Motional Effects in the Electron Spin Resonance Absorption Spectrum of CO2– Molecule-Ions in Single-Crystal Calcite

Abstract

The electron spin resonance absorption spectrum of CO₂^– molecule‐ion in single‐crystal calcite has been reinvestigated as a function of temperature. Between 77° and 250°K the spectrum exhibits orthorhombic symmetry and from the manner in which the widths of the spectral components behave, it is concluded that in this temperature range the CO₂^– molecule‐ion hops from one to another of three equivalent configurations each being rotationally displaced from the other two by $\text{\hspace{0.17em}±\hspace{0.17em}2π\hspace{0.17em}/\hspace{0.17em}3}$ radians in the (111) plane. Such hops involve the crossing of one of three equivalent potential wells whose depths are estimated to be 0.165 eV and whose widths are not more than 0.1 radians. At 250°K the spectrum associated with the (111) plane is wiped out. Above this temperature an axially symmetric spectrum appears whose parameters are simply related to those of the low‐temperature spectrum, and from the manner in which the widths depend upon temperature, it is concluded that in this temperature region the CO₂^– molecule‐ion undergoes hindered rotations about the [111] direction.