Exciton delocalization in the B850 light-harvesting complex: Comparison ofdifferent measures

A much debated topic in recent studies of ultrafast exciton dynamics in mol ecular aggregates has been the precise definition of the exciton coherence domain in the B850 aggregate of photosynthetic purple bacteria. In this wor k we compare several measures of exciton delocalization and their variation with temperature, inhomogeneous broadening, and time. We find that the exc iton wave packet prepared by a 40 fs pulse is initially delocalized over a substantial portion of the ring and contracts to a significantly smaller si ze on the time scale of a few hundreds of femtoseconds. Steady-state absorp tion and time-dependent pump-probe spectra of the B850 aggregate are simula ted and compare well with previously reported experimental data. The calcul ated thermalized exciton wave packet size is in good agreement with previou s experimental and theoretical estimates.