Exciton-Exciton Transitions in MnF2

Abstract

We have seen absorption lines involving the simultaneous creation of two excitons in MnF2. The observed lines are the pure electronic transitions associated with the double exciton band (6A1+6A1→ 4T1+4T1) previously observed by Ferguson.1 These double excitons are combinations of the E1 (18 419.6 cm−1) and E2 (18 436.6 cm−1) single excitons. Line transitions identified as E2+E2 (36 789 cm−1) and the doubly degenerate E1+E2 (36 917 cm−1) were seen; however, the transition to the E1+E1 state was not observed. The identification was made with the aid of [001] and [110] uniaxial stress measurements; the energy shifts of the double excitons under stress are simply related to those of the E1 and E2 excitons. We show, on the basis of one-electron molecular orbitals, that the absence of the transition E1+E1 is probably due to a weak exchange interaction between the electrons involved in this particular transition. The double exciton lines are strongly polarized, which is not indicated by the space group selection rules, but is consistent with the Mn pair symmetry.