Femtosecond Time Domain Research Articles

Primary charge separation within P870* in wild type and heterodimer mutants in femtosecond time domain

Primary charge separation dynamics in the reaction center (RC) of purple bacterium Rhodobacter sphaeroides and its P870 heterodimer mutants have been studied using femtosecond time-resolved spectroscopy with 20 and 40fs excitation at 870nm at 293K. Absorbance increase in the 1060–1130nm region that is presumably attributed to PAδ+ cation radical molecule as a part of mixed state with a charge transfer character P*(PAδ+PBδ−) was found. This state appears at 120–180fs time delay in the wild type RC and even faster in H(L173)L and H(M202)L heterodimer mutants and precedes electron transfer (ET) to BA bacteriochlorophyll with absorption band at 1020nm in WT. The formation of the PAδ+BAδ− state is a result of the electron transfer from P*(PAδ+PBδ−) to the primary electron acceptor BA (still mixed with P*) with the apparent time delay of ~1.1ps. Next step of ET is accompanied by the 3-ps appearance of bacteriopheophytin a− (HA−) band at 960nm. The study of the wave packet formation upon 20-fs illumination has shown that the vibration energy of the wave packet promotes reversible overcoming of an energy barrier between two potential energy surfaces P* and P*(PAδ+BAδ−) at ~500fs. For longer excitation pulses (40fs) this promotion is absent and tunneling through an energy barrier takes about 3ps. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Time-Resolved Resonance Raman Spectroscopy: Exploring Reactive Intermediates

The study of reaction mechanisms involves systematic investigations of the correlation between structure, reactivity, and time. The challenge is to be able to observe the chemical changes undergone by reactants as they change into products via one or several intermediates such as electronic excited states (singlet and triplet), radicals, radical ions, carbocations, carbanions, carbenes, nitrenes, nitrinium ions, etc. The vast array of intermediates and timescales means there is no single "do-it-all" technique. The simultaneous advances in contemporary time-resolved Raman spectroscopic techniques and computational methods have done much towards visualizing molecular fingerprint snapshots of the reactive intermediates in the microsecond to femtosecond time domain. Raman spectroscopy and its sensitive counterpart resonance Raman spectroscopy have been well proven as means for determining molecular structure, chemical bonding, reactivity, and dynamics of short-lived intermediates in solution phase and are advantageous in comparison to commonly used time-resolved absorption and emission spectroscopy. Today time-resolved Raman spectroscopy is a mature technique; its development owes much to the advent of pulsed tunable lasers, highly efficient spectrometers, and high speed, highly sensitive multichannel detectors able to collect a complete spectrum. This review article will provide a brief chronological development of the experimental setup and demonstrate how experimentalists have conquered numerous challenges to obtain background-free (removing fluorescence), intense, and highly spectrally resolved Raman spectra in the nanosecond to microsecond (ns-μs) and picosecond (ps) time domains and, perhaps surprisingly, laid the foundations for new techniques such as spatially offset Raman spectroscopy.

From electron energy-loss spectroscopy to multi-dimensional and multi-signal electron microscopy

This review intends to illustrate how electron energy-loss spectroscopy (EELS) techniques in the electron microscope column have evolved over the past 60 years. Beginning as a physicist tool to measure basic excitations in solid thin foils, EELS techniques have gradually become essential for analytical purposes, nowadays pushed to the identification of individual atoms and their bonding states. The intimate combination of highly performing techniques with quite efficient computational tools for data processing and ab initio modeling has opened the way to a broad range of novel imaging modes with potential impact on many different fields. The combination of Angström-level spatial resolution with an energy resolution down to a few tenths of an electron volt in the core-loss spectral domain has paved the way to atomic-resolved elemental and bonding maps across interfaces and nanostructures. In the low-energy range, improved energy resolution has been quite efficient in recording surface plasmon maps and from them electromagnetic maps across the visible electron microscopy (EM) domain, thus bringing a new view to nanophotonics studies. Recently, spectrum imaging of the emitted photons under the primary electron beam and the spectacular introduction of time-resolved techniques down to the femtosecond time domain, have become innovative keys for the development and use of a brand new multi-dimensional and multi-signal electron microscopy.

Femtosecond electron deflectometry for measuring transient fields generated by laser-accelerated fast electrons

The temporal evolution of the electric field generated near the surface of a solid target by a femtosecond laser pulse with intensity of 1 × 1016 W/cm2 has been investigated by electron deflectometry; in this technique, ultrashort electron pulses generated by intense femtosecond laser pulses are used as probes. We found that electric field of the order of 108 V/m along the target surface was generated and decayed within 400 fs. The results of this study demonstrate the potential of electron deflectometry for measuring ultrafast phenomena in the femtosecond time domain.

Ultrafast Dynamics of Surface-Enhanced Raman Scattering Due to Au Nanostructures

Ultrafast dynamics of surface-enhanced Raman scattering (SERS) was investigated at cleaved graphite surfaces bearing deposited gold (Au) nanostructures (∼10 nm in diameter) by using sensitive pump-probe reflectivity spectroscopy with ultrashort (7.5 fs) laser pulses. We observed enhancement of phonon amplitudes (C═C stretching modes) in the femtosecond time domain, considered to be due to the enhanced electromagnetic (EM) field around the Au nanostructures. Finite-difference time-domain (FDTD) calculations confirmed the EM enhancement. The enhancement causes drastic increase of coherent D-mode (40 THz) phonon amplitude and nanostructure-dependent changes in the amplitude and dephasing time of coherent G-mode (47 THz) phonons. This methodology should be suitable to study the basic mechanism of SERS and may also find application in nanofabrication.

Solvent effect on dynamical TPA and optical limiting of BDMAS molecular media for nanosecond and femtosecond laser pulses

The dynamical two-photon absorption (TPA) cross section as well as optical limiting of a 4,4′-bis(dimethylamino) stilbene (BDMAS) molecular medium for the nanosecond and femtosecond laser pulses is studied. This molecular medium can be described by a cascade three-level model in the visible light regime. Our numerical results show that the BDMAS molecular medium exhibits a strong optical limiting behaviour. The saturation TPA in the femtosecond time domain can be observed, and materials with larger nonlinear absorption cross sections would be much easier to saturate. Due to the contribution of the two-step TPA, the dynamical TPA cross section of BDMAS for nanosecond pulses is about three orders of magnitude larger than that for ultrashort femtosecond pulses. Special attention has been paid to the solvent effects on the optimal limiting performance. With an enhancement of the polarity of solvents, the dynamical optical limiting window becomes broader. In the origin of optical limiting, the dynamical TPA cross section of BDMAS decreases when the polarity of solvents increases, which is in good agreement with the experiment.

Synthesis and Characterization of Highly Soluble Two‐Photon‐Absorbing Chromophores with Multi‐Branched and Dendritic Architectures

AbstractA set of novel multi‐polar compounds composed of four fluorene‐based derivatives with systematic alteration of the molecular structure was synthesized and their nonlinear optical properties in the femtosecond and nanosecond time domain were examined. Preliminary experimental results show that the two‐photon activities of the model compounds are connected to structural parameters, such as the number of peripheral electron‐donating groups and/or the size of the π‐domain in a dye molecule. It is also found that these model chromophores possess large nonlinear attenuation under the irradiation of laser pulses working at nanosecond regimes, indicating that these compounds may have strong two‐photon‐assisted excited‐state absorption within the studied spectral region. Effective optical‐power‐limiting behaviors of the dendritic fluorophores were also demonstrated to show that such dye molecules can be potential materials for use as broadband and rapid‐responsive optical limiters, especially against laser lights with longer pulses.

Full-dimensional quantum dynamics of vibrationally highly excited NHD2

We report on full-dimensional vibrational quantum dynamics of the highly excited ammonia isotopologue NHD(2) using a newly developed potential energy surface and the MCTDH program package. The calculations allow to realistically simulate an infrared laser induced stereomutation reaction at the pyramidal nitrogen atom in the femtosecond time domain. Our results allow for a thorough qualitative and quantitative understanding of infrared photoinduced stereomutation kinetics, the underlying quantum dynamics, and the reaction mechanisms. Comparison is made with a previous, reduced dimensionality study of the same reaction [R. Marquardt, M. Quack, I. Thanopulos, and D. Luckhaus, J. Chem. Phys. 118, 643 (2003)], and it is shown that slight variances of reduced spaces lead to significantly different kinetics. Because the quantum dynamics depends subtly on variances of reduced spaces, reduced dimensionality treatments are not reliable even for qualitative predictions of the stereomutation kinetics. The first direct comparison between the Multiconfigurational Time Dependent Hartree [M. H. Beck, A. Jäckle, G. A. Worth et al., Phys. Rep. 324, 1 (2000)] and Unimolecular Reactions Induced by Monochromatic Infrared Radiation [M. Quack and E. Sutcliffe, QCPE Bulletin 6, 98 (1986)] program packages on a specific, four dimensional quantum dynamical problem allows for their full validation in the present work.

Concerted electron and proton transfer in ionic crystals mapped by femtosecond x-ray powder diffraction

X-ray powder diffraction, a fundamental technique of structure research in physics, chemistry, and biology, is extended into the femtosecond time domain of atomic motions. This allows for mapping (macro)molecular structure generated by basic chemical and biological processes and for deriving transient electronic charge density maps. In the experiments, the transient intensity and angular positions of up to 20 Debye Scherrer reflections from a polycrystalline powder are measured and atomic positions and charge density maps are determined with a combined spatial and temporal resolutions of 30 pm and 100 fs. We present evidence for the so far unknown concerted transfer of electrons and protons in a prototype material, the hydrogen-bonded ionic ammonium sulfate [(NH(4))(2)SO(4)]. Photoexcitation of ammonium sulfate induces a sub-100 fs electron transfer from the sulfate groups into a highly confined electron channel along the c-axis of the unit cell. The latter geometry is stabilized by transferring protons from the adjacent ammonium groups into the channel. Time-dependent charge density maps derived from the diffraction data display a periodic modulation of the channel's charge density by low-frequency lattice motions with a concerted electron and proton motion between the channel and the initial proton binding site. Our results set the stage for femtosecond structure studies in a wide class of (bio)molecular materials.

Ultrafast memory loss and relaxation processes in hydrogen-bonded systems

Structural memory of aqueous systems, such as neat water and biomolecules, in an aqueous environment is strongly influenced by hydrogen bond dynamics. Vibrational spectroscopy in the femtosecond (fs) time domain is applied to map structural dynamics in real-time and identify underlying molecular interactions. Neat liquid water displays an ultrafast loss of structural memory with the fastest decay of structural correlations occurring in the sub-100 fs regime. Both OH stretching and bending excitations of water molecules decay on a subpicosecond time scale, followed by dissipation of excess energy in the hydrogen bond network within a few picoseconds. Water shells around fully hydrated DNA show similar although slightly slower dynamics. A detailed study of hydration shells around ionic phosphate groups in the DNA backbone demonstrates a strong phosphate-water coupling and a subpicosecond rearrangement of hydrogen bonds upon energy disposal. Hydration shells serve as primary heat sinks for excess energy.

X-ray imaging beyond the limits

The intense, brief pulses of X-rays from upcoming free-electron lasers will greatly extend X-ray microscopy to the femtosecond time domain and to interatomic length scales. From recent experiments and simulations one can envisage imaging macromolecules with X-rays without the need for crystallization.

Two-photon absorption and optical power limiting properties in femtosecond regime of novel multi-branched chromophores based on tri-substituted olefinic scaffolds

Two new multi-branched model fluorophores based on tri-substituted olefinic skeletons had been synthesized and their degenerate two-photon absorption properties were characterized by ultra-short laser pulses in the femtosecond regime. The experimental results show that the studied chromophore structures exhibit relatively strong and wide-dispersed two-photon activities in the near infra-red (NIR) regime. It is demonstrated that molecular structures with expanded π-frameworks derived from multi-substituted olefin moieties would possess large molecular nonlinear absorptivities within the studied spectral region. Two-photon absorption based optical power limiting behaviors in the femtosecond time domain of these model compounds were also explored and the results indicate that such a structural motif could be a useful molecular design approach towards strong two-photon absorbing material systems for quick-responsive and broadband optical-suppressing related applications, especially when against ultra-short laser pulses.

Femtosecond electron and spin dynamics in ferromagnetic Ni probed by UV and soft X-ray pulses

Experiments are reviewed which extend ultraviolet photoemission and soft X-ray absorption spectroscopy to the femtosecond (fs) time domain. We show for the case of ferromagnetic Ni that the evolution of electronic temperature, exchange coupling, spin and orbital angular momentum following fs optical laser excitation can be individually probed. This provides new insights into ultrafast processes which involve energy and angular momentum transfer between charge, spin and lattice degrees of freedom.

Photoprotection of green plants: a mechanism of ultra-fast thermal energy dissipation in desiccated lichens

In order to survive sunlight in the absence of water, desiccation-tolerant green plants need to be protected against photooxidation. During drying of the chlorolichen Cladonia rangiformis and the cyanolichen Peltigera neckeri, chlorophyll fluorescence decreased and stable light-dependent charge separation in reaction centers of the photosynthetic apparatus was lost. The presence of light during desiccation increased loss of fluorescence in the chlorolichen more than that in the cyanolichen. Heating of desiccated Cladonia thalli, but not of Peltigera thalli, increased fluorescence emission more after the lichen had been dried in the light than after drying in darkness. Activation of zeaxanthin-dependent energy dissipation by protonation of the PsbS protein of thylakoid membranes was not responsible for the increased loss of chlorophyll fluorescence by the chlorolichen during drying in the light. Glutaraldehyde inhibited loss of chlorophyll fluorescence during drying. Desiccation-induced loss of chlorophyll fluorescence and of light-dependent charge separation are interpreted to indicate activation of a highly effective mechanism of photoprotection in the lichens. Activation is based on desiccation-induced conformational changes of a pigment-protein complex. Absorbed light energy is converted into heat within a picosecond or femtosecond time domain. When present during desiccation, light interacts with the structural changes of the protein providing increased photoprotection. Energy dissipation is inactivated and structural changes are reversed when water becomes available again. Reversibility of ultra-fast thermal dissipation of light energy avoids photo-damage in the absence of water and facilitates the use of light for photosynthesis almost as soon as water becomes available.

Breakdown of optical power limiting and dynamical two-photon absorption for femtosecond laser pulses in molecular medium

We solve numerically the Maxwell-Bloch equations using an iterative predictor-corrector finite-difference time-domain technique to study the propagation of femtosecond laser pulses in a strong two-photon absorption (TPA) organic molecular medium [4,4′-bis(dimethylamino) stilbene]. The hybrid density functional theory is used to calculate electronic structures of the compound. The molecular system is described by a three-level model in an optical regime and has demonstrated a good optical power limiting behavior in a certain intensity region. Thresholds for the breakdown of optical power limiting are observed that are dependent on the input pulse width and, slightly, the propagation distance. The dynamical two-photon absorption cross section is obtained, which is almost a linearly increasing function of the pulse width in the femtosecond time domain. The propagation distance also has an obvious influence on the measurement of the TPA cross section, and nonmonotonic dependence of the TPA cross section on propagation distance is observed. The input pulse width and the thickness of the molecular samples thus should be taken into account when the TPA cross section is measured.

Controlling the angular distribution of atomic photoelectrons in the region of laser-induced continuum structure in the femtosecond time domain

The influence of laser-induced continuum structure on the angular distribution of photoelectrons is studied in the femtosecond time domain by direct numerical solution of the time-dependent Schrödinger equation. Control of the photoelectron angular distribution is demonstrated for the hydrogen atom by coherent population transfer from the initial S-state to an excited D-state via the p-continuum and further ionization into the f-continuum states. A direct optimization procedure is used to find a domain of laser parameters, for which the efficiency of the control with respect to the time delay and energy detuning between the laser pulses is maximized.

Femtodynamics Reflected in Nanoseconds: Bimolecular Free Electron Transfer in Nonpolar Media

The (free) electron transfer (FET) from electron donor molecules to parent solvent radical cations of alkanes and alkyl chlorides exhibits mechanistic peculiarities that are conditioned by the low polarity of these solvents. Because of the negligible solvation of ions in such systems and the almost complete lack of an activation barrier, the electron jump takes place at the very first encounter of the reactants and, as such, in extremely short times of <or=10(-15) s. Molecular oscillations (deformation, bending) occurring within the femtosecond time domain result directly in significant changes of the pi- or n-electron distribution in the HOMO ground state of the donor molecule, thereby generating a distribution of conformers. This is considered to be a rationale for a possible generation of different product radical cations in the free electron transfer, which exhibit different spin and charge distribution and, consequently different stability. Experimentally, the latter has been verified for aromatic donors, substituted with mobile, i.e., not rigidly fixed heteroatom-centered groups (various phenol type compounds, thiophenols, aromatic amines, benzyltrimethylsilanes etc.). The individually characteristic product distribution could be visualized and quantified by time-resolved spectroscopy in the nanosecond time domain. On the basis of a manifold of experimental data and supported by quantum-chemical calculations, the free electron transfer phenomenon is analyzed and discussed in detail in this summarizing report. The results presented here stand also for a, seemingly paradox, situation in which the products of a diffusion-controlled bimolecular reaction are governed by femtosecond events.

Degenerate two-/three-photon absorption and optical power-limiting properties in femtosecond regime of a multi-branched chromophore

The degenerate two- and three-photon absorption properties of a multi-branched chromophore were investigated in the femtosecond regime utilizing white-light continuum and nonlinear transmission techniques. The experimental results show that the studied multi-branched structure exhibits relatively strong and broad two- and three-photon absorption bands in the near infra-red (IR) region. It is demonstrated that a highly conjugated molecular structure based on a symmetrically substituted skeleton would possess promoted molecular nonlinear absorptivities within the studied spectral region. Both two- and three-photon absorption-based optical power-limiting properties in the femtosecond time domain of this model compound were also characterized. The results indicate that a multi-chromophoric structure with expanded π-conjugation could be an effective multi-photon absorber and might be used as a single-component material system for quick-responsive and broadband optical-suppressing related applications, especially when under ultrashort laser pulses.

Ultrafast Intermolecular Hydrogen Bond Dynamics in the Excited State of Fluorenone

Steady-state fluorescence and time-resolved absorption measurements in pico- and femtosecond time domain have been used to investigate the dynamics of hydrogen bond in the excited singlet (S(1)) state of fluorenone in alcoholic solvents. A comparison of the features of the steady-state fluorescence spectra of fluorenone in various kinds of media demonstrates that two spectroscopically distinct forms of fluorenone in the S(1) state, namely the non-hydrogen-bonded (or free) molecule as well as the hydrogen-bonded complex, are responsible for the dual-fluorescence behavior of fluorenone in solutions of normal alcoholic solvents at room temperature (298 K). However, in 2,2,2-trifluoroethanol (TFE), a strong hydrogen bond donating solvent, emission from only the hydrogen-bonded complex is observed. Significant differences have also been observed in the temporal evolution of the absorption spectroscopic properties of the S(1) state of fluorenone in protic and aprotic solvents following photoexcitation using 400 nm laser pulses. An ultrafast component representing the solvent-induced vibrational energy relaxation (VER) process has been associated with the dynamics of the S(1) state of fluorenone in all kinds of solvents. However, in protic solvents, in addition to the VER process, further evolution of the spectroscopic and dynamical properties of the S(1) state have been observed because of repositioning of the hydrogen bonds around the carbonyl group. In normal alcohols, two different kinds of hydrogen-bonded complex of the fluorenone-alcohol system with different orientations of the hydrogen bond with respect to the carbonyl group and the molecular plane of fluorenone have been predicted. On the other hand, in TFE, formation of only one kind of hydrogen-bonded complex has been observed. These observations have been supported by theoretical calculations of the geometries of the hydrogen-bonded complexes in the ground and the excited states of fluorenone. Linear correlation between the lifetimes of the equilibration process occurring because of repositioning of the hydrogen bonds and Debye or longitudinal relaxation times of the normal alcoholic solvents establish the fact that, in weakly hydrogen bond donating solvents, the hydrogen bond dynamics can be described as merely a solvation process. Whereas, in TFE, hydrogen bond dynamics is better described by a process of conversion between two distinct excited states, namely, the non-hydrogen-bonded form and the hydrogen-bonded complex.

Photoinduced Multistep Electron Transfer in an Oligoaniline−Oligo(p-phenylene Vinylene)−Perylene Diimide Molecular Array

Photoinduced multistep electron transfer has been studied in two symmetrical oligoaniline−oligo(p-phenylene vinylene)−perylene diimide−oligo(p-phenylene vinylene)−oligoaniline (OAn−OPV−PERY−OPV−OAn) multichromophore arrays with fluorescence and transient absorption spectroscopy in the femtosecond and nanosecond time domains. The arrays consist of a symmetrical donor(2)−donor(1)−acceptor−donor(1)−donor(2) arrangement, with two OAn−OPV segments coupled to a central PERY unit via a direct linkage (1) or a saturated spacer (2). Photoexcitation gives the OAn−OPV+•−PERY-•−OPV−OAn as the primary charge-separated state. For 1 the transfer is extremely fast (kCS > 1000 ns-1) in polar and apolar solvents, while the rate constants for recombination differ and are significantly higher in polar solvents than in apolar solvents because recombination occurs in the Marcus inverted region. Charge separation and charge recombination are slower in 2, because the saturated spacer reduces the electronic coupling between OPV d...