A NEW CLASS OF COLLECTIVE EXCITATIONS - EXCITON STRINGS

Optical excitation in a strongly neutral quasi-one-dimensional mixed-s tack charge-transfer solid results in an exciton state, in which the e lectron and the hole are bound by electrostatic Coulomb interactions t hat are large, compared to the one-electron hopping. We present a join t theoretical-experimental demonstration of a new class of collective excitations, multiexcitons or exciton strings, consisting of a string of several (more than two) bound excitons, in a prototype neutral char ge-transfer solid. The stability of the multiexciton states arise from the combined effects of one dimensionality and strong Coulomb interac tions. Theoretically, we show that in narrow band one-dimensional semi conductors with long range Coulomb interactions, the occurrence of sta ble 2-exciton string (biexciton) necessarily implies stable higher mul tiexcitons. Experimentally, evidence for the multiexciton strings is d emonstrated by femtosecond pump-probe spectroscopy of anthracene pyrom ellitic acid dianhydride. Excellent qualitative agreement is found bet ween the calculated and the measured differential transmission spectra . Photoinduced absorptions to the 2-exciton string at low pump intensi ty and to the 3-exciton string at high pump intensity are observed, in agreement with the theory of excited state absorption. The 2-exciton string is confirmed also by a direct two-photon absorption measurement . The binding energies of the 2-exciton and the 3-exciton strings are obtained from the experimental data. The larger binding energy of the 3-exciton is in agreement with theory. (C) 1996 American Institute of Physics.