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Three new systems in the molecular spectrum of T h o have been rotationally analysed. They are located in the yellow region of the spectrum. Two bands at 5600 and 5615 .A are the 0-0 and 1-1 bands of a system named P-H. One band at 5613.A is another 0-0 band of a system called 0 -H . The states 0 and P both have = 0 with different symmetries. Another band at 5579A is also analysed. Its position in the term scheme of T h o is unknown.New measurements and an extended analysis of the previously investigated C-H transition have been performed.A term scheme and a table of molecular constants for all known states in T h o is given.

A new system of very simple Z-X bands in the region 4800-6000 A has been observed in flash discharges in CH4, both in absorption and emission. The rotational analysis shows conclusively that the molecule responsible is a diatomic nlolecule containing two carbon nuclei, a conclusion that is further confirmed by the observation of the corresponding system using 13CH4 instead of 12CH,. Neither the upper nor the lower state is one of the known states of C2. None of the predicted low-lying states of C, fits the observed new states. Two possibilities remain: either the new system corresponds to two fairly highly excited levels of C2 or it is due to the C,-ion. The latter alternative finds some support in the similarity of the new system to the near ultraviolet bands of N 2 + (which has the same number of electrons). However, the doublet splitting required by this interpretation has not been resolved, nor has the study of perturbations in the upper state given clear-cut evidence for doublet structure. On the other hand, a mass-spectronletric investigation of the ions in the flash discharge producing the spectrum shows a strong C,-peak, while C 2 + is weak. A definitive answer to the question of the carrier of the new spectrum cannot yet be given.

Perturbations in the A 1Π state of the isotopic 28Si16O and 28Si18O molecules are analyzed. The perturbing states, d 3Δ r , e 3Σ-, C 1Σ-, and D 1Δ, are identified and vibrationally assigned. Two independent methods of vibrationally numbering the perturbers, employing isotope shifts and vibrational variation of perturbation matrix elements, respectively, are described in detail and shown to be in complete agreement. Molecular constants for the eight lowest energy electronic states of SiO are presented. Semiempirical and isovalent relationships between spin-orbit and spin-spin constants are discussed.

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