Effects of excited state mixing on transient absorption spectra in dimers:Application to photosynthetic light-harvesting complex II

The excited state mixing effect is taken into account considering the diffe rence spectra of dimers. Both the degenerate (homo) dimer as well as the no ndegenerate (hetero) dimer are considered. Due to the higher excited state mixing with the two-exciton states in the homodimer, the excited state abso rption (or the difference spectrum) can be strongly affected in comparison with the results obtained in the Heitler-London approximation. The differen ce spectrum of the heterodimer is influenced by two resonance effects (i) m ixing of the ground state optical transitions of both monomers in the dimer and (ii) mixing of the excited state absorption of the excited monomer wit h the ground state optical transition in the nonexcited monomer. These effe cts have been tested by simulating the difference absorption spectra of the light-harvesting complex of photosystem II (LHC II) experimentally obtaine d with the 60 fs excitation pulses at zero delay times and various excitati on wavelengths. The pairs of coupled chlorophylls a and b for simulations h ave been taken from the best LHC II assignment model obtained by simulating the steady-state spectra and the transient absorption at various excitatio n wavelengths. Qualitatively the spectral peculiarities of the difference s pectra are explained by means of the resonance interpigment interactions, w hich are responsible for the excited state mixing. (C) 1999 American Instit ute of Physics. [S0021-9606(99)50430-3].