shown by Duffendack and Fox to comprise in reality two different systems. The relation of these various systems is indicated in Fig. 1 and Table I. The band systems of ionized carbon monoxide,---i.e., the comet-tail bands, the first negative group of carbon, and the Baldet-Johnson combination bands,---are also shown in this figure.The fourth positive group of carbon, as analyzed by the writer, comprises the former group of that name, and also Lyman's fifth positive group, and practically all the remaining unassigned bands found by Lyman in the ultra-violet spectrum of carbon monoxide. Full details of this analysis are given, and the results are tabulated in Table II and in Fig. 2. The bands found by Leifson in the absorption spectrum of cold carbon monoxide form a selected portion of this system, proving that it is the "resonance" system of CO. The system is also quantitatively related to the known infra-red absorption bands of CO in the manner demanded by the quantum theory. The general intensity distribution is in agreement with Franck's mechanical theory, as extended by Condon. The vibrational data obtained from this system have been used by Birge and Sponer to calculate the heat of dissociation of CO.The \AA{}ngstr\"om CO bands have as a final state the initial state of the fourth group, and this relation permits a definite assignment of vibrational quantum numbers for the \AA{}ngstr\"om bands. Recent work by Jass\'e on the fine structure of these bands is in agreement with this assignment. All of the measurements on the heads of the \AA{}ngstr\"om bands are collected in Table III and are compared with the calculated values. A puzzling discrepancy in one of the vibrational energy intervals is noted and discussed.The Cameron bands and the third positive group of carbon have been found by Johnson to have the same relation as the above two systems, and the Cameron bands form a second "resonance" system for CO. They have recently been found in absorption, by Hopfield. The common electronic level of these two systems is probably triple, although the bands of each system have either five-fold or six-fold heads. The various relations are discussed in detail, and equations are given in all cases.Recent work indicates that the electronic energy levels of molecules are similar to those found in atoms, and an assumed octet structure for CO suggests its correlation with Mg. The actual details of this correlation are given, but no attempt is made to discuss critically the underlying theory. The levels for CO are shown to be either single or triple, and the only two levels belonging to the same term sequence are found to fit a Rydberg formula with the known correct limit.