Ultrafast relaxation within excited doublet 2E state of 3d3 ions in ferroelectric Li2Ge7O15 crystals

The temperature dependencies of the widths of R1(Ē→4A2) and R2(2Ā→4A2) lines of Cr3+ and Mn4+ in ferroelectric Li2Ge7O15 (LGO) crystals were measured. It was observed that at low temperatures R2(2Ā→4A2), lines are significantly broader than R1(Ē→4A2) lines. The effect may be explained by a fast (∼ps) one-phonon 2Ā→Ē relaxation. The unusually large direct processes rates within 2E states may be explained by the specific vibrational spectrum of LGO, in which low-frequency optical vibrations are present, including the soft modes responsible for phase transition. The contribution of these modes to the vibrational density of states at frequencies close to 2Ā−Ē splitting Δ is observed in the vibronic sidebands of Cr3+ R1-lines.