Dynamics of the time-resolved stimulated Raman scattering spectrum in presence of transient vibronic inversion of population on the example of optically excited trans-β-apo-8'-carotenal

The full dynamics of energy relaxation in large organic molecules: from photo-excitation to solvent heating

In some molecular systems, such as nucleobases, polyenes or sunscreens, substantial amounts of photo-excitation energy are dissipated on a sub-picosecond time scale. Where does this energy go or among which degrees of freedom it is being distributed at such early times?

In some molecular systems, such as nucleobases, polyenes or the active ingredients of sunscreens, substantial amounts of photo-excitation energy are dissipated on a sub-picosecond time scale, raising questions such as: where does this energy go or among which degrees of freedom it is being distributed at such early times? Here we use transient absorption spectroscopy to track excitation energy dispersing from the optically accessible vibronic subsystem into the remaining vibrational subsystem of the solute and solvent. Monitoring the flow of energy during vibrational redistribution enables quantification of local molecular heating. Subsequent heat dissipation away from the solute molecule is characterized by classical thermodynamics and molecular dynamics simulations. Hence, we present a holistic approach that tracks the internal temperature and vibronic distribution from the act of photo-excitation to the restoration of the global equilibrium. Within this framework internal vibrational redistribution and vibrational cooling are emergent phenomena. We demonstrate the validity of the framework by examining a highly controversial example, carotenoids. We show that correctly accounting for the local temperature unambiguously explains their energetically and temporally congested spectral dynamics without the ad hoc postulation of additional ‘dark’ states. An immediate further application of this approach would be to monitor the excitation and thermal dynamics of pigment–protein systems.

Noncollinear and nonlinear pulse propagation

A novel method for numerical modelling of noncollinear and nonlinear interaction of femtosecond laser pulses is presented. The method relies on a separate treatment of each of the interacting pulses by it's own rotated unidirectional pulse propagation equation (UPPE). We show that our method enables accurate simulations of the interaction of pulses travelling at a mutual angle of up to 140°. The limit is imposed by the unidirectionality principal. Additionally, a novel tool facilitating the preparation of noncollinear propagation initial conditions - a 3D Fourier transform based rotation technique - is presented. The method is tested with several linear and nonlinear cases and, finally, four original results are presented: (i) interference of highly chirped pulses colliding at mutual angle of 120°, (ii) optical switching through cross-focusing of perpendicular beams (iii) a comparison between two fluorescence up-conversion processes in BBO with large angles between the input beams and (iv) a degenerate four-wave mixing experiment in a boxcar configuration.

A Unified Picture of S* in Carotenoids

In π-conjugated chain molecules such as carotenoids, coupling between electronic and vibrational degrees of freedom is of central importance. It governs both dynamic and static properties, such as the time scales of excited state relaxation as well as absorption spectra. In this work, we treat vibronic dynamics in carotenoids on four electronic states (|S0⟩, |S1⟩, |S2⟩, and |Sn⟩) in a physically rigorous framework. This model explains all features previously associated with the intensely debated S* state. Besides successfully fitting transient absorption data of a zeaxanthin homologue, this model also accounts for previous results from global target analysis and chain length-dependent studies. Additionally, we are able to incorporate findings from pump-deplete-probe experiments, which were incompatible to any pre-existing model. Thus, we present the first comprehensive and unified interpretation of S*-related features, explaining them by vibronic transitions on either S1, S0, or both, depending on the chain length of the investigated carotenoid.

Investigation of the S1/ICT equilibrium in fucoxanthin by ultrafast pump-dump-probe and femtosecond stimulated Raman scattering spectroscopy

Time-resolved multi-pulse spectroscopic methods-pump-dump-probe (PDP) and femtosecond stimulated Raman spectroscopy-were used to investigate the excited state photodynamics of the carbonyl group containing carotenoid fucoxanthin (FX). PDP experiments show that S1 and ICT states in FX are strongly coupled and that the interstate equilibrium is rapidly (<5 ps) reestablished after one of the interacting states is deliberately depopulated. Femtosecond stimulated Raman scattering experiments indicate that S1 and ICT are vibrationally distinct species. Identification of the FSRS modes on the S1 and ICT potential energy surfaces allows us to predict a possible coupling channel for the state interaction.

Identification of the Excited-State C═C and C═O Modes of trans-β-Apo-8'-carotenal with Transient 2D-IR-EXSY and Femtosecond Stimulated Raman Spectroscopy

Assigning the vibrational modes of molecules in the electronic excited state is often a difficult task. Here we show that combining two nonlinear spectroscopic techniques, transient 2D exchange infrared spectroscopy (T2D-IR-EXSY) and femtosecond stimulated Raman spectroscopy (FSRS), the contribution of the C═C and C═O modes in the excited-state vibrational spectra of trans-β-apo-8'-carotenal can be unambiguously identified. The experimental results reported in this work confirm a previously proposed assignment based on quantum-chemical calculations and further strengthen the role of an excited state with charge-transfer character in the relaxation pathway of carbonyl carotenoids. On a more general ground, our results highlight the potentiality of nonlinear spectroscopic methods based on the combined use of visible and infrared pulses to correlate structural and electronic changes in photoexcited molecules.