Interband absorption in a strongly correlated half-filled band Hubbard chain: Effect of long-range Coulomb interaction

We study the effect of a weak long-range Coulomb interaction on the interband absorption in a strongly correlated half-filled-band Hubbard chain such as low-dimensional charge-transfer organic salts by introducing a small nearest-neighbor Coulomb repulsion ($U_1$) to the Hubbard model. A long-range interaction gives rise to an attractive potential well between the doubly occupied site and the hole and distorts the interband absorption line shape for transitions to the upper Hubbard band. Also, it leads to formation of, and additional transitions to, a bound excitonic state. With an increasing value of $U_1$, the intensity leaks gradually from the former transitions to the latter, while the total intensity is conserved.