Pump–probe spectra of linear molecular aggregates: Effect of exciton–exciton interaction and higher molecular levels

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Abstract
The paper considers the differential pump–probe spectra due to excitons in linear molecular aggregates taking into account simultaneously effects of both exciton–exciton interaction and higher molecular levels. The theoreticalanalysis, carried out in terms of the Green function technique, provides analytical expressions for the line shape of the pump–probe spectrum valid for an arbitrary number N of molecules forming the aggregate. Furthermore, the theory can accommodate any number of molecular states with higher energies. This includes, inter alia, the most common situation in which the higher lying states form a dense set of sublevels of electronic, vibrational, etc. origin. It has been demonstrated that incorporation of such higher molecular levels introduces widths to biexciton peaks formed below the two-exciton continuum. In addition, the indirect interaction between the excitons via the higher molecular levels can facilitate formation of a biexciton at lower than usual values of the direct exciton–exciton coupling γ, in extreme cases even for negative γ values characterizing repulsion rather than attraction between the excitons. On the other hand, in the region around the exciton band-edge, the differential spectrum can be described reasonably well in terms of the model of noninteracting excitons for a wide range of parameters of the system, subject to the replacement of an actual number of molecules per aggregate N by the effective one N eff . The latter N eff is shown to be influenced both by the direct coupling between the excitons and also by the indirect coupling via the higher molecular levels.

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