  We examine how the electron correlation affects the electron-phonon (EP)
interaction in the linear optical absorption spectrum of the one-dimensional
(1D) extended Hubbard-Holstein model. A density matrix renormalization group
(DMRG) calculation shows that the effect of the EP interaction on an exciton is
enhanced by increasing the on-site Coulomb repulsion. This enhancement is in
contrast to the effect of the EP interaction on the ground state where the
Peierls instability is suppressed by the on-site Coulomb repulsion. The DMRG
data with the EP interaction fit with absorption experiments in 1D cuprates
better than those for the extended Hubbard model.
