Using Wave-Packet Interferometry to Monitor the External Vibrational Control of Electronic Excitation Transfer

We investigate the control of electronic energy transfer in molecular dimers through the preparation of specific vibrational coherences prior to electronic excitation, and its observation by nonlinear wave-packet interferometry. Laser-driven coherent nuclear motion can affect the instantaneous resonance between site-excited electronic states and thereby influence short-time electronic excitation transfer (EET). We first illustrate this control mechanism with calculations on a dimer whose constituent monomers undergo harmonic vibrations. We then consider the use of nonlinear wave-packet interferometry (nl-WPI) experiments to monitor the nuclear dynamics accompanying EET in general dimer complexes following impulsive vibrational excitation by a sub-resonant control pulse (or control pulse sequence). In measurements of this kind, two pairs of polarized phase-related femtosecond pulses following the control pulse generate superpositions of coherent nuclear wave packets in optically accessible electronic states. Interference contributions to the time- and frequency-integrated fluorescence signal due to overlaps among the superposed wave packets provide amplitude-level information on the nuclear and electronic dynamics. We derive the basic expression for a control-pulse-dependent nl-WPI signal. The electronic transition moments of the constituent monomers are assumed to have a fixed relative orientation, while the overall orientation of the complex is distributed isotropically. We include the limiting case of coincident arrival by pulses within each phase-related pair in which control-influenced nl-WPI reduces to a fluorescence-detected pump-probe difference experiment. Numerical calculations of pump-probe signals based on these theoretical expressions are presented in the following paper

Vibrational Coherence Transfer and Trapping as Sources for Long-Lived Quantum Beats in Polarized Emission from Energy Transfer Complexes

We entertain vibrational coherence transfer and related processes as possible sources for certain long-lived quantum beats observed in time-resolved polarized emission signals from photosynthetic light harvesting complexes. Signal calculations on a dimer model in which each chromophore supports a single vibrational mode show that coherence transfer to the acceptor and coherence trapping in the donor can increase the longevity of vibronic quantum beats beyond the time-scale for electronic energy exchange. These mechanisms imply an active role for coherent vibrational motion in the time-course of ultrafast energy transfer and suggest that external control over vibrations may provide a means for influencing the transport of electronic excitations. The effects of vibrational coherence transfer and trapping on excitation transfer are most vivid when the excitation-vibration coupling strength exceeds that for energy transfer. In light of the strong transfer coupling of photosynthetic light-harvesting complexes, we also examine the adiabatic energy-transfer regime, in which the relative coupling strengths are reversed