Coherent Vibronic Coupling in Light-Harvesting Complexes from Photosynthetic Marine Algae

Abstract

Observations of long-lived coherences in photosynthetic light-harvesting complexes utilize short pulses with broad spectral bandwidths to coherently excite multiple transitions and coherent superpositions. In order to identify the role that such quantum effects might play in efficient energy transfer, however, an alternative approach is required. We have developed a technique for two-color photon echo spectroscopy to selectively excite the pathway of interest and measure its evolution in the absence of any other excitation. We use this technique to excite a coherence pathway in phycocyanin-645 from cryptophyte algae and measure the dynamics of this coherence. A decoherence time of 500 fs was measured, and clear signatures for strong coupling between the electronic states and phonon modes were observed, allowing coherent coupling between otherwise nonresonant transitions. This provides detailed experimental evidence of the long-lived coherences and the nature of the quantum mechanical interactions between electronic states and phonon modes in phycocyanin-645 from cryptophyte marine algae.

Energy transfer in structured and unstructured environments: Master equations beyond the Born-Markov approximations

“…Top: Excitonic upper eigenstate population dynamics, ρ ee (t), for a system initiated in its canonical thermal state given in Eq. (28). The solid curve is calculated with the RCME and the points are obtained from the HEOM.…”

“…[16][17][18][19] In addition to the (often low frequency) continuum, the environmental spectral densities of pigment-protein complexes are generally structured; that is, there are specific underdamped vibrational modes of the environment that couple strongly to the excitonic degrees of freedom. There is now increasing evidence to suggest that these underdamped modes are an important contributing factor to the long-lived coherences observed in photosynthetic systems, [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] as well as links between the quantum mechanical nature of these vibrational modes and enhanced transfer rates. 25,37,39,40 A multitude of powerful computational methods have been developed to deal with the difficulties faced in modelling strongly dissipative quantum systems.…”

Coherence dynamics in light-harvesting complexes with two-colour spectroscopy

“…The oscillations observed in experiment are not due to polarization interference [1]. Additionally the experiment will not be sensitive to the type of coherent oscillations identified by previous two-dimensional spectroscopy, as we are measuring the intensity rather than electric field.…”

“…Additionally oscillations in the signal pathway give evidence for the coherent excitation of states outside the bandwidth of the laser pulse. This suggests strong coupling between the excited states and phonon modes [1]. …”

“…A two-colour four-wave mixing experiment allows us to isolate a coherence pathway and observe its evolution in the absence of other signals. We measured a decoherence time of 540fs for the coherence [1]. Additionally oscillations in the signal pathway give evidence for the coherent excitation of states outside the bandwidth of the laser pulse.…”