One-dimensional quantum rotator in solids: The para-ortho transition ofH2S−in KCl

Abstract

Trapping of a mobile interstitial hydrogen atom by a substitutional ${\mathrm{SH}}^{\mathrm{\ensuremath{-}}}$ impurity in KCl leads to the formation of a ${\mathrm{H}}_{2}$${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$ molecule on an anion site whose electron-spin-resonance (ESR) spectra exhibit axial symmetry around a 〈111〉 direction. A singlet line and a hydrogen hyperfine triplet both exhibiting the 〈111〉 symmetry but possessing slightly different g parameters are shown to belong to this ${\mathrm{H}}_{2}$${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$ center, and an Arrhenius analysis of the ESR intensities establishes that the triplet is excited at the expense of the singlet with a 16.2-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ activation energy. From this and from an analysis of the g and $^{33}\mathrm{S}$ hyperfine components incorporating higher-order corrections it is concluded that the ${\mathrm{H}}_{2}$${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$ molecule exhibits free or weakly hindered quantum rotation around its 〈111〉-oriented ${C}_{2v}$ axis: The proton-spin statistics only permit the nuclear singlet in the symmetric rotational ground state, while only the triplet nuclear state is allowed in the first-excited rotational state at 16.2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. As a corollary to this investigation, a vexing problem about the structure of the substitutional ${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$ center is settled. Furthermore, the isotropic part of the hyperfine interaction of the free ${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$(3${\mathrm{p}}^{5}$) ion is shown to be large and negative. Data on ${\mathrm{D}}_{2}$${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{HDS}}^{\mathrm{\ensuremath{-}}}$ centers are also presented.