We performed luminescence measurements in one-dimensional Mott insulators of $\mathrm{halogen}\mathrm{}\left(X\right)\mathrm{}$-bridged Ni compounds: $\left[\mathrm{Ni}\right(\mathrm{chxn}{)}_{2}X\left]Y_2\right(\mathrm{}(X,Y)=(\mathrm{Br},\mathrm{Br}),(\mathrm{Cl},\mathrm{Cl}),(\mathrm{Cl},{\mathrm{NO}}_3)).$ For all three compounds, we observe luminescence bands that have a small Stokes shift $E_{\mathrm{S}}$ of less than 0.05 eV. To understand such small values of $E_{\mathrm{S}},$ we applied an extended Peierls-Hubbard model to investigate lattice relaxation in the photoexcited states. The results demonstrate a different mechanism: a strong effective suppression of the electron-lattice interaction can occur by combining a basic property of Mott insulators and the present type of electron-lattice interaction. We argue that the negligible values of $E_{\mathrm{S}}$ and the small widths of the absorption spectra can be explained by this mechanism.

• Received 2 August 2002

DOI:https://doi.org/10.1103/PhysRevB.66.235103