Photoelectron spectra and electronic structure of diatomic alkali halides

Abstract

The He I photoelectron spectra of the chlorides, bromides and iodides of Na, K, Rb and Cs have been recorded in the vapour state by a molecular beam technique. The spectra exhibit features which have been interpreted on the basis of a completely ionic model in keeping with the treatments usually applied to ionic solids. To explain structure in the spectra of the lighter molecules it has been necessary to discuss polarization of the halogen atoms by the alkali metal ions. For the heavier molecules such as KI, RbI and CsI the spectra of the monomers consist of two fairly broad bands corresponding to ionization of the X$^{-}$ moiety to its $^{2}$P$_{\frac{3}{2}}$ and $^{2}$P$_{\frac{1}{2}}$ states. These have a 2/1 intensity ratio and a separation equal to 3/2 times the appropriate spin-orbit coupling coefficient. In the case of NaI the $^{2}$P$_{\frac{3}{2}}$ state is split by the strong electrostatic field of the Na$^{+}$ ion into states possessing $^{2}\Pi _{\frac{3}{2}}$ and $^{2}\Pi _{\frac{1}{2}}$ character. This is accompanied by a shift of the $^{2}$P$_{\frac{1}{2}}$ state to higher energies as it gradually assumes $^{2}\Sigma $ character. The result is an increase of the mean $^{2}$P$_{\frac{3}{2}}$-$^{2}$P$_{\frac{1}{2}}$ separation to a value greater than that to be expected from spin-orbit coupling alone. This increase becomes more pronounced for the bromides and chlorides where the spin-orbit coupling coefficient is smaller. For NaCl the separation of the $^{2}$P$_{\frac{3}{2}}$ and $^{2}$P$_{\frac{1}{2}}$ bands shows clearly that the $^{2}$P$_{\frac{1}{2}}$ band possesses considerable $^{2}\Sigma $ character. In terms of partial ionic character, increased polarization of the halogen atom is related to increased covalent character of the MX bond. The lighter molecules can therefore be said to be the least ionic in this description. The proportion of dimers to monomers observed in the spectra is found to fall as the molecules become more ionic, i.e. as the halogen atoms become less polarized.