Emission bands of SiCl₂ and SnCl₂

Abstract

The discharge through flowing vapour of SiCl₄, SnCl₄ and SnCl₂ shows a number of regions of selective continuous emission, and, intimately connected with them, several sets of new bands. These appear under conditions unfavourable for the production of the known bands of SiCl and SnCl respectively. They are broad and diffuse, and have not the appearance of bands due to a diatomic emitter of low molecular weight. They are quite different from the known SiCl and SnCl bands as far as the anharmonic factor of the ground level of the molecule is concerned. They are therefore attributed to the triatomic molecules SiCl₂ and SnCl₂ respectively. An analysis carried out on this assumption reveals two excited electronic levels of SiCl₂ (w = 28295 and 29952), and one of SnCl₂ (v = 22237). The presence of the deforming (breathing) vibration ω₁, and of the symmetrical valence vibration ω₂ are indicated both in the ground state and in the various excited states. The antisymmetric valence vibration is absent, in agreement with Herzberg-Teller's selection rules. For certain reasons ω₂' is smaller than ω₂", despite ω' being larger than ω" in the corresponding excited terms of SiCl and SnCl. The energies of excitation and the values of ω₂" of SiCl₂ and SnCl₂ show a close similarity to the corresponding constants of SiCl and SnCl respectively, and also the bond energies appear to be rather unaffected by the transition from the diatomic to the triatomic molecule. The later ones appear to be triangular.