## Abstract

The absorption spectra in the vacuum ultra-violet of the hydrides and deuterides of sulphur, selenium and tellurium, and methyl mercaptan and dimethyl sulphide are described. Well-developed Rydberg series leading to the following ionization potentials have been found: H$_{2}$S, 10$\cdot $4 V; MeSH, 9$\cdot $44 V; H$_{2}$Se, 9$\cdot $88 V; H$_{2}$Te, 9$\cdot $14 V. In the case of one series for H$_{2}$Se fifteen members of the series were observed. The spectra of the deuterides are almost identical with those of the hydrides, showing that virtually every band in the spectra is due to a separate electronic transition. This and the general nature of the rotational fine structure show the transitions concerned to be those of an electron from a non-bonding ground-state orbital, i.e. from the p lone-pair ground-state orbital. The nature of the upper orbitals of the various series is also interpreted and shown to provide explanations of certain peculiarities of the observations. The quantity I(X) - I(H$_{2}$X), where X is a group VI element, or I(Y)-I(HY), where Y is a group VII element, is shown to be positive and comparatively large when X or Y lies in the first period of the periodic table, but to change sign and to remain almost constant at a small negative value as one passes to elements in later periods. A plot of I(H$_{2}$X) against the first ionization potential of the corresponding inert gas is linear. Extrapolation enables the first ionization potential of H$_{2}$Po to be predicted at 8$\cdot $6 V. A similar plot for the halogen acids, if assumed linear, yields a predicted first ionization potential for HF of 17$\cdot $0 $\pm $ 0$\cdot $7 V.