No abstract

Complex multi-stage relaxation and reaction pathways after the optical excitation of molecules makes the disentanglement of the underlying mechanisms challenging. We present four examples that a new transient spectrometer with excitation fully tunable from the deep UV to the IR and 225 to 1700 nm probing allows for an analysis with greatly reduced ambiguity. The temporal resolution of about 50 fs allows us to resolve all relevant processes. For each example there is a new twist in the sequence of relaxation steps that had previously been overlooked. In malachite green it appears that the importance of the phenyl twisting has been overemphasized and rather a charge transfer state should be considered. In TINUVIN-P the predicted twisting as the driving motion for the ultrafast IC is confirmed and leads to a resolution of the earlier puzzle that the sub-5 ps regime shows kinetics deviating from a pure cooling process despite the sub-ps proton transfer cycle. For the bond cleavage of Ph 2 CH-Cl and Ph 2 CH-Br the degree of electron transfer within the radical pair can now be determined quantitatively and leads to a profound understanding of the long term cation yield. For the first time coherent wavepacket motion in the photoproducts is reported. Last but not least the measurement of the GSB recovery in the deep UV allows for the surprising result, that even after S 2 excitation of cyclopentenones the triplet states are reached with near unity probability within a few picoseconds.

The interaction of exciton and charge transfer (CT) states plays a central role in photo-induced CT processes in chemistry, biology, and physics. In this work, we use a combination of two-dimensional electronic spectroscopy (2D-ES), pump-probe measurements, and quantum chemistry to investigate the ultrafast CT dynamics in a lutetium bisphthalocyanine dimer in different oxidation states. It is found that in the anionic form, the combination of strong CT-exciton interaction and electronic asymmetry induced by a counter-ion enables CT between the two macrocycles of the complex on a 30 fs timescale. Following optical excitation, a chain of electron and hole transfer steps gives rise to characteristic cross-peak dynamics in the electronic 2D spectra, and we monitor how the excited state charge density ultimately localizes on the macrocycle closest to the counter-ion within 100 fs. A comparison with the dynamics in the radical species further elucidates how CT states modulate the electronic structure and tune fs-reaction dynamics. Our experiments demonstrate the unique capability of 2D-ES in combination with other methods to decipher ultrafast CT dynamics.

Diffraction and microscopy with ultrashort electron pulses can reveal atomic-scale motion during matter transformations. However, the spatiotemporal resolution is significantly limited by the achievable quality of the electron source. Here we report on the emission of femtosecond single/fewelectron pulses from a flat metal surface via two-photon photoemission at 50-100 kHz. As pump we use wavelength-tunable visible 40 fs pulses from a noncollinear optical parametric amplifier pumped by a picosecond thin-disk laser. We demonstrate the beneficial influence of photon energies close to the photocathode's work function for the coherence and duration of the electron pulses. The source's stability approaches the shot noise limit after removing second-order correlation with the driving laser power. Two-photon photoemission offers genuine advantages in minimizing emission duration and effective source size directly at the location of photoemission. It produces an unprecedented combination of coherent, ultrashort and ultrastable single/few-electron wave packets for timeresolving structural dynamics.

Newly generated frequencies during bulk continuum generation with femtosecond pump pulses do not fluctuate statistically and show strong correlations in spectrum and time. When a femtosecond continuum is used as probe light for transient spectroscopic measurements, these correlations result in a seemingly low noise level but largescale pseudo-structures that obscure the interpretation. We investigate the correlations for continua generated in YAG and calcium fluoride plates and incorporate the results into the design of our pump-probe setup. The high degree of correlation to the next pulse is utilized through chopping of the pump and referencing between successive laser shots. To suppress the adverse effect of the high degree of correlation to other wavelengths, we extend the detection by multichannel referencing with a second camera. The combination of both referencing schemes renders a precise spectral calibration unnecessary and increases the sensitivity of our spectrometer by a factor of 5 down to 20 μOD. This is already very close to the shot noise limit. To demonstrate the improvements, we present and discuss measurements on two different molecular solutions.