Fine Structure in the Absorption Spectra of KMnF3and RbMnF3

Abstract

Experimental studies of the absorption spectra in the 3900 Å region of KMn${\mathrm{F}}_3$ and RbMn${\mathrm{F}}_3$ at 4.2°K are presented. The spectra of the two substances are almost identical, and show the same number of lines and same type of fine structure. The ${}_{}{}^4{}_{}{}^{}\Gamma _3^{}{}_{}{}^{}$ levels, split by the tetragonal component of the crystalline field, are associated with observed transitions at ∼3939 and ∼3896 Å. Definite energy-level patterns are found for the weak vibrational lines accompanying these transitions. A sharp and very weak magnetic dipole line $M$ is found (with three components) at ∼3976 Å. An electric dipole line $P_1$ at ∼3966 Å appears to be a magnon sideband of the magnetic dipole line. $M$ is presumed to be an exciton line associated with the electric dipole transition $P_2$ at ∼3955 Å to the main ${}_{}{}^4{}_{}{}^{}\Gamma _1^{}{}_{}{}^{}$ level. The electric dipole lines $P_1$ and $P_2$, associated with the ${}_{}{}^4{}_{}{}^{}\Gamma _1^{}{}_{}{}^{}$ level, show a fine structure which is interpreted in terms of exchange-coupled pairs of ions with a spin Hamiltonian $-2J{\mathrm{S}}_1·{\mathrm{S}}_2$. The selection rule for these transitions is that the total spin of the pair is conserved.