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MULTISCOPE Report Summary

Project ID: 614623
Funded under: FP7-IDEAS-ERC
Country: Germany

Mid-Term Report Summary - MULTISCOPE (Multidimensional Ultrafast Time-Interferometric Spectroscopy of Coherent Phenomena in all Environments)

In project MULTISCOPE we develop and apply novel methods of nonlinear spectroscopy to investigate the significance and consequences of coherent effects for a variety of photophysical and photochemical molecular processes. We use coherent two-dimensional (2D) spectroscopy as an ideal tool to study electronic coherences. In contrast to most conventional schemes, we do not measure the coherently emitted field within a four-wave-mixing process but rather implement a range of incoherent observables (ion mass spectra, fluorescence, and photoelectrons). Yet we still extract all desired coherent information using “phase cycling” with collinear pulse sequences from a femtosecond pulse shaper. This opens up a new range of interdisciplinary experiments and allows a direct nonlinear-spectroscopic comparison of molecular systems in all states of matter (liquid phase, gas phase, surfaces).

In the first reporting period we have implemented a range of new 2D methods. For liquid-phase investigations we developed rapid-scan 2D fluorescence spectroscopy. This setup contains no movable parts and is thus compact and robust. All spectroscopic parameters are varied electronically on a 1-kHz shot-to-shot basis using a customized femtosecond pulse shaper. This allows us to acquire a 2D spectrum in just 6 s. Furthermore, we demonstrated optimized compressed sampling. We introduced a 4D von-Neumann representation to convey the essential information of 2D spectra and to generate optimized sampling patterns via a genetic algorithm. Thus one can reduce the number of data acquisition points to 25 % of the full set while retaining the essential information.

For gas-phase samples, we developed 2D spectroscopy of molecular beams using time-of-flight mass spectrometry. Thus it is possible to analyze interaction-free molecules. Recording a mass spectrum for each parameter setting of the pulse shaper and Fourier-transforming the ion signal over time delays, one obtains 2D spectra not only for the parent molecule but also for all fragments. This will make it possible to investigate the evolution of coherences during bond breakage. Initial demonstration experiments were carried out on NO_2.

In the case of surface-science studies, we engineered theoretically and proved experimentally coherent periodic energy transfer for widely separated nanoemitters. Strong coupling over a distance of twice the wavelength was achieved in a combination of localized and delocalized plasmonic modes. The resulting energy transfer efficiency is two orders of magnitude larger than with simpler plasmonic devices. Concerning method development, we designed and implemented in our laboratory a new setup for time-resolved photoemission electron microscopy (TR-PEEM) including aberration correction that offers high spatial resolution (3 nm) in combination with tunable (250 – 1000 nm) short-pulse excitation (< 30 fs) at high repetition rates (1 MHz). This was realized with a customized noncollinear optical parametric amplifier pumped by a fiber laser amplifier. The setup will be employed in the second reporting period for coherent 2D nanoscopy of molecular-plasmonic hybrid systems combining high spatial resolution, time resolution, and multidimensional spectral resolution.

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