Knowledge Bank

The electronic absorption spectrum of $KMnF3$ has been determined at room temperature, $77\circ K,20.4\circ K$. and $4.2\circ K$. In addition, a detailed study of the temperature dependence of the narrow absorption bands at about $4000\backslash AA(6A1g⟶4A1g,4Eg)$ has been made. The temperature shifts of the main lines correlate with the known structural and magnetic changes in this material. However, the absorption intensity of these bands is abnormally large $[f(300\circ K)=9.5\times 10-7,emax=0.61]$ and studies made with small amounts of Mn in $KZnF3$ and $KMgF3(1$ show that the majority of the absorption intensity comes from pure electronic transitions involving exchange coupled ions. The absorption spectrum of the simplest case, an isolated Mn-F-Mn pair, has been determined in both $KZnF3$ and $KMgF3$. In both crystals there are four lines in the strongest group. The temperature dependence of these lines and the energy intervals indicate that the exchange interaction is isotropic $(2JS\cdot S)$, the exchange interaction is antiferromagnetic in both ground and excited states and the selection rule $\Delta S=0$ holds rigorously. The concentration dependence of the absorption bands shows that the single ion contribution to the absorption intensity in pure $KMnF3$ is much smaller than the allowed transition involving exchange coupled ions.