QUANTUM-CONFINEMENT EFFECTS ON THE ORDERING OF THE LOWEST-LYING EXCITED-STATES IN CONJUGATED CHAINS

The symmetrized density-matrix renormalization-group approach is appli ed within the extended Hubbard-Peierls model (with parameters U/t, V/t , and bond alternation delta) to study the ordering of the lowest one- photon (1(1)B(u)(-)) and two-photon (2(1)A(g)(+)) states in one-dimens ional conjugated systems with chain lengths N up to N = 80 sites. Thre e different types of crossovers are studied, as a function of U/t, del ta, and N. The ''U crossover'' emphasizes the larger ionic character o f the 2A(g) state compared to the lowest triplet excitation. The ''del ta crossover'' shows strong dependence on both N and U/t. the ''N cros sover'' illustrates the more localized nature of the 2A(g) excitation relative to the 1B(u) excitation at intermediate correlation strengths .