Exciton-migration and three-pulse femtosecond optical spectroscopies of photosynthetic antenna complexes

Abstract

A theory for four-wave-mixing signals from molecular aggregates, which includes effects of two-exciton states, static disorder, and coupling to a phonon bath with an arbitrary spectral density, is developed. The third-order polarization is rigorously partitioned into a coherent and a sequential contribution. The latter is given by a sum of an exciton-hopping and a ground state ͑bleaching͒ terms, both expressed using the doorway-window representation. Applications are made to photon-echo and pump-probe spectroscopies of the B850 system of the LH2 antenna in purple bacteria. ͑1͒where B n † (B n ) are exciton creation ͑annihilation͒ operators for the n'th molecule, which satisfy the commutation relationsThe polarization operator P representing coupling of the aggregate to the optical field ϪE(t)• P has a form

Exciton (De)Localization in the LH2 Antenna of Rhodobacter sphaeroides As Revealed by Relative Difference Absorption Measurements of the LH2 Antenna and the B820 Subunit

“…Strong coupling may give rise to further reduction of the disorder-induced localization length due to polaron formation. [32][33][34][35] In this case the amplitude of nonlinear response will be determined by a combined action of the static and dynamic disorder. To describe the experimental ratio of amplitudes we most probably will need smaller values of the static disorder together with non-zero values of the dynamic disorder.…”

“…5 These reports have stimulated new studies of the excitonic structure of these light-harvesting complexes [6][7][8][9][10][11][12][13][14][15][16][17] including experimental studies and theoretical simulations of the nonlinear optical response of the antenna. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] However, the dynamics of electronic excitations in the antenna is still the subject of intensive debates. It is clear that the BChl arrangement in the form of a tightly-packed high-symmetry aggregate tends to delocalize electronic wavefunctions, but the intrinsic static and dynamic disorder of a pigment-protein complex may give rise to more localized states.…”

“…The strong coupling may give rise to further reduction of the exciton size due to polaron formation. [32][33][34][35] It can be significant if the phonon-induced localization length (polaron size) will be smaller than the disorder-induced localization length.…”

Parametric projection operator technique for second order non-linear response

“…With increasing details of the excited state dynamics revealed by the 2D spectroscopy [41][42][43] and with increasing size of the studied systems, it becomes more and more important to keep the numerical cost of simulations low, while simultaneously account for experimentally observed quantum effects. One of the possible research directions is to extend on existing reduced density matrix (RDM) theories [44,45] or relax certain approximations [46].…”

Subpicosecond Exciton Dynamics in Polyfluorene Films from Experiment and Microscopic Theory

“…Disentangling one from another is challenging and requires a combination of both experimental and theoretical approaches. Simulations of large-scale systems have been conducted using a Monte-Carlo simulation approach [29][30][31][32] or a Master-equation based approach [33][34][35][36][37] , while experiments have used spectral-and time-resolved photoluminescence decay 23,38,39 or transient absorption spectroscopy 39,40 to monitor the ultrafast photophysics. Furthermore a number of theoretical simulations model charge-separated excitations [41][42][43][44][45][46] , which are particularly relevant to current flow in devices such as photovoltaics and LEDs.…”