Coherent Multi-Quantum Multi-Dimensional Spectroscopy

• Applicant: Professor Dr. Tobias Brixner
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 423942615

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 third-order response function of light-matter interaction.Here we extend the concept systematically to higher nonlinear orders, higher quantum coherences, and higher dimensions. This offers unique information on excitons not accessible via other techniques. Using the developed methods, we plan to study the structure, dynamics, and correlations of electronically excited states in molecular and supramolecular species as well as quantum dots. This will be accomplished by femtosecond pulse shaping with shot-to-shot modulation at the 1-kHz repetition rate of the laser. Thus we can extract various nonlinear signal contributions via “phase cycling” from the same raw data set without changing the beam geometry. Rapid scanning significantly improves the signal-to-noise ratio and reduces the total data acquisition time.Our first approach is based on all-collinear excitation and fluorescence detection that avoids nonresonant solvent contributions. We will implement 1*4*4-, 1*6*6-, and 1*8*8-fold phase cycling to determine fourth-, sixth-, and eighth-order signals, respectively. Thus one can extract electron correlation energies or exciton binding energies. Moving to four-pulse sequences at 1*5*5*5=125-fold phase cycling reveals dynamic information within 15 different fourth- and sixth-order three-dimensional spectra that correlate various zero-, one-, two-, and three-quantum coherences with each other.A second approach employs a pump-probe geometry with measurement of the coherently emitted signal. Here we will use fifth-order exciton-exciton-interaction 2D spectroscopy to reveal exciton transport under systematic variation of supramolecular parameters such as chain length, geometry, or composition. We will also investigate systems in which transport may occur either along one, two, or three dimensions.Finally, we plan to compare the two excitation geometries by employing them on the same system using identical laser parameters. This should resolve a debate on the interpretation of 2D fluorescence spectra. In addition, we investigate the potential of four- and five-dimensional spectroscopy by deploying up to six pulses. This can disentangle all signal contributions up to sixth order and separate, for example, wavepacket dynamics on different electronic states that overlap in conventional 2D measurements.

• DFG Programme: Research Grants