We investigated the excited states of a one-dimensional charge-transfer Mott insulator ${\left[\mathrm{NiBr}\right(\mathrm{chxn})}_2{]\mathrm{Br}}_2$ by femtosecond luminescence spectroscopy. In the picosecond region, we observed a luminescence component which is different from that observed under steady-state excitation. This picosecond luminescence has a large Stokes shift and a broad bandwidth, suggesting the existence of an excited state with large lattice relaxation in addition to the excited state with a small relaxation. The dynamical behavior of the luminescence has been qualitatively interpreted in terms of a model assuming two adiabatic potential-energy surfaces.

• Received 19 March 2004

DOI:https://doi.org/10.1103/PhysRevB.70.081101