Sub-ps Time Scale Research Articles

Perspective: Watching low-frequency vibrations of water in biomolecular recognition by THz spectroscopy.

Terahertz (THz) spectroscopy has turned out to be a powerful tool which is able to shed new light on the role of water in biomolecular processes. The low frequency spectrum of the solvated biomolecule in combination with MD simulations provides deep insights into the collective hydrogen bond dynamics on the sub-ps time scale. The absorption spectrum between 1 THz and 10 THz of solvated biomolecules is sensitive to changes in the fast fluctuations of the water network. Systematic studies on mutants of antifreeze proteins indicate a direct correlation between biological activity and a retardation of the (sub)-ps hydration dynamics at the protein binding site, i.e., a "hydration funnel." Kinetic THz absorption studies probe the temporal changes of THz absorption during a biological process, and give access to the kinetics of the coupled protein-hydration dynamics. When combined with simulations, the observed results can be explained in terms of a two-tier model involving a local binding and a long range influence on the hydration bond dynamics of the water around the binding site that highlights the significance of the changes in the hydration dynamics at recognition site for biomolecular recognition. Water is shown to assist molecular recognition processes.

Tin-carbon clusters and the onset of microscopic level immiscibility: Experimental and computational study.

We report the experimental observation and computational analysis of the binary tin-carbon gas phase species. These novel ionic compounds are generated by impact of C60(-) anions on a clean tin target at some kiloelectronvolts kinetic energies. Positive Sn(m)C(n)(+) (m = 1-12, 1 ≤ n ≤ 8) ions were detected mass spectrometrically following ejection from the surface. Impact induced shattering of the C60(-) ion followed by sub-surface penetration of the resulting atomic carbon flux forces efficient mixing between target and projectile atoms even though the two elements (Sn/C) are completely immiscible in the bulk. This approach of C60(-) ion beam induced synthesis can be considered as an effective way for producing novel metal-carbon species of the so-called non-carbide forming elements, thus exploring the possible onset of molecular level miscibility in these systems. Sn2C2(+) was found to be the most abundant carbide cluster ion. Its instantaneous formation kinetics and its measured kinetic energy distribution while exiting the surface demonstrate a single impact formation/emission event (on the sub-ps time scale). Optimal geometries were calculated for both neutral and positively charged species using Born-Oppenheimer molecular dynamics for identifying global minima, followed by density functional theory (DFT) structure optimization and energy calculations at the coupled cluster singles, doubles and perturbative triples [CCSD(T)] level. The calculated structures reflect two distinct binding tendencies. The carbon rich species exhibit polyynic/cummulenic nature (tin end capped carbon chains) while the more stoichiometrically balanced species have larger contributions of metal-metal bonding, sometimes resulting in distinct tin and carbon moieties attached to each other (segregated structures). The Sn2C(n) (n = 3-8) and Sn2C(n)(+) (n = 2-8) are polyynic/cummulenic while all neutral Sn(m)C(n) structures (m = 3-4) could be described as small tin clusters (dimer, trimer, and tetramer, correspondingly) attached to a nearly linear carbon chain. For example, the 1:1 (Sn:C) Sn3C3 and Sn4C4 clusters are composed of all-tin triangle and rhombus, correspondingly, with a short carbon chain (C3, C4) attached on top. The cationic Sn3C(n)(+) (n = 1-5) and Sn4C(n)(+) (n = 1-4) species exhibit various intermediate geometries. Structure calculations at the CCSD(T) level are essential since the segregation effect is not as easily evident based on the most stable structures calculated by DFT alone. Dependences of bond energies (per atom) reflect the evolution of the segregation effect. The mass spectral abundances could be reasonably rationalized in terms of calculated stabilities of the cluster ions with respect to various dissociation channels.

Ultrafast photo-induced nuclear relaxation of a conformationally disordered conjugated polymer probed with transient absorption and femtosecond stimulated Raman spectroscopies.

A combination of transient absorption (TAS) and femtosecond stimulated Raman (FSRS) spectroscopies were used to interrogate the photo-induced nuclear relaxation dynamics of poly(3-cyclohexyl,4-methylthiophene) (PCMT). The large difference in inter-ring dihedral angles of ground and excited-state PCMT make it an ideal candidate for studying large-amplitude vibrational relaxation associated with exciton trapping. Spectral shifting in the S1 TA spectra on sub-ps timescales (110 ± 20 and 800 ± 100 fs) is similar to spectroscopic signatures of excited-state relaxation observed with related photoexcited conjugated polymers and which have been attributed to exciton localization and a combination of resonant energy transfer and torsional relaxation, respectively. Measurements made with both techniques reveal fast PCMT S1 decay and triplet formation (τS1 = 25-32 ps), which is similar to the excited-state dynamics of short oligothiophenes and highly twisted polyconjugated molecules. On ultrafast timescales FSRS of S1 PCMT offers a new perspective on the nuclear dynamics that underlie localization of excitons in photoexcited conjugated polymers: Spectral dynamics in the C=C stretching region (1400-1600 cm(-1)) include a red-shift of the in-phase C=C stretching frequency, as well as a change in the relative intensity of in-phase and out-of-phase stretch intensities on a timescale of ∼100 fs. Both changes indicate an ultrafast vibrational distortion that increases the conjugation length in the region of the localized excitation and are consistent with exciton self-localization or trapping. Wavelength-dependent excited-state FSRS measurements further demonstrate that the C=C stretching frequency provides a useful spectroscopic handle for interrogating the degree of delocalization in excited conjugated polymers given the selectivity achieved via resonance enhancement.

Charge carrier dynamics in organic semiconductors and their donor-acceptor composites: Numerical modeling of time-resolved photocurrent

We present a model that describes nanosecond (ns) time-scale photocurrent dynamics in functionalized anthradithiophene (ADT) films and ADT-based donor-acceptor (D/A) composites. By fitting numerically simulated photocurrents to experimental data, we quantify contributions of multiple pathways of charge carrier photogeneration to the photocurrent, as well as extract parameters that characterize charge transport (CT) in organic films including charge carrier mobilities, trap densities, hole trap depth, and trapping and recombination rates. In pristine ADT films, simulations revealed two competing charge photogeneration pathways: fast, occurring on picosecond (ps) or sub-ps time scales with efficiencies below 10%, and slow, which proceeds at the time scale of tens of nanoseconds, with efficiencies of about 11%–12%, at the applied electric fields of 40–80 kV/cm. The relative contribution of these pathways to the photocurrent was electric field dependent, with the contribution of the fast process increasing with applied electric field. However, the total charge photogeneration efficiency was weakly electric field dependent exhibiting values of 14%–20% of the absorbed photons. The remaining 80%–86% of the photoexcitation did not contribute to charge carrier generation at these time scales. In ADT-based D/A composites with 2 wt.% acceptor concentration, an additional pathway of charge photogeneration that proceeds via CT exciton dissociation contributed to the total charge photogeneration. In the composite with the functionalized pentacene (Pn) acceptor, which exhibits strong exciplex emission from a tightly bound D/A CT exciton, the contribution of the CT state to charge generation was small, ∼8%–12% of the total number of photogenerated charge carriers, dependent on the electric field. In contrast, in the composite with PCBM acceptor, the CT state contributed about a half of all photogenerated charge carriers. In both D/A composites, the charge carrier mobilities were reduced and trap densities and average trap depths were increased, as compared to a pristine ADT donor film. A considerably slower recombination of free holes with trapped electrons was found in the composite with the PCBM acceptor, which led to slower decays of the transient photocurrent and considerably higher charge retention, as compared to a pristine ADT donor film and the composite with the functionalized Pn acceptor.

Observation of direct vibrational excitation in gas-surface collisions of CO with Au(111): a new model system for surface dynamics

We report vibrational excitation of CO from its ground (v = 0) to first excited (v = 1) vibrational state in collision with Au(111) at an incidence energy of translation of E(I) = 0.45 eV. Unlike past work, we can exclude an excitation mechanism involving temporary adsorption on the surface followed by thermalization and desorption. The angular distributions of the scattered CO molecules are narrow, consistent with direct scattering occurring on a sub-ps time scale. The absolute excitation probabilities are about 3% of those expected from thermal accommodation. The surface temperature dependence of excitation, which was measured between 373 and 973 K, is Arrhenius-like with an activation energy equal to the energy required for vibrational excitation. Our measurements are consistent with a vibrational excitation mechanism involving coupling of thermally excited electron-hole pairs of the solid to CO vibration.

Femtosecond Time-Resolved Fluorescence Study of TiO2-Coated ZnO Nanorods/P3HT Photovoltaic Films

The photovoltaic performance of organic–inorganic hybrid films based on regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) and ZnO nanorods depends strongly on the quality of the interfaces between the organic polymer and the inorganic nanostructures. A thin, solution-processed TiO2 interlayer was introduced on the surface of ZnO nanorods to improve the interface between ZnO and P3HT. In order to understand the effects of the TiO2 interlayer on the initial photophysical processes after photoexcitation, femtosecond time-resolved fluorescence dynamics of the hybrid ZnO nanorods/P3HT (ZnO/P3HT) films with or without TiO2 interlayer coating were comparatively studied using a fluorescence up-conversion technique. Because of the well-aligned P3HT nanodomains in these annealed hybrid films, energetically downhill excitation energy transfer was found to be the dominant process in the subps time scale with an excitation at 400 nm. The fluorescence decay times for the ZnO–TiO2/P3HT film were significantly extended...

Propagation of a laser-driven relativistic electron beam inside a solid dielectric

Laser probe diagnostics: shadowgraphy, interferometry, and polarimetry were used for a comprehensive characterization of ionization wave dynamics inside a glass target induced by a laser-driven, relativistic electron beam. Experiments were done using the 50-TW Leopard laser at the University of Nevada, Reno. We show that for a laser flux of ∼2 × 10(18) W/cm2 a hemispherical ionization wave propagates at c/3 for 10 ps and has a smooth electron-density distribution. The maximum free-electron density inside the glass target is ∼2 × 10(19) cm-3, which corresponds to an ionization level of ∼0.1%. Magnetic fields and electric fields do not exceed ∼15 kG and ∼1 MV/cm, respectively. The electron temperature has a hot, ringlike structure with a maximum of ∼0.7 eV. The topology of the interference phase shift shows the signature of the "fountain effect", a narrow electron beam that fans out from the propagation axis and heads back to the target surface. Two-dimensional particle-in-cell (PIC) computer simulations demonstrate radial spreading of fast electrons by self-consistent electrostatic fields driven by laser. The very low ionization observed after the laser heating pulse suggests a fast recombination on the sub-ps time scale.

Solvated electrons at the water–air interface: surface versus bulk signal in low kinetic energy photoelectron spectroscopy

Time-resolved photoelectron spectroscopy at low kinetic energies (≲5 eV) is applied to dilute iodide solutions with different surface and bulk contributions. The results indicate a pronounced surface sensitivity. Signals assigned to solvated electrons near the liquid surface decay rapidly on a sub-ps timescale. In contrast to the literature, a long-lived surface solvated electron at 1.6 eV binding energy is not observed.

Fountain effect of laser-driven relativistic electrons inside a solid dielectric

Ultrafast interferometry with sub-ps resolution has been applied for the direct measurement of an electron density induced by a laser-driven relativistic electron beam inside a solid dielectric. The topology of the interference phase shift shows the signature of the “fountain effect,” a narrow electron beam that fans out from the propagation axis and heads back to the target surface. Two-dimensional particle-in-cell (PIC) computer simulations demonstrate radial spreading of fast electrons by self-consistent electrostatic fields. The very low ionization, ∼0.1%, observed after the heating pulse suggests a fast recombination at the sub-ps time scale.

Ultrafast Manipulation of Strong Coupling in Metal−Molecular Aggregate Hybrid Nanostructures

We demonstrate an ultrafast manipulation of the Rabi splitting energy Ω(R) in a metal-molecular aggregate hybrid nanostructure. Femtosecond excitation drastically alters the optical properties of a model system formed by coating a gold nanoslit array with a thin J-aggregated dye layer. Controlled and reversible transient switching from strong (Ω(R) ≃ 55 meV) to weak (Ω(R) ≈ 0) coupling on a sub-ps time scale is directly evidenced by mapping the nonequilibrium dispersion relations of the coupled excitations. Such a strong, externally controllable coupling of excitons and surface plasmon polaritons is of considerable interest for ultrafast all-optical switching applications in nanoscale plasmonic circuits.

Ultrafast studies of single semiconductor and metal nanostructures through transient absorption microscopy

The past decade has seen tremendous progress in the development of absorption based techniques for studying single semiconductor and metal nanoparticles. These techniques complement the more common emission and Rayleigh scattering measurements. The advantages of absorption-based detection are that a wider range of materials can be examined compared to emission, and smaller particles can be studied than is possible with Rayleigh scattering. These advantages are somewhat offset by increased difficulty. Importantly, by employing absorption based detection with a pump and probe beam, transient absorption experiments can be performed on single nanostructures. This allows dynamics measurements on ultrafast (sub-ps) timescales, and is the focus of this mini-review. These experiments provide information about how processes such as electron–phonon coupling, energy dissipation, and charge carrier trapping vary between nanostructures, as well as how they depend on environment.

Disentangling thermal and nonthermal excited states in a charge-transfer insulator by time- and frequency-resolved pump-probe spectroscopy

Time-and-frequency resolved pump-probe optical spectroscopy is used to investigate the effect of the impulsive injection of delocalized excitations through a charge-transfer process in insulating CuGeO3. A large broadening of the charge-transfer edge is observed on the sub-ps timescale. The modification of this spectral feature can not be attributed to the local increase of the effective temperature, as a consequence of the energy absorbed by the pump pulse. The measured modifications of the optical properties of the system are consistent with the creation of a non-thermal state, metastable on the ps timescale, after the pump-induced impulsive modification of the electron interactions.

Synthesized femtosecond laser pulse source for two-wavelength contouring with simultaneously recorded digital holograms

A dual-wavelength femtosecond laser pulse source and its application for digital holographic single-shot contouring are presented. The synthesized laser source combines sub-picosecond time scales with a wide reconstruction range. A center wavelength distance of the two separated pulses of only 15 nm with a high contrast was demonstrated by spectral shaping of the 50 nm broad seed spectrum centered at 800 nm. Owing to the resulting synthetic wavelength, the scan depth range without phase ambiguity is extended to the 100-microm-range. Single-shot dual-wavelength imaging is achieved by using two CMOS cameras in a Twyman-Green interferometer, which is extended by a polarization encoding sequence to separate the holograms. The principle of the method is revealed, and experimental results concerning a single axis scanner mirror operating at a resonance frequency of 0.5 kHz are presented. Within the synthetic wavelength, the phase difference information of the object was unambiguously retrieved and the 3D-shape calculated. To the best of our knowledge, this is the first time that single-shot two-wavelength contouring on a sub-ps time scale is reported.

Ultrafast transient absorption spectroscopy of the photo-induced Z– E isomerization of a photochromic furylfulgide

The photo-induced Z– E isomerization of a photochromic furylfulgide following excitation at 387 nm was studied using time-resolved transient absorption spectroscopy with probing from 410 to 690 nm in ethanol as solvent. The excited-state absorption decayed dominantly on a time scale of 220 fs. An oscillatory behavior was interpreted as vibrational coherence involving a low-frequency torsional motion. A slow component with a decay time of 4.1 ps was assigned to hot ground state absorption. At 25 ps, the transient absorption resembled the stationary E- Z difference spectrum. The results indicate that the Z– E photoisomerization proceeds on a sub-ps time scale.

An experiment to measure the electron–ion thermal equilibration rate in a strongly coupled plasma

We present the most recent results from an experiment aimed at obtaining the temperature equilibration rate between ions and electrons in a strongly coupled plasma by directly measuring the temperature of each component. The plasma is formed by heating a sonic gas jet with a 10 ps laser pulse. The electrons are preferentially heated by the short pulse laser (we are aiming for Te ~ 100 eV), while the ions, after undergoing very rapid (sub-ps timescale) disorder-induced heating, should only reach a temperature of 10–15 eV. This results in a strongly coupled ion plasma with a Γii ~ 3–5. We plan to measure the electron and ion temperatures of the resulting plasma independently during and after heating, using collective Thomson scattering for electrons and a high-resolution x-ray spectrometer for the ions (measuring Doppler-broadened absorption lines). Theory indicates that the equilibration rate could be significantly lower than that given by the usual weakly coupled model (Landau–Spitzer) due to coupled collective modes present in the dense plasma.

Polar Solvation Dynamics in Zn(II)-Substituted Cytochrome c: Diffusive Sampling of the Energy Landscape in the Hydrophobic Core and Solvent-Contact Layer

We employed picosecond time-resolved fluorescence spectroscopy to characterize polar diffusive protein and solvent dynamics in Zn(II)-substituted cytochrome c. The solvent-response function, obtained from the dynamic Stokes shift of the Zn(II) porphyrin's fluorescence 0-0 transition energy over the 100-ps to 12-ns time scale, exhibits a double-exponential decay (250 ps and 1.45 ns time constants). The addition of glycerol to the external solvent medium slows only the slower of the two exponential components; in the presence of 50% (v/v) glycerol, the second time constant was lengthened to 2.2 ns. The magnitude of the decay accounts for 85% of the reorganization energy and leaves room for only a minor inertial component on the sub-ps time scale. The results show that two types of polypeptide motions are sensed by the intrinsic Zn(II) porphyrin. The slower of the motions arises from polypeptide motions in the interfacial, solvent-contact portion of the protein. The faster, solvent-independent part of the response arises from motions of the polypeptide in the hydrophobic core of the protein. The two classes of motion are apparently not strongly coupled, nor is the dynamic Stokes shift coupled to the nonpolar reorganization that accompanies the excited-state photodissociation of the Zn(II) porphyrin's protein-derived axial ligands. The finding that the inertial response is limited to a small fraction of the total reorganization energy suggests that the hydrophobic core of the protein is largely incapable of the free-rotor or librational character of polypeptide and solvent motions that is observed near the solvent interface.

Ultrafast photoinduced reflectivity transients in(Nd0.5Sr0.5)MnO3

The temperature dependence of ultrafast photoinduced reflectivity transients is reported in Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ thin film. The photoinduced reflectivity shows a complex response with very different temperature dependences on different timescales. The response on the sub-ps timescale appears to be only weakly sensitive to the 270K-metal-insulator phase transition. Below $\sim 160$ K the sub-ps response displays a two component behavior indicating inhomogeneity of the film resulting from the substrate induced strain. On the other hand, the slower response on the 10-100 ps timescale is sensitive only to the metal-insulator phase transition and is in agreement with some previously published results. The difference in the temperature dependences of the responses on nanosecond and $\mu $s timescales indicates that thermal equilibrium between the different degrees of fredom is established relatively slowly - on a nanosecond timescale.

Fundamentals and advantages of ultrafast micro-structuring of transparent materials

Experimental investigations using femtosecond and picosecond laser pulses at 800 nm illuminate the distinctions between the dynamics and nature of ultrafast processing of dielectrics compared with semiconductors and metals. Dielectric materials are strongly charged at the surface on the sub-ps time scale and undergo an impulsive Coulomb explosion prior to thermal ablation. Provided the laser pulse width remains in the ps or sub-ps time domain, this effect can be exploited for processing. In the case of thermal ablation alone, the high localization of energy accompanied by ultrafast laser micro-structuring is of great advantage also for high quality processing of thin metallic or semiconducting layers, in which the surface charge is effectively quenched.

Isotope effect of the photoinduced H(D)-transfer reaction in indole–ammonia clusters

The intracluster reaction dynamics initiated by femtosecond laser pulses at 263 nm has been studied in nondeuterated and deuterated indole–ammonia clusters. No isotope effect is observed on the sub-ps timescale. On the long-timescale (of few 100 ps), however, the structural reorientation dynamics of the parent as well as the product clusters is prolongated (up to about 3 times) for the deuterated complexes. This effect is interpreted as due to the difference of the torsion modes and due to low energetic barriers between different isomers of both isotopic complexes.

Stimulation of electrical conductivity in a π-conjugated polymeric conductor with infrared light

Irradiation with infrared light is found to stimulate the electrical conductivity of a film of an organic polymeric conductor [poly(3,4-ethylenedioxythiopene) with polystyrene sulfonate]. The change in conductivity is found to be linear in the intensity of the irradiation (4–400 mW/cm2). Both frequency and time domain measurements reveal that the change in resistance induced by irradiation, relaxes according to ΔR(t)∝(1/t)0.6, with t as the time after excitation. As a possible mechanism for this relaxation, we model the diffusion of heat from the polymer film to the supporting glass substrate. Assuming that the change in resistance is linear with the raise in temperature caused by the infrared irradiation, one predicts a ΔR(t)∝(1/t)0.5 dependence. The similarity between the model and experimental behavior is taken as an indication that the relaxation is limited by heat transport from the polymer film and that the thermalization of the charge carriers occurs on a shorter time scale. Electrical characterization is complemented with optical measurements. These show infrared-induced transient absorption of the polymer film with practically the same relaxation behavior as the change in resistance. This suggests that the optical transients are also due to thermal excitations. In the sub-ps time domain, measurements of the change in optical transmission (ΔT/T) induced by the infrared pulse show a very short-lived component with a lifetime close to the instrumental resolution (∼500 fs). The rapid response is followed by a slow component that decays according to (ΔT/T)(t)∝(1/t)0.65. This is interpreted in terms of cooling of the excited charge carriers limited by heat transport, indicating that the thermalization of the carriers occurs on the sub-ps time scale.