Electron and ion spectroscopy of azobenzene in the valence and core shells.

Azobenzene is a prototype and a building block of a class of molecules of extreme technological interest as molecular photo-switches. We present a joint experimental and theoretical study of its response to irradiation with light across the UV to x-ray spectrum. The study of valence and inner shell photo-ionization and excitation processes combined with measurement of valence photoelectron-photoion coincidence and mass spectra across the core thresholds provides a detailed insight into the site- and state-selected photo-induced processes. Photo-ionization and excitation measurements are interpreted via the multi-configurational restricted active space self-consistent field method corrected by second order perturbation theory. Using static modeling, we demonstrate that the carbon and nitrogen K edges of azobenzene are suitable candidates for exploring its photoinduced dynamics thanks to the transient signals appearing in background-free regions of the NEXAFS and XPS.

Time and frequency resolved transient-absorption and stimulated-Raman signals of stochastic light.

Covariance spectroscopy signals based on the transmission of broadband stochastic probe light undergoing a nonlinear optical process with matter are studied. The resulting signal depends on intensity correlation functions of the probe pulse. Application is made to transient absorption and stimulated Raman signals, where an ultraviolet pump and a delayed stochastically modulated infrared or optical Raman probe are used for monitoring vibrational excitations. We show that the stochastic properties of light can be exploited to circumvent the limitations on the joint temporal and spectral resolution of the signal. We study a model system with a time-dependent frequency and show that the dynamical information can be fully extracted from the covariance signal. This information is heavily suppressed upon averaging of noisy signals and practically inaccessible in conventional stimulated Raman measurements.

Energy flow between spectral components in 2D broadband stimulated Raman spectroscopy.

We introduce a general theoretical description of non resonant impulsive femtosecond stimulated Raman spectroscopy in a multimode harmonic model. In this technique an ultrashort actinic pulse creates coherences of low frequency modes and is followed by a paired narrowband Raman pulse and a broadband probe pulse. Using closed-time-path-loop (CTPL) diagrams, the response on both the red and the blue sides of the broadband pulse with respect to the narrowband Raman pulse is calculated, the process couples high and low frequency modes, which share the same ground state. The transmitted intensity oscillates between the red and the blue side, while the total number of photons is conserved. The total energy of the probe signal is periodically modulated in time by the coherence created in the low frequency modes.

Probing chirality fluctuations in molecules by nonlinear optical spectroscopy.

Symmetry breaking caused by geometric fluctuations can enable processes that are otherwise forbidden. An example is a perylene bisimide dyad whose dipole moments are perpendicular to each other. Förster-type energy transfer is thus forbidden at the equilibrium geometry since the dipolar coupling vanishes. Yet, fluctuations of the geometric arrangement have been shown to induce finite energy transfer that depends on the dipole variance, rather than the mean. We demonstrate an analogous effect associated with chirality symmetry breaking. In its equilibrium geometry, this dimer is non-chiral. The linear chiral response which depends on the average geometry thus vanishes. However, we show that certain 2D chiral optical signals are finite due to geometric fluctuations. Furthermore, the correlation time of these fluctuations can be experimentally revealed by the waiting time dependence of the 2D signal.

Tracking the mechanism of fibril assembly by simulated two-dimensional ultraviolet spectroscopy.

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the accumulation of plaque deposits in the human brain. The main component of these plaques consists of highly ordered structures called amyloid fibrils, formed by the amyloid ß-peptide (Aß). The mechanism connecting Aß and AD is yet undetermined. In a previous study, a coarse-grained united-residue model and molecular dynamics simulations were used to model the growth mechanism of Aß amyloid fibrils. On the basis of these simulations, a dock/lock mechanism was proposed, in which Aß fibrils grow by adding monomers at either end of an amyloid fibril template. To examine the structures in the early time-scale formation and growth of amyloid fibrils, simulated two-dimensional ultraviolet spectroscopy is used. These early structures are monitored in the far ultraviolet regime (λ = 190-250 nm) in which the computed signals originate from the backbone nπ* and ππ* transitions. These signals show distinct cross-peak patterns that can be used, in combination with molecular dynamics, to monitor local dynamics and conformational changes in the secondary structure of Aß-peptides. The protein geometry-correlated chiral xxxy signal and the non-chiral combined signal xyxy-xyyx were found to be sensitive to, and in agreement with, a dock/lock pathway.

Simulations of the two-dimensional electronic spectroscopy of the photosystem II reaction center.

We report simulations of the two-dimensional electronic spectroscopy of the Q(y) band of the D1-D2-Cyt b559 photosystem II reaction center at 77 K. We base the simulations on an existing Hamiltonian that was derived by simultaneous fitting to a wide range of linear spectroscopic measurements and described within modified Redfield theory. The model obtains reasonable agreement with most aspects of the two-dimensional spectra, including the overall peak shapes and excited state absorption features. It does not reproduce the rapid equilibration from high energy to low energy excitonic states evident by a strong cross-peak below the diagonal. We explore modifications to the model to incorporate new structural data and improve agreement with the two-dimensional spectra. We find that strengthening the system-bath coupling and lowering the degree of disorder significantly improves agreement with the cross-peak feature, while lessening agreement with the relative diagonal/antidiagonal width of the 2D spectra. We conclude that two-dimensional electronic spectroscopy provides a sensitive test of excitonic models of the photosystem II reaction center and discuss avenues for further refinement of such models.

Element- and enantiomer-selective visualization of molecular motion in real-time.

Ultrafast optical-domain spectroscopies allow to monitor in real time the motion of nuclei in molecules. Achieving element-selectivity had to await the advent of time resolved X-ray spectroscopy, which is now commonly carried at X-ray free electron lasers. However, detecting light element that are commonly encountered in organic molecules, remained elusive due to the need to work under vacuum. Here, we present an impulsive stimulated Raman scattering (ISRS) pump/carbon K-edge absorption probe investigation, which allowed observation of the low-frequency vibrational modes involving specific selected carbon atoms in the Ibuprofen RS dimer. Remarkably, by controlling the probe light polarization we can preferentially access the enantiomer of the dimer to which the carbon atoms belong.

Trapping photon-dressed Dirac electrons in a quantum dot studied by coherent two dimensional photon echo spectroscopy.

We study the localization of dressed Dirac electrons in a cylindrical quantum dot (QD) formed on monolayer and bilayer graphene by spatially different potential profiles. Short lived excitonic states which are too broad to be resolved in linear spectroscopy are revealed by cross peaks in the photon-echo nonlinear technique. Signatures of the dynamic gap in the two-dimensional spectra are discussed. The effect of the Coulomb induced exciton-exciton scattering and the formation of biexciton molecules are demonstrated.

Distinguishing amyloid fibril structures in Alzheimer's disease (AD) by two-dimensional ultraviolet (2DUV) spectroscopy.

Understanding the aggregation mechanism of amyloid fibrils and characterizing their structures are important steps in the investigation of several neurodegenerative disorders associated with the misfolding of proteins. We report a simulation study of coherent two-dimensional chiral signals of three NMR structures of Aß protein fibrils associated with Alzheimer's Disease, two models for Aß(8-40) peptide wild-type (WT) and one for the Iowa (D23N) Aß(15-40) mutant. Both far-ultraviolet (FUV) signals (λ = 190-250 nm), which originate from the backbone nπ* and ππ* transitions, and near-ultraviolet (NUV) signals (λ ≥ 250 nm) associated with aromatic side chains (Phe and Tyr) show distinct cross-peak patterns that can serve as novel signatures for the secondary structure.

The proton momentum distribution in strongly H-bonded phases of water: a critical test of electrostatic models.

Water is often viewed as a collection of monomers interacting electrostatically with each other. We compare the water proton momentum distributions from recent neutron scattering data with those calculated from two electronic structure-based models. We find that below 500 K these electrostatic models, one based on a multipole expansion, which includes the polarizability of the monomers, are not able to even qualitatively account for the sizable vibrational zero-point contribution to the enthalpy of vaporization. This discrepancy is evidence that the change in the proton well upon solvation cannot be entirely explained by electrostatic effects alone, but requires correlations of the electronic states on the molecules involved in the hydrogen bonds to produce the observed softening of the well.