Optical Bloch equations for light harvesting complexes: pump probe spectra and saturation dynamics at high light intensity excitation

Abstract

The light harvesting complex of photosystem II (LHC II) of green plants is the pigment-protein complex, that binds the majority of chlorophyll on earth. It transfers the excitation energy absorbed from solar radiation to the photosystem II core complex. The monomeric subunit of the trimeric LHC-II complex contains 14 coupled chlorophyll molecules, embedded in a protein matrix, which provides a vibronic bath for electronic excitations [1]. We focus on the regime of higher laser intensities, where theories that consider the optical field as a pertubation and treat the phonon-lineshape accurately[2] cannot be used . For this purpose Bloch equations are derived using the correlation expansion method [3]. They include the electron-vibron interactions (pigment-protein coupling), the electron-electron interaction caused by the Coulomb coupling between the different pigments [4] and the applied optical pulses. The different interactions leads to effects like formation of delocalized excited states, excitation relaxation, exciton-exciton annihilation as well as Pauli blocking. All parameters for the Bloch equations are independently determined: the Coulomb matrix elements from quantum chemical calculations [4], the spectral density of pigment-protein coupling from fluorescence line narrowing measurements [2]. To illustrate the application of Bloch equations for high pulse intensities, we focus on pump-probe spectra (Fig. 1 a)) and the fluorescence quantum yield: Fig. 1 a) shows the intensity dependence of the pump probe signal of LHC II. Interestingly, the transfer rate from chlorophyll b at 645 nm to chlorophylla at 680 nm was found to be almost unaffected by the intensity. Only the energy that is able to relax towards chlorophylls with lower site energies is reduced for higher intensities (see. Fig. 1 a)) due to Pauli blocking effects.