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Concept: Femtosecond


Protein hydration is essential to its structure, dynamics, and function, but water-protein interactions have not been directly observed in real time at physiological temperature to our awareness. By using a tryptophan scan with femtosecond spectroscopy, we simultaneously measured the hydration water dynamics and protein side-chain motions with temperature dependence. We observed the heterogeneous hydration dynamics around the global protein surface with two types of coupled motions, collective water/side-chain reorientation in a few picoseconds and cooperative water/side-chain restructuring in tens of picoseconds. The ultrafast dynamics in hundreds of femtoseconds is from the outer-layer, bulk-type mobile water molecules in the hydration shell. We also found that the hydration water dynamics are always faster than protein side-chain relaxations but with the same energy barriers, indicating hydration shell fluctuations driving protein side-chain motions on the picosecond time scales and thus elucidating their ultimate relationship.

Concepts: Oxygen, Energy, Light, Water, Orders of magnitude, Picosecond, Femtosecond, Femtochemistry


A variety of organisms have evolved mechanisms to detect and respond to light, in which the response is mediated by protein structural changes after photon absorption. The initial step is often the photoisomerization of a conjugated chromophore. Isomerization occurs on ultrafast time scales and is substantially influenced by the chromophore environment. Here we identify structural changes associated with the earliest steps in the trans-to-cis isomerization of the chromophore in photoactive yellow protein. Femtosecond hard x-ray pulses emitted by the Linac Coherent Light Source were used to conduct time-resolved serial femtosecond crystallography on photoactive yellow protein microcrystals over a time range from 100 femtoseconds to 3 picoseconds to determine the structural dynamics of the photoisomerization reaction.

Concepts: DNA, Electron, X-ray, Energy, Optics, Light, Electromagnetic radiation, Femtosecond


The technological demand to push the gigahertz (10(9) hertz) switching speed limit of today’s magnetic memory and logic devices into the terahertz (10(12) hertz) regime underlies the entire field of spin-electronics and integrated multi-functional devices. This challenge is met by all-optical magnetic switching based on coherent spin manipulation. By analogy to femtosecond chemistry and photosynthetic dynamics–in which photoproducts of chemical and biochemical reactions can be influenced by creating suitable superpositions of molecular states–femtosecond-laser-excited coherence between electronic states can switch magnetic order by ‘suddenly’ breaking the delicate balance between competing phases of correlated materials: for example, manganites exhibiting colossal magneto-resistance suitable for applications. Here we show femtosecond (10(-15) seconds) photo-induced switching from antiferromagnetic to ferromagnetic ordering in Pr0.7Ca0.3MnO3, by observing the establishment (within about 120 femtoseconds) of a huge temperature-dependent magnetization with photo-excitation threshold behaviour absent in the optical reflectivity. The development of ferromagnetic correlations during the femtosecond laser pulse reveals an initial quantum coherent regime of magnetism, distinguished from the picosecond (10(-12) seconds) lattice-heating regime characterized by phase separation without threshold behaviour. Our simulations reproduce the nonlinear femtosecond spin generation and underpin fast quantum spin-flip fluctuations correlated with coherent superpositions of electronic states to initiate local ferromagnetic correlations. These results merge two fields, femtosecond magnetism in metals and band insulators, and non-equilibrium phase transitions of strongly correlated electrons, in which local interactions exceeding the kinetic energy produce a complex balance of competing orders.

Concepts: Electron, Magnetic field, Fundamental physics concepts, Condensed matter physics, Paramagnetism, Magnetism, Ferromagnetism, Femtosecond


Observing the crucial first few femtoseconds of photochemical reactions requires tools typically not available in the femtochemistry toolkit. Such dynamics are now within reach with the instruments provided by attosecond science. Here, we apply experimental and theoretical methods to assess the ultrafast nonadiabatic vibronic processes in a prototypical complex system-the excited benzene cation. We use few-femtosecond duration extreme ultraviolet and visible/near-infrared laser pulses to prepare and probe excited cationic states and observe two relaxation timescales of 11 ± 3 fs and 110 ± 20 fs. These are interpreted in terms of population transfer via two sequential conical intersections. The experimental results are quantitatively compared with state-of-the-art multi-configuration time-dependent Hartree calculations showing convincing agreement in the timescales. By characterising one of the fastest internal conversion processes studied to date, we enter an extreme regime of ultrafast molecular dynamics, paving the way to tracking and controlling purely electronic dynamics in complex molecules.

Concepts: Scientific method, Molecule, Chemistry, Atom, Chemical bond, Hypothesis, Physical chemistry, Femtosecond


The effect of acceptor strength variation on ultrafast twisting relaxation dynamics was investigated by comparing the ultrafast relaxation dynamics in a series of dimethylaminochalcone (DMAC) derivatives. Employing femtosecond resolved transient absorption and fluorescence experiments, the twisted intramolecular charge transfer (TICT) relaxation rate is shown to vary from 2.0 picoseconds in a weak electron accepting system to 420 femtoseconds in a strong electron accepting system. The strength of the acceptor, empirically expressed as Hammett’s constant, is shown to exhibit a linear free energy relationship (LFER) with the twisting rate. It is proposed that variation in the charge pulling capacity of the acceptor modifies the torsional barrier along the TICT coordinate in the S1 state, resulting in a tunable TICT relaxation rate, depending on the acceptor strength.

Concepts: Electron, Orders of magnitude, Derivative, Gibbs free energy, Free-energy relationship, Picosecond, Hammett equation, Femtosecond


Flavoproteins, containing flavin chromophores, are enzymes capable of transferring electrons at very high speeds. The ultrafast photoinduced electron transfer (ET) kinetics of riboflavin binding protein to the excited riboflavin was studied by femtosecond spectroscopy and found to occur within a few hundreds femtoseconds [PNAS 98.21(2001):11867-11872]. This ultrafast kinetics was attributed to the presence of two aromatic rings that could transfer the electron to riboflavin: the side chains of tryptophan 156 and tyrosine 75. However, the underlying ET mechanism remained unclear. Here, using a hybrid quantum mechanical-molecular dynamics approach, we perform ET dynamics simulations taking into account the motion of the protein and the solvent upon ET. This approach reveals that ET occurs via a major reaction channel involving tyrosine 75 (83%) and a minor one involving tryptophan 156 (17%). We also show that the protein environment is designed to assure the fast quenching of the riboflavin excited state.

Concepts: Electron, Quantum mechanics, Atom, Benzene, Excited state, Riboflavin, Conjugated system, Femtosecond


With power conversion efficiency (PCE) exceeding 22%, perovskite solar cells (PSCs) have thrilled the photovoltaic research. However, interface behavior is still not understood and a hot subject of research: different processes occur over a hierarchy of timescales, from femtoseconds to seconds, making the perovskite interface physics intriguing. Here, by femtosecond transient absorption spectroscopy with spectral coverage extended to the crucial IR region, we interrogate the ultrafast interface- specific processes at standard as well as newly molecularly-engineered perovskite interfaces used in state of the art PSCs. We demonstrate that ultrafast interfacial charge injection happens with a time constant of 100 fs, resulting in hot transfer from energetic charges and setting the time-scale for the first step involved in the complex charge transfer process. This also stems true for 20% efficient devices measured under real operation, where the femtosecond injection is followed by a slower picosecond component. Our findings provide compelling evidence of the femtosecond interfacial charge injection step demonstrating a robust method for a straightforward identification of the interfacial non-equilibrium processes at ultrafast timescale.

Concepts: Solar cell, Object-oriented programming, Robust statistics, Picosecond, Time scale, Femtosecond


Protein surface hydration is fundamental to its structure, flexibility, dynamics and function, but it has been challenging to disentangle their ultimate relationships. Here, we report our systematic characterization of hydration dynamics around a β-barrel protein, rat liver fatty acid-binding protein (rLFABP), to reveal the effect of different protein secondary structures on hydration water. We employed a tryptophan scan to the protein surface one at a time and examined total 17 different sites. We observed three types of hydration water relaxations with distinct time scales, from hundreds of femtoseconds to a hundred of picoseconds. We also examined the anisotropy dynamics of the corresponding tryptophan sidechains and observed two distinct relaxations from tens to hundreds of picoseconds. Integrating our previous findings on α-helical proteins, we conclude that (1) the hydration dynamics is highly heterogeneous around the protein surface of both α-helical and β-sheet proteins. The outer-layers water of the hydration shell behaves bulk-type and relaxes in hundreds of femtoseconds. The inner-layers water collectively relaxes in two time scales, reorientation motions in a few picoseconds and network restructuring in tens-to-a hundred of picoseconds; (2) the hydration dynamics are always faster than local protein relaxations and in fact drive the protein fluctuations on the picosecond time scale; (3) the hydration dynamics in general are more retarded around β-sheet structures than α-helical motifs. A thicker hydration shell and a more rigid interfacial hydration network are observed in the β-sheet protein. Overall, these findings elucidate the intimate relationship of water-protein interactions and dynamics on the ultrafast timescale.

Concepts: Secondary structure, Interpersonal relationship, Hydrolysis, Picosecond, Time scale, Time scales, Time standard, Femtosecond


Excited-state relaxation dynamics and energy-transfer processes in the chlorophyll a (Chl a) manifold of the light-harvesting complex II (LHCII) were examined at physiological temperature using femtosecond two-dimensional electronic spectroscopy (2DES). The experiments were done under conditions free from singlet-singlet annihilation and anisotropic decay. Energy transfer between the different domains of the Chl a manifold was found to proceed on timescales from hundreds of femtoseconds to five picoseconds, before reaching equilibration. No component slower than 10 ps was observed in the spectral equilibration dynamics. We clearly observe the bidirectional (uphill and downhill) energy transfer of the equilibration process between excited states. This bidirectional energy flow, although implicit in the modelling and simulation of the EET processes, has not been observed in any prior transient absorption studies. Furthermore, we identified the spectral forms associated with the different energy transfer lifetimes in the equilibration process.

Concepts: Photon, Orders of magnitude, Vector space, Force, Philosophy of science, Manifold, Picosecond, Femtosecond


Efficient dye-sensitized solar cells are based on highly diffusive mesoscopic layers that render these devices opaque and unsuitable for ultrafast transient absorption spectroscopy measurements in transmission mode. We developed a novel sub-200 femtosecond time-resolved diffuse reflectance spectroscopy scheme combined with potentiostatic control to study various solar cells in fully operational condition. We studied performance optimized devices based on liquid redox electrolytes and opaque TiO2 films, as well as other morphologies, such as TiO2 fibers and nanotubes. Charge injection from the Z907 dye in all TiO2 morphologies was observed to take place in the sub-200 fs time scale. The kinetics of electron-hole back recombination has features in the picosecond to nanosecond time scale. This observation is significantly different from what was reported in the literature where the electron-hole back recombination for transparent films of small particles is generally accepted to occur on a longer time scale of microseconds. The kinetics of the ultrafast electron injection remained unchanged for voltages between +500 mV and -690 mV, where the injection yield eventually drops steeply. The primary charge separation in Y123 organic dye based devices was clearly slower occurring in two picoseconds and no kinetic component on the shorter femtosecond time scale was recorded.

Concepts: Orders of magnitude, Solar cell, Absorption, Titanium dioxide, Dye-sensitized solar cell, Nanosecond, Picosecond, Femtosecond