Excitonicn-string in linear chains: Electronic structure and optical properties

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Abstract
The electronic structure and the optical properties of an excitonic n-string which is a bound state of n Frenkel excitons (n=2,3,. . .) are theoretically clarified. The binding mechanism may be attributed to the electrostatic dipolar interaction among the Frenkel excitons, which is in contrast to the formation mechanism of the otherwise analogous excitonic molecules in a system of Wannier excitons; it makes it possible to form bound states of more than two Frenkel excitons. The effects of the n-string states on the electronic structure and optical properties are studied. The two-photon absorption and the differential transmission spectra are shown to depend strongly on the direction of the induced static dipole moment, which determines whether the interaction between excitons is attractive or repulsive.