CHEMICAL BONDING AND SIZE SCALING OF NONLINEAR POLARIZABILITIES OF CONJUGATED POLYMERS

Using sum rules for spectral moments of linear and nonlinear absorptio n we show that the jth order off-resonant polarizabilities of polyacet ylene chains scale as similar to j Delta(- j)N(j + 1)/(N + L)(j); N is the number of carbon atoms, Delta the bond-length alternation, and L( Delta) the exciton coherence size. This result is in excellent agreeme nt with time-dependent Hartree-Fock calculations performed for chains with up to 200 carbon atoms.