A new class of collective excitations: Exciton strings

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Abstract
Optical excitation in a strongly neutral quasi‐one‐dimensional mixed‐stack charge‐transfer solid results in an exciton state, in which the electron and the hole are bound by electrostatic Coulomb interactions that are large compared to the one‐electron hopping. We present a joint 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 charge‐transfer solid. The stability of the multiexciton states arise from the combined effects of one dimensionality and strong Coulomb interactions.Theoretically, we show that in narrow band one‐dimensional semiconductors with long range Coulomb interactions, the occurrence of stable 2‐exciton string (biexciton) necessarily implies stable higher multiexcitons. Experimentally, evidence for the multiexciton strings is demonstrated by femtosecond pump–probe spectroscopy of anthracene pyromellitic acid dianhydride. Excellent qualitative agreement is found between the calculated and the measured differential transmission spectra. Photoinduced absorptions to the 2‐exciton string at low pump intensity 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.