Temperature dependence of infrared-absorption lines ofCo2+in cadmium halides

Abstract

Temperature-dependent infrared-absorption measurements were performed on the spin orbit and trigonal-crystal-field split levels of the ${}^{4}{\mathrm{T}}_{1g}$ cubic-field ground term of ${\mathrm{Co}}^{2+}{(3d}^{7})$ in the cadmium halides $\mathrm{Cd}{X}_{2}$ (X: Cl, Br, and I). Besides ${\ensuremath{\gamma}}_{4}^{+}\ensuremath{\rightarrow}{\ensuremath{\gamma}}_{4}^{+}$ and ${\ensuremath{\gamma}}_{4}^{+}\ensuremath{\rightarrow}{\ensuremath{\gamma}}_{5,6}^{+}$ electronic transitions, sharp vibronic lines are observed. From temperature-dependent line shift and line-broadening measurements of these lines, electron-phonon coupling parameters were determined. For ${\mathrm{Co}}^{2+}$ in ${\mathrm{CdCl}}_{2},$ where the energy separation of the ${\ensuremath{\gamma}}_{4}^{+}$ and ${\ensuremath{\gamma}}_{5,6}^{+}$ levels studied is comparable to the phonon energies, a contribution from direct one-phonon processes needs to be included. Further line-shift measurements were made on ${\mathrm{Co}}^{2+}$ pair lines to determine whether the electron-phonon coupling differed for ${\mathrm{Co}}^{2+}$ pairs, with no significant difference being observed. The infrared-absorption spectrum for ${\mathrm{Co}}^{2+}$ in ${\mathrm{CdI}}_{2}$ is reported.