Two-dimensional femtosecond optical spectroscopy of trapping dynamics in a charge-transfer process

Optically Induced Charge Transfer in Organic Mixed-Valence Systems: Wave Packet Dynamics and Femtosecond Transient Spectroscopy

We theoretically study the dynamics of charge transfer induced by femtosecond laser-pulse excitation. Models involving coupled electronic states of symmetrically bridged organic mixed-valence molecules are investigated, where the motion proceeds along two reaction coordinates. Linear absorption spectra of two species that differ in the energetical position of the bridge, relative to acceptor and donor states, are determined and compared to experimental results. From the wave packet dynamics it emerges that relaxation dominates the charge transfer. This behavior is reflected in transient absorption spectra, which are obtained from a directional decomposition of the time-dependent polarization. Due to the nature of the coupled dynamics the extraction of the relevant contributions needs an extension of well-known techniques for the decomposition.

Time-Domain Line-Shape Analysis from 2D Spectroscopy to Precisely Determine Hamiltonian Parameters for a Photosynthetic Complex

Optical signals come from coherences between quantum states, with spectral line widths determined by the coherences' dephasing dynamics. Using a 2D electronic spectrometer, we observe weak coherence- and rephasing-time-domain signals persisting to 1 ps in the Fenna-Matthews-Olson complex at 77 K. These are coherences between the ground and excited states prepared after the complex interacts once or three times with light, rather than zero-quantum coherences that are more frequently investigated following two interactions. Here, we use these small but persistent signal components to isolate spectral contributions with narrowed peaks and reveal the system's eigenenergies.

Exciton-exciton annihilation in a molecular trimer: Wave packet dynamics and 2D spectroscopy

We theoretically study the exciton-exciton annihilation (EEA) in a molecular trimer MMM. The system is treated within a model of electronic states, and the coupling to a bath is incorporated using the quantum jump method. Two situations of initial excitation are compared. In the first one, a two-photon process populates configurations M^*M^*M and MM^*M^* so that two excitons reside on neighboring monomers M. Then, EEA can immediately proceed. In contrast, if the trimer initially is in the local configuration M^*MM^*, exciton diffusion must occur before the annihilation process can take place. For the trimer, this excitonic motion takes place on a very short time scale. In both cases, wave packets are prepared which show a different quantum dynamics where the latter depends on the couplings and decay rates. It is documented how fifth-order coherent two-dimensional spectroscopy can be used to directly map the EEA as a function of time.

A wave packet picture of exciton-exciton annihilation: Molecular dimer dynamics

The usual view of exciton-exciton annihilation (EEA) processes in molecular aggregates is based on locally excited states of the monomer units. However, the corresponding localized configurations can only be assumed if the system is in a coherent superposition of eigenstates, i.e., a wave packet. We study a molecular dimer and focus on the characterization of EEA by a wave packet motion induced in the system by ultrashort pulse excitation. Here, coherences that appear are destroyed by dissipation processes. We discuss the influence of interband and intraband relaxation on the dynamics. The states that participate in the annihilation process are directly accessible by fifth-order optical two-dimensional spectroscopy. Such spectra are calculated, and spectral features are related to the annihilation process.

Optically Induced Electron Transfer in Mixed-Valence States: A Model Study on Electronic Transitions, Relaxation Dynamics, and Transient Absorption Spectroscopy

Quantum dynamical model calculations are performed on the optically induced electron transfer in a mixed-valence system interacting with different solvents. The simultaneously occurring processes of population transfer between electronic states and relaxation are studied in detail. Transient absorption traces, as recently recorded in our laboratory, are simulated, and the features of the spectra are related to the dynamics. The agreement with the experiment hints at the fact that the employed one-dimensional models catch the essentials of the photochemistry of the investigated systems and that they can be used for the interpretation of the transient absorption spectra. It is inferred that the ultrafast electron transfer processes take place on a sub-picosecond time scale and afterward relaxation occurs within several picoseconds.