Search

Conical intersections (CoIns) of multidimensional potential energy surfaces are ubiquitous in nature and control pathways and yields of many photo-initiated intramolecular processes. Such topologies can be potentially involved in the energy transport in aggregated molecules or polymers but are yet to be uncovered. Here, using ultrafast two-dimensional electronic spectroscopy (2DES), we reveal the existence of intermolecular CoIns in molecular aggregates relevant for photovoltaics. Ultrafast, sub-10-fs 2DES tracks the coherent motion of a vibrational wave packet on an optically bright state and its abrupt transition into a dark state via a CoIn after only 40 fs. Non-adiabatic dynamics simulations identify an intermolecular CoIn as the source of these unusual dynamics. Our results indicate that intermolecular CoIns may effectively steer energy pathways in functional nanostructures for optoelectronics. Two-dimensional electronic spectroscopy reveals the existence of intermolecular conical intersections in molecular aggregates relevant for photovoltaics.

We studied and compared the hydrodeoxygenation (HDO) and depolymerization of aldehyde-stabilized lignin and 4-propylguaiacol (PG), a model lignin monomer. We demonstrated by liquid phase adsorption...

Halide perovskites are promising optoelectronic materials. Despite impressive device performance, especially in photovoltaics, the femtosecond dynamics of elementary optical excitations and their interactions are still debated. Here we combine ultrafast two-dimensional electronic spectroscopy (2DES) and semiconductor Bloch equations (SBEs) to probe the room-temperature dynamics of nonequilibrium excitations in CsPbBr3 crystals. Experimentally, we distinguish between excitonic and free-carrier transitions, extracting a ∼30 meV exciton binding energy, in agreement with our SBE calculations and with recent experimental studies. The 2DES dynamics indicate remarkably short

The role of molecular vibrations for the persistence of quantum coherences, recently observed in photoinduced charge transfer reactions in both biological and artificial energy conversion systems at room temperature, is currently being intensely discussed. Experiments using two-dimensional electronic spectroscopy (2DES) suggest that vibrational motion – and its coupling to electronic degrees of freedom – may play a key role for such coherent dynamics and potentially even for device function. In organic photovoltaics materials, strong coupling of electronic and vibrational motion is predicted, especially for ubiquitous C=C stretching vibrations. The signatures of such strong vibronic couplings in 2DES are, however, debated. Here we analyse the effect of strong vibronic coupling in model simulations of 2DES spectra and dynamics for an electronic dimer coupled to a single high-frequency vibrational mode. This system represents the simplest conceivable model for a prototypical donor–acceptor interface in the active layer of organic solar cells. The vibrational mode is chosen to mimic C=C stretching vibrations with typical large vibronic couplings predicted in organic photovoltaics materials. Our results show that the decisive signatures of strong vibronic coupling mediating coherent charge transfer between donor and acceptor are not only temporally oscillating cross-peaks, but also most importantly characteristic peak splittings in the 2DES spectra. The 2DES pattern thus directly reflects the new eigenstates of the system that are formed by strong mixing of electronic states and vibrational mode.

Niobia is a well-known solid acid catalyst owing to its intrinsic Brønsted and Lewis acidity. However, catalyst development with this oxide has been limited by our ability to control its pore structure and thermal stability. Here we report a novel post-synthetic approach for preparing mesoporous niobia catalysts. This method relies on controlling the kinetics of niobium(V) ethoxide to deposit conformal Nb2O5 overcoats on SBA-15 in a typical Stöber solution. Full Nb2O5 coverage over the mesopores of SBA-15 was achieved by adding 4 monolayer equivalents of precursor (4Nb2O5@SBA-15), which was verified by X-ray photoelectron spectroscopy. This overcoated SBA-15 had a high surface area and retained a narrow as well as ordered pore size distribution. Importantly, the typical structural transition from the amorphous to pseudo-hexagonal Nb2O5 phase did not occur with the overcoat after calcination at 773 K. Limiting this crystallization imparts an unprecedented thermal stability to our niobia overcoat, which enables the acid sites to be well preserved after catalyst regeneration. Furthermore, 4Nb2O5@SBA-15 showed higher yields than commercial niobia (HY-340) and lab-synthesized bulk niobia in two probe reactions: xylose dehydration to furfural and Friedel–Crafts alkylation. In both cases, the improvement could be explained by the unique structural features of the niobia overcoat, including a favorable ratio of Brønsted and Lewis acid sites in the case of xylose dehydration and a high proportion of isolated Nb–OH groups for the alkylation reaction. Such structural features and unprecedented thermal stability provide additional tools for synthetizing unique solid acid catalysts using a simple post-synthesis deposition method.

Plasmon-plasmon interactions are key in controlling light at the nanoscale and in the development of high-performance plasmonic devices. Sophisticated design and efficient dynamic control of such devices requires a precise description of both spectral responses and ultrafast temporal dynamics in plasmon-plasmon coupling systems. Here, a microscopic model based on the Heisenberg-Langevin formalism is developed for generic plasmon-plasmon interacting systems. The validity of the model was experimentally supported by precisely reproducing the output spectra of interacting plasmons in a one-dimensional plasmonic crystal. We discussed the inheritance from an initial phase to the Rabi phase, which is accompanied by the vacuum fluctuations dephasing the incoherent coupling channel. We further derived the generalized Rabi frequency between two coupled plasmonic modes. We demonstrate that the interplay between the Rabi phase and the coupling-induced incoherent damping process modulates the ultrafast dynamics of the polariton modes, resulting in distinctly different spectral responses including the degeneracy and reversing in the polariton branches. Our model can be readily extended to any other bosonic coupling systems such as interacting nanoparticles and photonic/plasmonic cavities, thus holding great promise and potential applications in nanocircuits and cavity QED.

Atomically thin transition metal dichalcogenides (TMDs) and halide perovskites have rapidly grown as promising materials for efficient optoelectronic devices. In both material classes, the dynamics of optical excitations and their interactions on ultrafast timescales are still debated. Using a newly developed theory, we discuss the interaction of TMD excitons and unbound charge carriers with optical phonons in the dielectric environment of the 2d layer. We find a significant reduction of exciton binding energies as well as linewidth broadening due to the dynamical coupling to environmental phonons. Moreover, we investigate near-band-edge optical transitions in CsPbBr3 single crystals at room temperature by combining ultrafast two-dimensional electronic spectroscopy and semiconductor Bloch equation calculations. An exciton binding energy of ~30 meV and remarkably short

We track ultrafast charge carrier relaxation and the concurrent build-up of dynamical exciton screening on a 30-fs timescale, and probe strong electron-phonon couplings in halide perovskite crystals by temperature-dependent two-dimensional electronic spectroscopy.

The cascaded Auger decay following ionization or excitation with 94 eV soft-X-ray pulses from the 3d subshell in krypton has been energy-and-time-resolved for the first time. The decay time of the 4s −14p −2 nl → 4p −3 + e and/or 4s −14p −1 nl → 4p −2 + e transition is measured to be 74 ± 20 fs. Furthermore, our data shows that the electrons with a kinetic energy around 25 eV (generally assigned as 3d −1 np → 4s −2 n′p + e) emitted after core excitation are emitted in a second decay step suggesting an alternative assignment of the form 4s −2 np 4p −2 + e.

We show that attosecond metrology has evolved from proof-of- principle experiments to a level where complex processes can be resolved in time that cannot be accessed using any other existing technique. The cascaded Auger decay following ionization and excitation of the 3d-subshell in Kr with subfemtosecond 94eV soft x-ray pulses has been energy- and time-resolved in an x-ray pump-infrared probe experiment. This Auger cascade reveals rich multi- electron dynamics, which despite the fact that there are many experimental and theoretical data available, is not yet fully understood. We present time-resolved data showing the sequence of the temporal dynamics in the cascaded Auger

In the presence of an IR-field, high-energy Auger-electrons are emitted with the same kinetic energy as IR-streaked electrons directly XUV-ionized from the 4s-subshell in Krypton. Interference of the two electronic wave-packets is experimentally observed.

The cascaded Auger decay following excitation with 92-eV soft-X-ray pulses from the 3p-subshell in Krypton has been energy-and-time-resolved for the first time. The decay time of the 4s-14p-1np→4p-2+e transition is measured to be 50±10 fs.

We report persistent 100-fs period Rabi oscillations between 1s and 2p excitons in halide perovskite single crystals driven by off-resonant low-frequency phonon modes. This contrasts with prevailing models for the electron-phonon coupling in these materials.