Evidence of two-phonon vibronic progressions in layered3d-metal dihalides

Abstract

Experimental evidence of two-phonon vibronic progressions due to second-order linear electron-phonon interactions is obtained in Ni${\mathrm{Cl}}_2$ and Ni${\mathrm{Br}}_2$ crystals, by comparing the phonon structure observed in ${}_{}{}^3{}_{}{}^{}A_2^{}{}_{}{}^{}\mathrm{}\left(F\right)\mathrm{}\to {}_{}{}^1{}_{}{}^{}E\mathrm{}\left(D\right)\mathrm{}$ transition with Raman data. The vanishing of the spin-dependent first-order electron-phonon interaction is shown to be a necessary condition for observing prominent second-order phonon structures in isoconfigurational crystal-field transitions. According to this rule we predict and discuss the appearance of second-order progressions in other $d$-metal layered compounds.