Time-resolved photoelectron spectroscopy of IR-driven electron dynamics in a charge transfer model system.

If the adiabatic approximation is valid, electrons smoothly adapt to molecular geometry changes. In contrast, as a characteristic of diabatic dynamics, the electron density does not follow the nuclear motion. Recently, we have shown that the asymmetry in time-resolved photoelectron spectra serves as a tool to distinguish between these dynamics [Falge et al., J. Phys. Chem. Lett., 2012, 3, 2617]. Here, we investigate the influence of an additional, moderately intense infrared (IR) laser field, as often applied in attosecond time-resolved experiments, on such asymmetries. This is done using a simple model for coupled electronic-nuclear motion. We calculate time-resolved photoelectron spectra and their asymmetries and demonstrate that the spectra directly map the bound electron-nuclear dynamics. From the asymmetries, we can trace the IR field-induced population transfer and both the field-driven and intrinsic (non-)adiabatic dynamics. This holds true when considering superposition states accompanied by electronic coherences. The latter are observable in the asymmetries for sufficiently short XUV pulses to coherently probe the coupled states. It is thus documented that the asymmetry is a measure for phases in bound electron wave packets and non-adiabatic dynamics.

Communication: Vibrational and vibronic coherences in the two dimensional spectroscopy of coupled electron-nuclear motion.

We theoretically investigate the photon-echo spectroscopy of coupled electron-nuclear quantum dynamics. Two situations are treated. In the first case, the Born-Oppenheimer (adiabatic) approximation holds. It is then possible to interpret the two-dimensional (2D) spectra in terms of vibrational motion taking place in different electronic states. In particular, pure vibrational coherences which are related to oscillations in the time-dependent third-order polarization can be identified. This concept fails in the second case, where strong non-adiabatic coupling leads to the breakdown of the Born-Oppenheimer-approximation. Then, the 2D-spectra reveal a complicated vibronic structure and vibrational coherences cannot be disentangled from the electronic motion.

Extended quantum jump description of vibronic two-dimensional spectroscopy.

We calculate two-dimensional (2D) vibronic spectra for a model system involving two electronic molecular states. The influence of a bath is simulated using a quantum-jump approach. We use a method introduced by Makarov and Metiu [J. Chem. Phys. 111, 10126 (1999)] which includes an explicit treatment of dephasing. In this way it is possible to characterize the influence of dissipation and dephasing on the 2D-spectra, using a wave function based method. The latter scales with the number of stochastic runs and the number of system eigenstates included in the expansion of the wave-packets to be propagated with the stochastic method and provides an efficient method for the calculation of the 2D-spectra.

Quantum diffusion wave-function approach to two-dimensional vibronic spectroscopy.

We apply the quantum diffusion wavefunction approach to calculate vibronic two-dimensional (2D) spectra. As an example, we use a system consisting of two electronic states with harmonic oscillator potentials which are coupled to a bath and interact with three time-delayed laser pulses. The first- and second-order perturbative wave functions which enter into the expression for the third-order polarization are determined for a sufficient number of stochastic runs. The wave-packet approach, besides being an alternative technique to calculate the spectra, offers an intuitive insight into the dissipation dynamics and its relation to the 2D vibronic spectra.

Theoretical analysis of the relaxation dynamics in perylene bisimide dimers excited by femtosecond laser pulses.

We present a model for the relaxation dynamics in perylene bisimide dimers, which is based on ab initio electronic structure and quantum dynamics calculations including effects of dissipation. The excited-state dynamics proceeds via a mixing of electronic states of local Frenkel and charge-transfer characters, which becomes effective upon a small distortion of the dimer geometry. In this way, it is possible to explain the fast depopulation of the photoexcited state, which we characterize by femtosecond transient absorption measurements. The combined theoretical and experimental analysis hints at a trapping mechanism, which involves nonadiabatic and dissipative dynamics in an excited-state vibronic manifold and provides an atomistic picture that might prove valuable for future design of photovoltaic materials.

Quantum wave-packet dynamics in spin-coupled vibronic states.

Extending the Shin-Metiu two-electron Hamiltonian, we construct a new Hamiltonian with effective singlet-triplet couplings. The Born-Oppenheimer electronic potentials and couplings are obtained for different parameters, and the laser-free dynamics is calculated with the full Hamiltonian and in the adiabatic limit. We compare the dynamics of the system using nuclear wave packets for different numbers of Born-Oppenheimer potentials and vibronic wave packets on a full 3-dimensional (two electron coordinates plus one nuclear coordinate) grid. Using strong fields, we show that it is possible to dynamically lock the spin state of the system by decoupling the singlet-triplet transition via a nonresonant dynamic Stark effect in the adiabatic limit. Although a similar spin-locking mechanism is observed in the dynamics of vibronic wave packets, multiphoton ionization cannot be neglected leading to the breakdown of the control scheme.

Fingerprints of Adiabatic versus Diabatic Vibronic Dynamics in the Asymmetry of Photoelectron Momentum Distributions.

When the Born-Oppenheimer approximation is valid, electrons adiabatically follow the nuclear motion in molecules. For strong nonadiabatic coupling between electronic states, one encounters a diabatic motion where the electrons remain local and do not adapt to molecular geometry changes. We show that the mentioned limiting cases are reflected differently in the asymmetry of time-resolved photoelectron momentum distributions. Whereas for adiabatic dynamics, the asymmetry directly maps the time-dependent average nuclear momentum, in the diabatic case, the asymmetry is determined by a nonclassical interference effect arising from the mixing of wave function components in different electronic states, which is present at times nonadiabatic transitions take place.

Time-resolved photoelectron spectroscopy of coupled electron-nuclear motion.

We investigate pump-probe electron detachment spectroscopy in a model system which is ideally suited to study coupled electronic and nuclear wave-packet dynamics. Time-resolved photoelectron spectra are calculated within the adiabatic approximation and a discretization of the detachment continuum. These spectra are compared to those which derive from a non-Born-Oppenheimer description and a numerically exact treatment of the detachment process. In this way it is possible to identify the influence of non-adiabatic effects on the spectra in a systematic way and also to test commonly applied approximations.

Detergent-like activity and alpha-helical structure of warnericin RK, an anti-Legionella peptide.

Warnericin RK is the first antimicrobial peptide known to be active against Legionella pneumophila, a pathogen bacterium that is responsible for severe pneumonia. Strikingly, this peptide displays a very narrow range of antimicrobial activity, almost limited to the Legionella genus, and a hemolytic activity. A similar activity has been described for delta-lysin, a well-known hemolytic peptide of Staphylococci that has not been described as antimicrobial. In this study we aimed to understand the mode of action of warnericin RK and to explain its particular target specificity. We found that warnericin RK permeabilizes artificial membranes in a voltage-independent manner. Osmotic protection experiments on erythrocytes showed that warnericin RK does not form well-defined pores, suggesting a detergent-like mode of action, as previously described for delta-lysin at high concentrations. Warnericin RK also permeabilized Legionella cells, and these cells displayed a high sensitivity to detergents. Depending on the detergent used, Legionella was from 10- to 1000-fold more sensitive than the other bacteria tested. Finally, the structure of warnericin RK was investigated by means of circular dichroism and NMR spectroscopy. The peptide adopted an amphiphilic alpha-helical structure, consistent with the proposed mode of action. We conclude that the specificity of warnericin RK toward Legionella results from both the detergent-like mode of action of the peptide and the high sensitivity of these bacteria to detergents.

Sensitivity-optimized experiment for the measurement of residual dipolar couplings between amide protons.

High signal to noise is a necessity for the quantification of NMR spectral parameters to be translated into accurate and precise restraints on protein structure and dynamics. An important source of long-range structural information is obtained from (1)H-(1)H residual dipolar couplings (RDCs) measured for weakly aligned molecules. For sensitivity reasons, such measurements are generally performed on highly deuterated protein samples. Here we show that high sensitivity is also obtained for protonated protein samples if the pulse schemes are optimized in terms of longitudinal relaxation efficiency and J-mismatch compensated coherence transfer. The new sensitivity-optimized quantitative J-correlation experiment yields important signal gains reaching factors of 1.5 to 8 for individual correlation peaks when compared to previously proposed pulse schemes.