The absorption spectra of the halogen acids in the vacuum ultra-violet

Abstract
Although the halogen acids have long been known to Possess regions of continuous absorption, no discrete electronic spectra have previously been reported for the neutral molecules. Their emission spectra occurring between 2800 and 4000 A really belong to the molecular ions. In the work to be described here, new absorption spectra of the halogen acids, which consist of very intense system of relatively discrete bands, have been dis-covered in the Schumann region. The spectra are essentially in the nature of strong resonance bands and are very similar in this respect to corresponding bands of the alkyl halides (price 1936). Extensive absorption systems of I 2 , Br 2 and Cl 2 have also been found in the same region. These will be described in a later publication. The continuous background against which the bands were observed was provided by the Lyman continuum. Other experimental details have been reported previously (Collins and Price 1934). The absorption spectrum of hydrogen iodide is shown in fig. 1 a (Plate 11). This particular photograph corresponds to a pressure of about 0·01 mm. in a path length of 50 cm. The most striking feature of the spectrum is the presence of a large number of bands with strong sharp Q branches accompanied on either side by weaker P and R branches (fig. 1 a, b, c ), the rotational structures of allied are only slightly degraded. This indicates that a relatively non-bonding electron is being excited, a fact which is also substantiated by the absence of any pronounced vibrational progressions. For example, the first strong band at 1762 A is unaccompanied by any vibrational bands, there doing a transparent region more than 5000 cm. -1 wide to the short wave-length side of it. Though the rotational structures of the P and R branches of this band are diffuse, the sharp nature of the Q branch is easily discernible (see fig. 1 a, c). The spectra of HBr and HCl each begin with a very similar band occurring at 1491 A for HBr (fig. 1 d ) and at 1331A for HCl (fig. 1 e ). It will be shown that these bands are related to one another and are analogous to the B bands of the alkyl halides. The next strong bands on the short wave-length side of them can further be linked with the C bands of the alkyl halides. Because of certain peculiarities which the above bands of the halogen acids (fig. 1 c, d, e ) exhibit, they will be discussed together in a later paragraph.

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