Studies in the Infra-red region of the spectrum. Part IV.-Discussion of absorption bands of ammonia, phosphine and arsine

Of absorption bands in the infra-red which are partially or more completely resolved, the following four types (fig. 9) have been observed in the work of others and ourselves. Type 1 occurs in dipoles such as HC1, HBr and HF* and is absent in NH3, PH3 and AsH3. Type 2 is characteristic of the Main Sequence of bands common to NH3, PH3 and AsH3, and appears in the infra-red spectrum of CH4 ; type 3, so far found only in the band of NH3 at 10·55 ft, contains two Q branches ; while type 4 exhibits the fusion of two distinct bands, of which the zero branches in PH3 and AsH3 are the first members of a distinct series of harmonics.