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Combining experimental set up and ab initio molecular dynamics simulations, we were able to follow the time evolution of the X-ray absorption near edge spectrum (XANES) of a dense copper plasma. This provides a deep insight into femtosecond laser interaction with a metallic copper target. This paper presents a review of the experimental developments we made to reduce the X-ray probe duration, from approximately 10 ps to fs duration with table-top laser systems. Moreover, we present microscopic scale simulations, performed with Density Functional Theory, as well as macroscopic simulations considering the Two-Temperature Model. These tools allow us to get a complete picture of the evolution of the target at a microscopic level, from the heating process to the melting and expansion stages, with a clear view of the physics involved during these processes. This article is part of the theme issue 'Dynamic and transient processes in warm dense matter'. [ABSTRACT FROM AUTHOR]

Controlling how electromagnetic waves interact with complex media is critical for applications in imaging and focusing. Such lightwave interactions with complex media can lead to dramatic optical effects like lasing. While much work in random lasing focus on understanding how gain and scattering co‐operatively generate lasing, little work has focused on how to manipulate the lasing threshold without modifying the structural disorder. Here, a simple, mostly unexplored, strategy is demonstrated that employs atomic layer deposition (ALD) to tune the local near‐field environment while preserving the underpinning disorder—controlling lasing in a nanoscale complex medium on a large scale (>cm2). The nanoscale complex medium is a quasi‐2D system of coupled zinc oxide nanospheres with overall thickness deep in the sub‐wavelength regime (≈λ/4). Near‐ultraviolet femtosecond spectroscopy probes the broadband response of the gain nanomaterial, details how ALD process fundamentally modifies the fast‐picosecond and slow‐nanosecond carrier dynamics, and informs on the relevant timescales critical for lasing. Full‐field electromagnetic simulations provide critical insights about how near‐field dielectric environment modifies the nanostructure's scattering cross‐section, which ultimately results in enhanced lasing. These results highlight a simple path to control how electromagnetic waves interact in a complex medium, a key step toward large‐scale implementation of complex lasers. [ABSTRACT FROM AUTHOR]

Photoexcited molecules convert light into chemical and mechanical energy through changes in electronic and nuclear structure that take place on femtosecond timescales. Gas phase ultrafast electron diffraction (GUED) is an ideal tool to probe the nuclear geometry evolution of the molecules and complements spectroscopic methods that are mostly sensitive to the electronic state. GUED is a weak and passive probing tool that does not alter the molecular properties during the probing process and is sensitive to the spatial distribution of charge in the molecule, including both electrons and nuclei. Improvements in temporal resolution have enabled GUED to capture coherent nuclear motions in molecules in the excited and ground electronic states with femtosecond and subangstrom resolution. Here we present the basic theory of GUED and explain what information is encoded in the diffraction signal, review how GUED has been used to observe coherent structural dynamics in recent experiments, and discuss the advantages and limitations of the method. [ABSTRACT FROM AUTHOR]

Time- and angle-resolved photoemission spectroscopy (trARPES) using femtosecond extreme ultraviolet high harmonics has recently emerged as a powerful tool for investigating ultrafast quasiparticle dynamics in correlated-electron materials. However, the full potential of this approach has not yet been achieved because, to date, high harmonics generated by 800 nm wavelength Ti:Sapphire lasers required a trade-off between photon flux, energy and time resolution. Photoemission spectroscopy requires a quasi-monochromatic output, but dispersive optical elements that select a single harmonic can significantly reduce the photon flux and time resolution. Here we show that 400 nm driven high harmonic extreme-ultraviolet trARPES is superior to using 800 nm laser drivers since it eliminates the need for any spectral selection, thereby increasing photon flux and energy resolution to < 150 meV while preserving excellent time resolution of about 30 fs. © 2014 The Authors.

Surface plasmon polaritons (SPPs) can confine and guide light in nanometer volumes and are ideal tools for achieving electric field enhancement and the construction of nanophotonic circuitry. The realization of the highest field strengths and fastest switching requires confinement also in the temporal domain. Here, we demonstrate a tapered plasmonic waveguide with an optimized grating structure that supports few-cycle surface plasmon polaritons with >70 THz bandwidth while achieving >50% light-field-to-plasmon coupling efficiency. This enables us to observe the─to our knowledge─shortest reported SPP wavepackets. Using time-resolved photoelectron microscopy with suboptical-wavelength spatial and sub-10 fs temporal resolution, we provide full spatiotemporal imaging of co- and counter-propagating few-cycle SPP wavepackets along tapered plasmonic waveguides. By comparing their propagation, we track the evolution of the laser-plasmon phase, which can be controlled via the coupling conditions.

Photochromic ring closure reaction dynamics of 1,2‐bis(2‐methylbenzo[b]thiophene‐3‐yl)hexafluoro cyclopentene is investigated by means of time‐resolved fluorescence spectra. Compared with a single wavelength probe, direct measurement of the emission spectra during the reaction provides unambiguous and straightforward picture for the ring closure reaction pathway. We observe two different emission bands in the fluorescence of the open ring isomer. From the global analysis for the spectra, we obtain <1 ps time constant for the ring closure reaction in polar solvent. Moreover, the ring closure reaction of the compound takes place entirely on the ultrafast timescale. The unreactive parallel conformer decays to the ground state by 150 ps time constant. [ABSTRACT FROM AUTHOR]

The time-resolved photodynamics of the methyl iodide cation (CH _3 I ^+ ) are investigated by means of femtosecond XUV–IR pump–probe spectroscopy. A time-delay-compensated XUV monochromator is employed to isolate a specific harmonic, the 9th harmonic of the fundamental 800 nm (13.95 eV, 88.89 nm), which is used as a pump pulse to prepare the cation in several electronic states. A time-delayed IR probe pulse is used to probe the dissociative dynamics on the first excited $\tilde {A}\enspace {}^{2}\mathrm{A}_{1}$ state potential energy surface. Photoelectrons and photofragment ions— ${\mathrm{C}\mathrm{H}}_{3}^{+}$ and I ^+ —are detected by velocity map imaging. The experimental results are complemented with high level ab initio calculations for the potential energy curves of the electronic states of CH _3 I ^+ as well as with full dimension on-the-fly trajectory calculations on the first electronically excited state $\tilde {A}\enspace {}^{2}\mathrm{A}_{1}$ , considering the presence of the IR pulse. The ${\mathrm{C}\mathrm{H}}_{3}^{+}$ and I ^+ pump–probe transients reflect the role of the IR pulse in controlling the photodynamics of CH _3 I ^+ in the $\tilde {A}\enspace {}^{2}\mathrm{A}_{1}$ state, mainly through the coupling to the ground state $\tilde {X}\enspace {}^{2}\mathrm{E}_{3/2,1/2}$ and to the excited $\tilde {B}\enspace {}^{2}\mathrm{E}$ state manifold. Oscillatory features are observed and attributed to a vibrational wave packet prepared in the $\tilde {A}\enspace {}^{2}\mathrm{A}_{1}$ state. The IR probe pulse induces a coupling between electronic states leading to a slow depletion of ${\mathrm{C}\mathrm{H}}_{3}^{+}$ fragments after the cation is transferred to the ground $\tilde {X}\enspace {}^{2}\mathrm{E}_{3/2,1/2}$ states and an enhancement of I ^+ fragments by absorption of IR photons yielding dissociative photoionization.

The femtosecond-laser induced photodissociation of CH 3 Br adsorbed at sub-monolayer coverage on a solid surface was investigated by time-resolved pump-probe mass spectrometry. To tune the interaction of the CH 3 Br molecules with the substrate, an Mo(1 0 0) surface was covered with ultrathin insulating MgO layers of variable thickness. By gradually decreasing the magnesia layer thickness to the 2D limit the photodissociation dynamics observed by detection of the methyl fragment indicates an energetic lowering of the relevant methyl bromide excited states due to the increasing spatial proximity of the metallic support. Potential orientational effects of the methyl bromide adsorption geometry are also considered. [ABSTRACT FROM AUTHOR]

Heavy-enzyme isotope effects (15N-, 13C-, and ²H-labeled protein) explore mass-dependent vibrational modes linked to catalysis. Transition path-sampling (TPS) calculations have predicted femtosecond dynamic coupling at the catalytic site of human purine nucleoside phosphorylase (PNP). Coupling is observed in heavy PNPs, where slowed barrier crossing caused a normal heavyenzyme isotope effect (kchem light/kchem heavy > 1.0). We used TPS to design mutant F159Y PNP, predicted to improve barrier crossing for heavy F159Y PNP, an attempt to generate a rare inverse heavyenzyme isotope effect (kchem light/kchem heavy < 1.0). Steady-state kinetic comparison of light and heavy native PNPs to light and heavy F159Y PNPs revealed similar kinetic properties. Pre-steadystate chemistry was slowed 32-fold in F159Y PNP. Pre-steady-state chemistry compared heavy and light native and F159Y PNPs and found a normal heavy-enzyme isotope effect of 1.31 for native PNP and an inverse effect of 0.75 for F159Y PNP. Increased isotopic mass in F159Y PNP causes more efficient transition state formation. Independent validation of the inverse isotope effect for heavy F159Y PNP came from commitment to catalysis experiments. Most heavy enzymes demonstrate normal heavy-enzyme isotope effects, and F159Y PNP is a rare example of an inverse effect. Crystal structures and TPS dynamics of native and F159Y PNPs explore the catalytic-site geometry associated with these catalytic changes. Experimental validation of TPS predictions for barrier crossing establishes the connection of rapid protein dynamics and vibrational coupling to enzymatic transition state passage. [ABSTRACT FROM AUTHOR]

With the decrease of solvent hydrogen bond interaction, the proportion of ultrafast quenched conformations increases gradually. Ultrafast decay will dominate the shape of the time-resolved emission spectra in DMF. [Display omitted] • The ultrafast fluorescence dynamics of NADH free in aprotic solvents (DMSO/DMF) in detail have been reported by up-conversion technology. • The proportion of various components of ultrafast fluorescence dynamics in NADH was revealed by DAS (decay associated spectra) and TRES (time-resolved emission spectra). • This confirms the theoretical model for NADH that the ultrafast quenching conformation is covered by "biological water". • This provides a reliable case and preliminary experimental data for the more refined correction of FLIM. We have recently provided some experimental evidence that there are ultrafast quenched conformation(s) ("dark states") of NADH. In this paper, the ultrafast fluorescence dynamics of NADH free in aprotic solvents (DMSO/DMF) has been investigated, using both a femtosecond up-conversion spectrophotofluorometer and a picosecond time-correlated single-photon counting (TCSPC) apparatus. The fluorescence kinetics in three solvents were recovered by constructing DAS (decay associated spectra) and TRES (time-resolved emission spectra). With the decrease of the ability of solvent to provide hydrogen bond interaction, apparent slow solvent relaxation (SSR) amplitudes were reduced, and the signal of "pure" quasi-static self-quenching (QSSQ) emerges. This fully positive DAS (positive at all emission wavelengths) appeared only in DMF, and this component accounts for about 20% of the total DAS. This confirms a model in which the ultrafast quenching portion of DAS is masked at times by the negative-going signal from slow solvent relaxation (SSR). Further, we have shown that the ratio of any slow solvent relaxation and QSSQ terms should be accounted for when quantifying NADH via fluorescence lifetime imaging microscopy (FLIM). Eventually, the other properties of the solvent (not only hydrogen bonding, but also polarity, viscosity, etc.) incorporated in QM-MM simulation must be fully considered to predict this ultrafast quenching vs SSR mixture more accurately. For now, we exploit the greatly reduced H-bonding to decrypt the QSSQ in a mixture. [ABSTRACT FROM AUTHOR]

Long regarded as a model system for studying insulator-to-metal phase transitions, the correlated electron material vanadium dioxide (VO2) is now finding novel uses in device applications. Two of its most appealing aspects are its accessible transition temperature (~341 K) and its rich phase diagram. Strain can be used to selectively stabilize different VO2 insulating phases by tuning the competition between electron and lattice degrees of freedom. It can even break the mesoscopic spatial symmetry of the transition, leading to a quasiperiodic ordering of insulating and metallic nanodomains. Nanostructuring of strained VO2 could potentially yield unique components for future devices. However, the most spectacular property of VO2--its ultrafast transition--has not yet been studied on the length scale of its phase heterogeneity. Here, we use ultrafast near-field microscopy in the mid-infrared to study individual, strained VO2 nanobeams on the 10 nm scale. We reveal a previously unseen correlation between the local steady-state switching susceptibility and the local ultrafast response to below-threshold photoexcitation. These results suggest that it may be possible to tailor the local photoresponse of VO2 using strain and thereby realize new types of ultrafast nano-optical devices. [ABSTRACT FROM AUTHOR]

This article reviews recent progress in the application of core hole clock approach in the framework of resonant Auger electron spectroscopy to the monomolecular assembles of alkyl, oligophenyl, and oligo(phenylene–ethynylene) based molecules on Au(1 1 1) substrates, referring mostly to the work by the author et al. The major goal was to study electron transfer (ET) dynamics in these systems serving as prototypes of molecular electronics (ME) devices. The ET pathway to the conductive substrate was unambiguously defined by resonant excitation of the nitrile tailgroup attached to the molecular backbone. Characteristic ET times within the femtosecond domain were determined, along with the attenuation factors for the ET dynamics, analogous to the case of the static transport. The above parameters were found to exhibit strong dependence on the character of the molecular orbital which mediates the ET process. In addition, certain spectral features, which can be associated with an inverse ET from the molecular backbone to the excitation site, were observed upon exchange of the nitrile group by strongly electronegative nitro moiety. The reported results represent a valuable input for theory and a certain potential for applications such as ME devices where optimization of ET can have significant technological impact. [ABSTRACT FROM AUTHOR]

1T-TaS 2 is a 2D metallic compound which undergoes a series of electronically driven phase transitions toward charge density wave and Mott phases. Its intricate electron–phonon interactions and electron–electron correlations have been promising peculiar out-of-equilibrium dynamics. In this paper, we provide the first direct information on the atomic structure response to an ultra-fast infrared laser pulse in the commensurate phase of 1T-TaS 2 , by using femtosecond time-resolved X-ray diffraction. We show that ultra-fast excitation with near-infrared photons drives a displacive excitation of the amplitude mode of the commensurate charge density wave. About 3 ps after laser excitation, the system reaches a new, photo-induced state that is maintained for at least 10 ps. We give evidence that this long-lived state exhibits the same structural modulation as in the thermodynamically stable commensurate phase, with a large correlation length. Only the average amplitude of the modulation is found to decrease. We propose that the long-lived state is formed from the commensurate phase by reducing the modulation amplitude on few superlattice nodes. The underlying mechanism proposed is the annihilation of self-trapped polarons. [ABSTRACT FROM AUTHOR]

Time- and angle-resolved photoemission spectroscopy ( tr ARPES) using femtosecond extreme ultraviolet high harmonics has recently emerged as a powerful tool for investigating ultrafast quasiparticle dynamics in correlated-electron materials. However, the full potential of this approach has not yet been achieved because, to date, high harmonics generated by 800 nm wavelength Ti:Sapphire lasers required a trade-off between photon flux, energy and time resolution. Photoemission spectroscopy requires a quasi-monochromatic output, but dispersive optical elements that select a single harmonic can significantly reduce the photon flux and time resolution. Here we show that 400 nm driven high harmonic extreme-ultraviolet tr ARPES is superior to using 800 nm laser drivers since it eliminates the need for any spectral selection, thereby increasing photon flux and energy resolution to <150 meV while preserving excellent time resolution of about 30 fs. [ABSTRACT FROM AUTHOR]

The unoccupied electronic structure of epitaxial Pb films on Si(1 1 1) is analyzed by angle-resolved two-photon photoemission in the Γ ¯ → M ¯ direction close to the Brillouin zone center. The experimental results are compared to density functional theory calculations and we focus on the nature of the interaction of the 6 p z states with the Si substrate. The experimentally obtained dispersion E ( k || ) of the unoccupied quantum well states is weaker than expected for freestanding films, in good agreement with their occupied counterparts. Following E ( k || ) of quantum well states as a function of momentum at different energies, which are degenerate and non-degenerate with the Si conduction band, we observe no influence of the Si bulk band and conclude a vanishing direct interaction of the Pb 6 p z states with the Si band. However, the momentum range at which mixing of 6 p z and 6 p x , y derived subbands is found to occur in the presence of the Si substrate is closer to Γ ¯ than in the corresponding freestanding film, which indicates a substrate-mediated enhancement of the mixing of these states. Additional femtosecond time-resolved measurements show a constant relaxation time of hot electrons in unoccupied quantum well states as a function of parallel electron momentum which supports our conclusion of a p x , y mediated interaction of the p z states with the Si conduction band. [ABSTRACT FROM AUTHOR]

The flavin cofactor in photoenzyme photolyase and photoreceptor cryptochrome may exist in an oxidized state and should be converted into reduced state(s) for biological functions. Such redox changes can be efficiently achieved by photoinduced electron transfer (ET) through a series of aromatic residues in the enzyme. Here, we report our complete characterization of photoreduction dynamics of photolyase with femtosecond resolution. With various site-directed mutations, we identified all possible electron donors in the enzyme and determined their ET timescales. The excited cofactor behaves as an electron sink to draw electron flow from a series of encircling aromatic molecules in three distinct layers from the active site in the center to the protein surface. The dominant electron flow follows the conserved tryptophan triad in a hopping pathway across the layers with multiple tunneling steps. These ET dynamics occur ultrafast in less than 150 ps and are strongly coupled with local protein and solvent relaxations. The reverse electron flow from the flavin is slow and in the nanosecond range to ensure high reduction efficiency. With 12 experimentally determined elementary ET steps and 6 ET reaction pairs, the enzyme exhibits a distinct reduction-potential gradient along the same aromatic residues with favorable reorganization energies to drive a highly unidirectional electron flow toward the active-site center from the protein surface. [ABSTRACT FROM AUTHOR]

The flavin adenine dinucleotide cofactor has an unusual bent configuration in photolyase and cryptochrome, and such a folded structure may have a functional role in initial photochemistry. Using femtosecond spectroscopy, we report here our systematic characterization of cyclic intramolecular electron transfer (ET) dynamics between the flavin and adenine moieties of flavin adenine dinucleotide in four redox forms of the oxidized, neutral, and anionic semiquinone, and anionic hydroquinone states. By comparing wild-type and mutant enzymes, we have determined that the excited neutral oxidized and semiquinone states absorb an electron from the adenine moiety in 19 and 135 ps, whereas the excited anionic semiquinone and hydroquinone states donate an electron to the adenine moiety in 12 ps and 2 ns, respectively. All back ET dynamics occur ultrafast within 100 ps. These four ET dynamics dictate that only the anionic hydroquinone flavin can be the functional state in photolyase due to the slower ET dynamics (2 ns) with the adenine moiety and a faster ET dynamics (250 ps) with the substrate, whereas the intervening adenine moiety mediates electron tunneling for repair of damaged DNA. Assuming ET as the universal mechanism for photolyase and cryptochrome, these results imply anionic flavin as the more attractive form of the cofactor in the active state in cryptochrome to induce charge relocation to cause an electrostatic variation in the active site and then lead to a local conformation change to initiate signaling. [ABSTRACT FROM AUTHOR]

Abstract: Vibrational dynamics of the water shell around DNA oligomers are addressed by femtosecond two-dimensional (2D) infrared spectroscopy in a wide range of water concentrations. In the 2D spectra, NH stretching excitations of DNA are clearly separated from OH stretching excitations of water. For full hydration, OH stretching excitations display spectral diffusion on a time scale of 500fs. Vibrational relaxation results in the build-up of a hot water ground state on a 1–2ps time scale. A comparison of center line slopes of the 2D spectra with the frequency–time correlation function of bulk water suggests a decay of frequency correlations distinctly slower than in bulk H2O. We attribute this behavior to local DNA–water interactions which reduce structural fluctuations and to the smaller rate of resonant OH stretch energy transfer. In contrast, the kinetics of excess energy dissipation are similar in the two systems. [Copyright &y& Elsevier]

The photochemically induced electrocyclic ring-opening reaction of 1,3-cyclohexadiene to 1,3,5-hexatriene serves as a prototype for many important reactions in chemistry and in biological systems. Based on experimental and computational studies, a detailed picture of the reaction has now emerged: Excitation to the Franck-Condon region places the molecule on a steeply repulsive part of the 1B potential energy surface, which propels the molecule in exactly the conrotatory direction that conforms to the Woodward-Hoffmann rules of orbital symmetry. Bypassing a cusp in a symmetry-breaking direction, the wave packet enters the 2A state within 55 fs. It continues to move directly toward the 2A//1A conical intersection, where it crosses in approximately 80 fs to the ground state. This article summarizes the published experimental and theoretical work to describe the current understanding of the reaction while pointing to important questions that remain to be addressed. [ABSTRACT FROM AUTHOR]

Abstract: Using sub-5fs visible laser pulses, ultrafast time-resolved absorption change was observed for RuII (TPP)(CO) (TPP=5,10, 15, 20 tetraphenylporphirine) solved in chloroform. Exponential fitting of the time traces estimated decay life times of intermediates as 230±70 fs, 1150±260 fs, 2150±360 fs, and ≫4.8 ps for , , and , respectively. Real-time 4.8 change of instantaneous molecular vibration frequency was studied by spectrogram analysis, which reflects gradual vibration mode change in the intersystem crossing from singlet state to triplet state. [Copyright &y& Elsevier]

Abstract: Resonant X-ray Raman scattering is a powerful tool to study molecular dynamics and subtle chemical effects like the molecular field beyond vibrational and lifetime limitations. Using this technique in the tender X-ray region, gas phase HCl is studied as a benchmark molecule for other compounds like freons, which play an important role in physical–chemical properties of the ozone layer of atmosphere. [ABSTRACT FROM AUTHOR]

In this review we attempt to sketch an overview of the various methods currently being used or under development to enable ultra-fast dynamic imaging of matter. We concentrate on those techniques which combine atomic scale spatial resolution and femtosecond or even sub-femtosecond temporal resolution. In part this review was inspired and informed by the material presented at the 'Ultrafast Dynamic Imaging II' workshop held in Ischia, Italy in April 2009, but we also have drawn on a wider background of material especially when discussing the emerging laser-based methods. [ABSTRACT FROM AUTHOR]

To celebrate the 10 anniversary of the Nobel Prize for Femtochemistry, the field is overviewed with several classic examples and the new extension to Femtobiology. The revolutionary breakthrough in 4D electron microscopy is briefly introduced here and a new age to structural dynamics is rising on the horizon, an exciting time and a great opportunity for China and for the world. [ABSTRACT FROM AUTHOR]

Abstract: Quenching of tryptophan fluorescence in proteins has been critical to the understanding of protein dynamics and enzyme reactions using tryptophan as a molecular optical probe. We report here our systematic examinations of potential quenching residues with more than 40 proteins. With site-directed mutation, we placed tryptophan to desired positions or altered its neighboring residues to screen quenching groups among 20 amino acid residues and of peptide backbones. With femtosecond resolution, we observed the ultrafast quenching dynamics within 100ps and identified two ultrafast quenching groups, the carbonyl- and sulfur-containing residues. The former is glutamine and glutamate residues and the later is disulfide bond and cysteine residue. The quenching by the peptide-bond carbonyl group as well as other potential residues mostly occurs in longer than 100ps. These ultrafast quenching dynamics occur at van der Waals distances through intraprotein electron transfer with high directionality. Following optimal molecular orbital overlap, electron jumps from the benzene ring of the indole moiety in a vertical orientation to the LUMO of acceptor quenching residues. Molecular dynamics simulations were invoked to elucidate various correlations of quenching dynamics with separation distances, relative orientations, local fluctuations and reaction heterogeneity. These unique ultrafast quenching pairs, as recently found to extensively occur in high-resolution protein structures, may have significant biological implications. [Copyright &y& Elsevier]

Abstract: The time evolution of electronically excited vitamin B12 (cyanocobalamin) has been observed for the first time in the gas phase. It reveals an ultrafast decay to a state corresponding to metal excitation. This decay is interpreted as resulting from a ring to metal electron transfer. This opens the observation of the excited state of other complex biomimetic systems in the gas phase, the key to the characterisation of their complex evolution through excited electronic states. [Copyright &y& Elsevier]

Abstract: Rotational mechanisms of a chiral molecular motor driven by femtosecond laser pulses were investigated on the basis of results of a quantum control simulation. A chiral molecule, (R)-2-methyl-cyclopenta-2,4-dienecarboaldehyde, was treated as a molecular motor within a one-dimensional model. It was assumed that the motor is fixed on a surface and driven in the low temperature limit. Electric fields of femtosecond laser pulses driving both regular rotation of the molecular motor with a plus angular momentum and reverse rotation with a minus one were designed by using a global control method. The mechanism of the regular rotation is similar to that obtained by a conventional pump–dump pulse method: the direction of rotation is the same as that of the initial wave packet propagation on the potential surface of the first singlet excited state . A new control mechanism has been proposed for the reverse rotation that cannot be driven by a simple pump–dump pulse method. In this mechanism, a coherent Stokes pulse creates a wave packet localized on the ground state potential surface in the right hand side. The wave packet has a negative angular momentum to drive reverse rotation at an early time. [Copyright &y& Elsevier]

Abstract: Quantum wavepacket dynamics at the X 2 A 1/A 2 B 2 conical intersection of NO2 molecule following a femtosecond pump pulse is computed, and subsequent probing of the dynamics with time-resolved femtosecond photoelectron spectroscopy is investigated theoretically. Diabatic representation of the ground and first excited state potential energy surfaces are constructed, and is used in time propagation of the vibrational wave function through pump and probe pulses in three dimensions. The Hamiltonian explicitly includes the pump interaction between the neutral states and probe interaction between the neutral and (discretized) ion continuum. Constant ionization matrix elements are used in this study. The computed photoelectron kinetic energy spectra are seen to reflect ultrafast wavepacket motion through the conical intersection region at early times. [Copyright &y& Elsevier]

A time-dependent generalized non-linear Schrödinger equation (GNLSE) of motion was earlier derived in our laboratory by combining density functional theory and quantum fluid dynamics in three-dimensional space. In continuation of the work reported previously, the GNLSE is applied to provide additional knowledge on the femtosecond dynamics of the electron density in the hydrogen molecule interacting with high-intensity laser fields. For this purpose, the GNLSE is solved numerically for many time-steps over a total interaction time of 100 fs, by employing a finite-difference scheme. Various time-dependent (TD) quantities, namely, electron density, ground-state survival probability and dipole moment have been obtained for two laser wavelengths and four different intensities. The high-order harmonics generation (HHG) is also examined. The present approach goes beyond the linear response formalism and, in principle, calculates the TD electron density to all orders of change. [ABSTRACT FROM AUTHOR]

Abstract: Image-potential states provide a model system to study electron scattering at surfaces. With time-, energy- and angle-resolved two-photon photoemission quasi-elastic intraband and resonant interband scattering processes can be identified and resolved. The scattering sources are related to phonons and to imperfections of the surface such as defects and steps. [Copyright &y& Elsevier]

We report our direct study of complex protein dynamics in human thioredoxin by dissecting into elementary processes and determining their relevant time scales. By combining site-directed mutagenesis with femtosecond spectroscopy, we have distinguished four partly time-overlapped dynamical processes at the active site of thioredoxin. Using intrinsic tryptophan as a molecular probe and from mutation studies, we ascertained the negligible contribution to solvation by protein sidechains and observed that the hydration dynamics at the active site occur in 0.47–0.67 and 10.8–13.2 ps. With reduced and oxidized states, we determined the electron-transfer quenching dynamics between excited tryptophan and a nearby disulfide bond in 10–1 7.5 ps for three mutants. A robust dynamical process in 95–114 ps, present in both redox states and all mutants regardless of neighboring charged, polar, and hydrophobic residues around the probe, is attributed to the charge transfer reaction with its adjacent peptide bond. Site-directed mutations also revealed the electronic quenching dynamics by an aspartate residue at a hydrogen bond distance in 275–615 ps. The local rotational dynamics determined by the measurement of anisotropy changes with time unraveled a relatively rigid local configuration but implies that the protein fluctuates on the time scale of longer than nanoseconds. These results elucidate the temporal evolution of hydrating water motions, electron-transfer reactions, and local protein fluctuations at the active site, and show continuously synergistic dynamics of biological function over wide time scales. [ABSTRACT FROM AUTHOR]

Water motion at protein surfaces is fundamental to protein structure, stability, dynamics, and function. By using intrinsic tryptophans as local optical probes, and with femtosecond resolution, it is possible to probe surface-water motions in the hydration layer. Here, we report our studies of local hydration dynamics at the surface of the enzyme Staphylococcus nuclease using site-specific mutations. From these studies of the WT and four related mutants, which change local charge distribution and structure, we are able to ascertain the contribution to solvation by protein side chains as relatively insignificant. We determined the time scales of hydration to be 3–5 ps and 100–150 ps. The former is the result of local librational/rotational motions of water near the surface; the latter is a direct measure of surface hydration assisted by fluctuations of the protein. Experimentally, these hydration dynamics of the WT and the four mutants are also consistent with results of the total dynamic Stokes shifts and fluorescence emission maxima and are correlated with their local charge distribution and structure. We discuss the role of protein fluctuation on the time scale of labile hydration and suggest reexamination of recent molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

Abstract: The lifetime of electronic excitations at surfaces play an important role for desorption processes induced by electronic transitions. Two-photon photoemission with time, energy, and angular resolution allows to study the scattering processes of excited electrons in detail. For electronic surface states various scattering channels can be identified, such as elastic vs. inelastic or intraband vs. interband. As examples the scattering of electrons in image-potential states by copper adatoms on Cu(001) and the decay and exciton formation in the dangling-bond surface conduction band at the Si(100) c(4×2) surface are presented. These studies of the carrier dynamics at surfaces imply that photochemical or electronic desorption processes are influenced by surface defects and are more efficient at semiconductor than on metal surfaces. [Copyright &y& Elsevier]

Recent experimental work has reached an accurate determination of surface-state linewidth by scanning tunneling spectroscopy, photoemission and directly in the time domain by two-photon photoemission. The concomitant progress in the theoretical description of surface-state decay has led to an excellent agreement between theory and experiment for low-index noble-metal surfaces. The understanding of the behavior of s–p-like surface states and image-potential states on these surfaces constitutes the basis for the application to more complicated systems. [Copyright &y& Elsevier]

A time-of-flight (TOF) mass spectrometry gating technique is applied to a study of methanol clusters subjected to ionizations via intense femtosecond laser pulses. The resulting high charged species (C2+, C3+/O4+) acquire large amounts of kinetic energy resulting from Coulomb repulsion of multicharged atomic ions that reside in close proximity to one another. Protons which are of two kinds, methyl and hydroxyl, also acquire large amounts of kinetic energy. When compared with protons generated from the Coulomb explosion of water clusters ((H2O)n, n≤20), protons from methanol clusters ((CH3OH)n, n≤10) acquire less overall average kinetic energy, which is in agreement with earlier findings that suggest greater clustering yields higher energy. Interestingly, despite the lower average kinetic energy released, the methanol protons peak at a higher value of energy than those generated in the water cluster system, an effect attributed to the presence of both methyl and hydroxyl groups. [Copyright &y& Elsevier]

The relaxation of the Q1(π—π*) excited state of the nonprotonated Fc4PH2 and diprotonated Fc4PH4 2+ forms of meso-tetraferrocenylporphyrin was studied by femtosecond laser absorption spectroscopy. Transition from the Q1(π—π*) state to the charge-transfer state was shown to occur within 208±10 fs for Fc4PH2 and 9±3 ps for Fc4PH4 2+. A fast vibrational relaxation with a characteristic time of 120—140 fs was found for both forms. The relaxation time of Fcδ+—Pδ– charge-transfer state for Fc4PH2 was 17±4 ps. [ABSTRACT FROM AUTHOR]

Relaxation of photoexcited states in nanosized semiconductor particles of iron oxides was studied by femtosecond laser photolysis techniques: (1) in an aqueous colloidal solution of α-Fe2O3; (2) in Fe2O3 particles in the Nafion® cation-exchange polymeric membrane; (3) in an aqueous colloid of γ-Fe2O3; and (4) in nanocrystals of ferrihydrite 5Fe2O3·9H2O, which are contained in the protein shell of ferritine. The photoinduced excited states relax at the femtosecond and picosecond time scale. The spectra of photoinduced absorption of photoexcited states and the relaxation dynamics in the studied iron oxides weakly depend on the structure and surface environment of a nanoparticle. [ABSTRACT FROM AUTHOR]