We demonstrate how diverse femtosecond spectroscopy approaches coalesce to a comprehensive understanding of photochemical reaction pathways, exemplarily for the ring-open isomers of merocyanine compounds. Pump-probe transient absorption spectroscopy discloses photo-induced ring closure, whereas coherent two-dimensional (2D) electronic spectra directly visualize whether there is photoisomerization. We further introduce coherent triggered-exchange 2D electronic spectroscopy, a versatile tool for analyzing excited states and associated reaction pathways, with the information from where the reaction started intrinsically preserved. Beyond that, third-order three-dimensional spectroscopy provides an intuitive picture for which reactants can be turned into which products, additionally exposing the reactive molecular modes connecting them.
We report on ultrafast visible pump/mid-infrared probe spectroscopy of the carboxy form of heme proteins by
employing the recently developed chirped-pulse upconversion technique, which allows both high resolution and
sensitivity over an extremely broad spectral range. Commonly, the bleach signal due to ligand dissociation and
the incipient docking-site absorption signal, being about 200 cm-1 apart and differing by more than an order of
magnitude in absorptivity, are studied in separate experiments. We here monitor them simultaneously, allowing
a direct observation and a concurrent analysis of the initial processes after photoinduced ligand dissociation, for
instance, the formation of hot vibrational bands.
We employ a 128-pixel liquid-crystal spatial light modulator to generate variable pulse sequences from a titanium-sapphire femtosecond laser amplifier system centered at 800 nm. By applying phase modulations based on triangular-shaped spectral phase patterns, pulse sequences can be generated whose overall spectrum is still
conserved but whose subpulses differ in their spectral composition. We further use nonlinear crystals to analyze the shape of these pulses after frequency conversion. Our experiments show that it is possible to transfer these pulse sequences into the ultraviolet at central wavelengths of 400 nm and 267 nm without losing the ability
to spectrally distinguish between the femtosecond subpulses. This is affirmed by measurements using cross-correlation and XFROG (cross-correlation frequency resolved optical gating) techniques with an unmodulated laser pulse. Simulations of the experiments are performed for comparison. We also discuss promising applications in spectroscopy or information encoding.
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