Signatures of Exciton Delocalization and Exciton-Exciton Annihilation in Fluorescence-Detected Two-Dimensional Coherent Spectroscopy

We present calculations of the fluorescence-detected coherent two-dimensional (F-2DES) spectra of a molecular heterodimer. We compare how the F-2DES technique differs from the standard coherently detected two-dimensional (2DES) spectroscopy in measuring exciton delocalization. We analyze which processes contribute to cross-peaks in the zero-waiting-time spectra obtained by the two methods. Based strictly on time-dependent perturbation theory, we study how in both methods varying degree of cancellation between perturbative contributions gives rise to cross-peaks, and identify exciton annihilation and exciton relaxation contributions to the cross-peak in the zero-waiting-time F-2DES. We propose that time-gated fluorescence detection can be used to isolate the annihilation contribution to F-2DES both to retrieve information equivalent to 2DES spectroscopy and to study annihilation contribution itself.Comment: 12 pages including SI, 4 figures and 1 tabl

Enhancement of Vibronic and Ground-State Vibrational Coherences in 2D Spectra of Photosynthetic Complexes

A vibronic-exciton model is applied to investigate the mechanism of enhancement of coherent oscillations due to mixing of electronic and nuclear degrees of freedom recently proposed as the origin of the long-lived oscillations in 2D spectra of the FMO complex [Christensson et al. J. Phys. Chem. B 116 (2012) 7449]. We reduce the problem to a model BChl dimer to elucidate the role of resonance coupling, site energies, nuclear mode and energy disorder in the enhancement of vibronic-exciton and ground-state vibrational coherences, and to identify regimes where this enhancement is significant. For a heterodimer representing the two coupled BChls 3 and 4 of the FMO complex, the initial amplitude of the vibronic-exciton and vibrational coherences are enhanced by up to 15 and 5 times, respectively, compared to the vibrational coherences in the isolated monomer. This maximum initial amplitude enhancement occurs when there is a resonance between the electronic energy gap and the frequency of the vibrational mode. The bandwidth of this enhancement is about 100 cm-1 for both mechanisms. The excitonic mixing of electronic and vibrational DOF leads to additional dephasing relative to the vibrational coherences. We evaluate the dephasing dynamics by solving the quantum master equation in Markovian approximation and observe a strong dependence of the life-time enhancement on the mode frequency. Long-lived vibronic-exciton coherences are found to be generated only when the frequency of the mode is in the vicinity of the electronic resonance. Although the vibronic-exciton coherences exhibit a larger initial amplitude compared to the ground-state vibrational coherences, we conclude that both type have a similar magnitude at long time for the present model. The ability to distinguish between vibronic-exciton and ground-state vibrational coherences in the general case of molecular aggregate is discussed.Comment: 16 pages, 6 figure

Parametric Projection Operator Technique for Second Order Non-linear Response

We demonstrate the application of the recently introduced parametric projector operator technique to a calculation of the second order non-linear optical response of a multilevel molecular system. We derive a parametric quantum master equation (QME) for the time evolution of the excited state of an excitonic system after excitation by the first two pulses in the usual spectroscopic four-wave-mixing scheme. This master equation differs from the usual QME by a correction term which depends on the delay \tau between the pulses. In the presence of environmental degrees of freedom with finite bath correlation time and in the presence of intramolecular vibrations we find distinct dynamics of both the excite state populations and the electronic coherence for different delays \tau.Comment: 15 pages, 8 figure