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Coherent Multi-Quantum Multi-Dimensional Spectroscopy

Subject Area Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Term from 2019 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 423942615
 
Final Report Year 2023

Final Report Abstract

Coherent two-dimensional (2D) electronic spectroscopy is a powerful method for investigating exciton dynamics, energy and charge transport, or chemical reactions on ultrafast time scales. Providing frequency resolution both for the excitation and the detection separates contributions that overlap in one-dimensional transient absorption. The 2D method reveals the full thirdorder response function of light–matter interaction. Here, we extended the concept systematically to higher nonlinear orders and higher quantum coherences. This was accomplished by femtosecond pulse shaping with rapid shot-to-shot modulation at the 1-kHz repetition rate of the laser. With that, we extracted various nonlinear signal contributions via “phase cycling” from the same raw data set without changing the beam geometry. We employed an all-collinear excitation and fluorescence detection that avoided nonresonant solvent contributions. We have implemented various phase-cycling schemes like 1×4×4 and 1×6×6, and adapted socalled “cogwheel phase cycling”, a procedure from NMR spectroscopy, to obtain fourth-, sixth-, and eighth-order multi-quantum signals. With that, we extracted multiexciton binding energies, the homogeneous biexciton linewidth, and multiexciton correlations. By transferring this approach to 2D nanoscopy, we were also able to identify plasmon-polariton wave packets. Moving to four-pulse sequences at 1×5×5×5=125-fold phase cycling enabled us to reveal dynamic information within 15 different fourth- and sixth-order three-dimensional spectra that correlated various zero-, one-, two-, and three-quantum coherences with each other. A second approach employed a pump–probe geometry with measurement of the coherently emitted signal. Here we used fifth-order exciton–exciton-interaction 2D spectroscopy to reveal the time and length scale as well as the character of exciton diffusion under systematic variation of supramolecular parameters such as chain length, geometry, or composition. We then compared the two excitation geometries by employing them on same molecular systems using identical laser parameters. Finally, we developed so-called “intensity cycling” for pump–probe spectroscopy, which yields nonlinear signals that are not contaminated by higher orders, resolving a problem that persisted for decades in ultrafast spectroscopy. Altogether, this project successfully established several novel spectroscopic methods to study the structure, dynamics, and correlations of higher electronically excited states in a wide range of systems.

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