Examination of two new low-lying electronic states of SiS observed in chemiluminescent flame spectra

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Diatomic silicon sulfide molecules have been produced in the chemiluminescent reaction of silicon atoms with OCS. Spectra of the resulting flame consist mainly of two new band systems in the region 350–400 and 385–600 nm. These systems have been assigned as b 3 Π r-X 1 Σ + and a 3 Σ +-X 1 Σ + on the basis of band structure, spin-orbit splitting, molecular constants, and comparison with chemiluminescent spectra of isovalent molecules. Vibrational assignments were made with the help of the isotope effect and vibrational constants were obtained. Rotational structure was observed in some a- X bands and a partial analysis yielded an approximate rotational constant, B ∼ 0.247 ± 0.007 cm −1, for the a 3 Σ + state. Franck-Condon factors, calculated for the a- X system, are shown to fit the general trend of the intensity distribution. Irregularities in spin-orbit splitting and in relative intensities of the spin-orbit components of the b 3 Π-X 1 Σ + system were observed and an attempt is made to explain them in terms of interactions with neighboring states. Addition of active nitrogen to the flame was shown to greatly increase the intensity of the b- X system relative to the a- X system. Constants (in cm −1) obtained for the new state are: a 3σ +: T e=24 582.1 ± 1.3, ω e=503.8±1.0, ω ex e=1.86±0.21 b 3σ +: T e=27 314.5 ± 2.2, ω e=619.4±2.0, ω ex e=5.75±0.52 b 3σ +: T e=27 407.9 ± 1.1, ω e=524.3±1.2, ω ex e=3.97±0.28