Ultrafast Time Resolution Research Articles

Site-Specific Coupling of Hydration Water and Protein Flexibility Studied in Solution with Ultrafast 2D-IR Spectroscopy

There is considerable evidence for the slaving of biomolecular dynamics to the motions of the surrounding solvent environment, but to date there have been few direct experimental measurements capable of site-selectively probing both the dynamics of the water and the protein with ultrafast time resolution. Here, two-dimensional infrared spectroscopy (2D-IR) is used to study the ultrafast hydration and protein dynamics sensed by a metal carbonyl vibrational probe covalently attached to the surface of hen egg white lysozyme dissolved in D(2)O/glycerol solutions. Surface labeling provides direct access to the dynamics at the protein-water interface, where both the hydration and the protein dynamics can be observed simultaneously through the vibrational probe's frequency-frequency correlation function. In pure D(2)O, the correlation function shows a fast initial 3 ps decay corresponding to fluctuations of the hydration water, followed by a significant static offset attributed to fluctuations of the protein that are not sampled within the <20 ps experimental window. Adding glycerol increases the bulk solvent viscosity while leaving the protein structurally intact and hydrated. The hydration dynamics exhibit a greater than 3-fold slowdown between 0 and 80% glycerol (v/v), and the contribution from the protein's dynamics is found to slow in a nearly identical fashion. In addition, the magnitude of the dynamic slowdown associated with hydrophobic hydration is directly measured and shows quantitative agreement with predictions from molecular dynamics simulations.

Photodetachment and electron reactivity in 1-methyl-1-butyl-pyrrolidinium bis(trifluoromethylsulfonyl)amide

The transient absorption spectrum in the range 500 nm-1000 nm was measured with ultrafast time resolution on a flowing neat, aliphatic, room-temperature ionic liquid following anion photodetachment. In this region the spectrum was shown to be a combination of absorption from the electron and the hole. Spectrally-resolved electron quenching determined a bimodal shape for the hole spectrum in agreement with recent computational predictions on a smaller aliphatic ionic liquid [Margulis et al., J. Am. Chem. Soc. 133, 20186 (2011)]. For time delays beyond 15 ps, spectral evolution qualitatively agrees with recent radiolysis experiments [Wishart et al., Faraday Discuss. 154, 353 (2012)]. However, the shape of the spectrum is different, reflecting the contrast in ionization energy between the two methods. Previously unobserved reactivity of the electron was found with a time constant of 300 fs. The results demonstrate solvent control of the rate coefficient for reaction between the electron and proton, with a rapid decline in the rate within the first picosecond.

Ultrafast continuum mid-infrared spectroscopy: probing the entire vibrational spectrum in a single laser shot with femtosecond time resolution

Until now, ultrafast IR spectroscopy has been limited by the bandwidth of optical parametric amplifiers, typically 100-400 cm(-1). Here we present the first example of transient IR spectroscopy using a continuum laser source to probe the entire mid-IR region with ultrafast time resolution. The continuum source is based on focusing the fundamental, second harmonic, and third harmonic of 1 mJ, 25 fs, 800 nm pulses in air, generating ∼150 fs continuum mid-IR pulses that span the frequency range of <400 to >5000 cm(-1) or, conversely, <2 to >25 μm. We characterize the spectral and temporal properties of dicarbonylacetonato rhodium(I) in hexane. We further demonstrate the versatility of the method by measuring the very fast and broad (>1500 cm(-1)) spectral changes following IR excitation associated with the 7-azaindole-acetic acid heterodimer in carbon tetrachloride.

Visualizing charge movement near organic heterojunctions with ultrafast time resolution via an induced Stark shift

We introduce a method to monitor photoinduced charge separation processes in organic donor-acceptor heterostructures. This approach utilizes a transient Stark shift of the exciton band of a molecular J-aggregate, deposited as a thin probe layer adjacent to the organic heterojunction. The high temporal dynamic range of this approach, from 100 femtoseconds to nanoseconds and longer, enables the entire charge separation process to be followed in both space and time. More broadly, this method can be applied to characterize photoinduced charge injection and separation processes in different materials and architectures, where sub-picosecond time resolution is needed at high spatial resolution.

Spectroscopic characterization of the first ultrafast catalytic intermediate in protochlorophyllide oxidoreductase

The enzyme NADPH:protochlorophyllide oxidoreductase (POR) catalyses the reduction of protochlorophyllide into chlorophyllide, a precursor of chlorophyll a in photosynthetic organisms. The enzyme binds the substrate and the cofactor in the dark and catalysis is initiated by the absorption of light by the substrate. We have carried out spectroscopic measurements with ultrafast time resolution under single pulse conditions, which reveal a biphasic formation of the first catalytic intermediate, I675* with average rates of (3.7 ± 0.7 ps)(-1) and (177 ± 78 ps)(-1), as obtained from a systematic analysis of 15 datasets. Measurements in the mid-IR absorption spectral region show that I675* is associated with a decrease of the PChlide C[double bond, length as m-dash]O keto oscillator strength. The spectroscopic changes in the visible and mid-IR regions are specific for the enzyme reaction as they do not occur in the photoexcited substrate alone. In deuterated samples, the rates of I675* formation are reduced by a factor of 1.5-2 compared to protonated samples, suggesting the involvement of a proton movement in this reaction step. The quantum yield of this step is determined to be 0.64 ± 0.11, and the quantum yield of the final reaction product formed on a later time scale, chlorophyllide, is 0.26 ± 0.06. Several possible interpretations of these data are discussed.

Unraveling the Solid-Liquid-Vapor Phase Transition Dynamics at the Atomic Level with Ultrafast X-Ray Absorption Near-Edge Spectroscopy

X-ray absorption near-edge spectroscopy (XANES) is a powerful probe of electronic and atomic structures in various media, ranging from molecules to condensed matter. We show how ultrafast time resolution opens new possibilities to investigate highly nonequilibrium states of matter including phase transitions. Based on a tabletop laser-plasma ultrafast x-ray source, we have performed a time-resolved (∼3 ps) XANES experiment that reveals the evolution of an aluminum foil at the atomic level, when undergoing ultrafast laser heating and ablation. X-ray absorption spectra highlight an ultrafast transition from the crystalline solid to the disordered liquid followed by a progressive transition of the delocalized valence electronic structure (metal) down to localized atomic orbitals (nonmetal-vapor), as the average distance between atoms increases.

3D reconstruction of the complex dielectric function of glass during femtosecond laser micro-fabrication

We report on a new technique to reconstruct the 3D dielectric function change in transparent dielectric materials and the application of the technique for on-line monitoring of refractive index modification in BK7 glass during direct femtosecond laser microfabrication. The complex optical field scattered from the modified region is measured using two-beam, single-shot interferogram and the distribution of the modified refractive index is reconstructed by numerically solving the inverse scattering problem in Born approximation. The optical configuration suggested is further development of digital holographic microscopy. It takes advantage of high spatial resolution and almost the same optical paths for both interfering beams, and allows ultrafast time resolution. © Springer Science+Business Media, LLC. 2011.

Ultrafast Time-Resolved Vibrational Spectroscopy at University of Zurich

Vibrational spectroscopy uniquely combines ultrafast time-resolution with chemical selectivity and molecular structure resolution. A list of examples of experiments and applications pursued at University of Zurich highlights the wide-spread applicability of time-resolved vibrational spectroscopy for molecular sciences.

Broadband near UV to visible optical activity measurement using self-heterodyned method

We demonstrate that broadband electronic optical activity can be measured with supercontinuum light pulse generated by a femtosecond pump (800 nm). It is the self-heterodyned detection technique that enables us to selectively measure the real (optical rotatory dispersion, ORD) or imaginary (circular dichroism, CD) part of the chiroptical susceptibility by controlling the incident polarization state. The single-shot-based measurement that is capable of correcting power fluctuations of the continuum light is realized by using a fast CCD detector and a polarizing beam splitter. Particularly, non-differential scheme used does not rely on any polarization-switching components. We anticipate that this broadband CD/ORD spectrometry with intrinsically ultrafast time-resolution will be applied to a variety of ultrafast chiroptical dynamics studies.

Optical properties of III-nitrides in electric fields

The influence of intermediate to high electric fields on the optical properties of direct-gap strongly-polar III-Nitrides is characterized. It is manifested through the dependence on the electric field of the nonequilibrium thermodynamic state of the system, which is characterized by a nonequilibrium effective temperature (quasi-temperature), quasi-chemical potentials and drift velocities of the excited carriers driven away from equilibrium, and the quasi-temperatures of the phonons in the different branches. In particular, we analyze the processes of absorption and luminescence, and a field-dependent Roosbroeck-Shockley relation is derived. It is shown that it is possible to measure the carriers’ drift velocity and quasi-temperature, in the steady state or with ultrafast time resolution, resorting to luminescence together with Raman scattering experiments.

Measuring the intensity and phase of high-order harmonic emission from aligned molecules

High-order harmonic emission from aligned molecules can be used as a probe of molecular structure with ultrafast time resolution. In this paper, we review past work, and also present new measurements of the intensity and phase of harmonic emission from N 2O and N 2 molecules. We find that high harmonic emission from CO 2 and N 2O molecules is well described by a simple two-center interference model. In contrast, high harmonic emission from N 2 exhibits a very different behavior, not readily explained by assuming that the electron responsible for the high-order harmonic emission recollides with the molecule as a plane wave. These data clearly emphasize the need for more comprehensive models of high harmonic generation from molecules.

2D-IR spectroscopy: ultrafast insights into biomolecule structure and function

Ultrafast 2D-IR spectroscopy is rapidly becoming a valuable tool for examining the relationship between structure and function of biomolecules. The unique combination of molecular-level structural information and ultrafast time resolution gives previously inaccessible insights relating to the rapid structural fluctuations, vibrational dynamics and solvent-solute interactions of proteins, all of which have important implications for the biological function of these species. In this tutorial review, the method and development of ultrafast 2D-IR spectroscopy is discussed, including an introduction to the two main experimental approaches, double resonance and photon echo 2D-IR, and the extension of the technique to non-equilibrium or transient 2D-IR measurements. The scope of the new information available through 2D-IR spectroscopy is then demonstrated by reference to the current state of the art of 2D-IR studies of molecules of biological interest.

Nanoscale topography of dynamic surfaces with ultrafast time resolution

We describe an optical system for detecting the movement of a surface with subnanosecond temporal and nanometer vertical displacement resolution. The system is fielded on an experiment to determine the distortion of a laser-ablated metal layer and compare the results with hydrodynamic simulations. We also discuss errors that can arise and potential means to mitigate them. The resultant data show one can examine dynamic changes to a reflective surface with accuracy down to tens of nanometers at hundreds of picoseconds time resolution.

Observation of Singlet Cycloreversion of Thymine Oxetanes by Direct Photolysis

An ultrafast broadband transient absorption spectroscopic study of the direct photolysis of oxetane DMT-BP [which is the oxetane adduct of 1,3-dimethylthymine (DMT) with benzophenone (BP)] is presented. Previous nanosecond time-resolved absorption studies by other researchers observed that direct photolysis of such oxetanes results in a rare, adiabatic photochemical reaction to produce a triplet excited-state carbonyl species. However, the mechanism for this adiabatic photochemical reaction remained unclear for the reaction sequence of the bond scission and the intersystem crossing (ISC) because of the time resolution for the experiments, and this prompted us to further study its mechanism with ultrafast time-resolution. The ultrafast time-resolved spectra presented here indicate that the cycloreversion reaction occurs in a stepwise manner on a singlet excited-state, and then intersystem crossing (ISC) occurs to produce the triplet carbonyl product observed in the previously reported nanosecond time-resolved experiments.

Two-Dimensional Infrared Spectroscopy of Photoswitchable Peptides

We present a detailed discussion of the complimentary fields of the application of two-dimensional infrared (2D-IR) spectroscopy in comparison with two-dimensional nuclear magnetic resonance (2D-NMR) spectroscopy. Transient 2D-IR (T2D-IR) spectroscopy of nonequilibrium ensembles is probably one of the most promising strengths of 2D-IR spectroscopy, as the possibilities of 2D-NMR spectroscopy are limited in this regime. T2D-IR spectroscopy uniquely combines ultrafast time resolution with microscopic structural resolution. In this article we summarize our recent efforts to investigate the ultrafast structural dynamics of small peptides, such as the unfolding of peptide secondary structure motifs. The work requires two ingredients: 2D-IR spectroscopy and the possibility of triggering a structural transition of a peptide on an ultrafast timescale using embedded or intrinsic photoswitches. Several photoswitches have been tested, and we discuss our progress in merging these two pathways of research.

Prospects for electron imaging with ultrafast time resolution

Many pivotal aspects of material science, biomechanics, and chemistry would benefit from nanometer imaging with ultrafast time resolution. Here the authors demonstrate the feasibility of short-pulse electron imaging with 10nm∕10ps spatiotemporal resolution, sufficient to characterize phenomena that propagate at the speed of sound in materials (1–10km∕s) without smearing. The authors outline resolution-degrading effects that occur at high current density followed by strategies to mitigate these effects. Finally, the authors present a model electron imaging system that achieves 10nm∕10ps spatiotemporal resolution.

Femtosecond Time- and Wavelength-Resolved Fluorescence and Absorption Spectroscopic Study of the Excited States of Adenosine and an Adenine Oligomer

By employing broadband femtosecond Kerr-gated time-resolved fluorescence (KTRF) and transient absorption (TA) techniques, we report the first (to our knowledge) femtosecond combined time- and wavelength-resolved study on an ultraviolet-excited nucleoside and a single-stranded oligonucleotide (namely adenosine (Ado) and single-stranded adenine oligomer (dA)(20)) in aqueous solution. With the advantages of the ultrafast time resolution, the broad spectral and temporal probe window, and a high sensitivity, our KTRF and TA results enable the real time monitoring and spectral characterization of the excited-state relaxation processes of the Ado nucleoside and (dA)(20) oligonucleotide investigated. The temporal evolution of the 267 nm excited Ado KTRF spectra indicates there are two emitting components with lifetimes of approximately 0.13 ps and approximately 0.45 ps associated with the L(a) and L(b) pipi* excited states, respectively. These Ado results reveal no obvious evidence for the involvement of the npi* state along the irradiative internal conversion pathway. A distinct mechanism involving only the two pipi* states has been proposed for the ultrafast Ado deactivation dynamics in aqueous solution. The time dependence of the 267 nm excited (dA)(20) KTRF and TA spectra reveals temporal evolution from an ultrafast "A-like" state (with a approximately 0.39 ps decay time) to a relatively long-lived E(1) "excimer" (approximately 4.3 ps decay time) and an E(2) "excimer-like" (approximately 182 ps decay time) state. The "A-like" state has a spectral character closely resembling the excited state of Ado. Comparison of the spectral evolution between the results for Ado and (dA)(20) provides unequivocal evidence for the local excitation character of the initially photoexcited (dA)(20). The rapid transformation of the locally excited (dA)(20) component into the delocalized E(1) "excimer" state which then further evolves into the E(2) "excimer-like" state indicates that base stacking has a high ability to modify the excited-state deactivation pathway. This modification appears to occur by suppressing the internal conversion pathway of an individually excited base component where the stacking interaction mediates efficient interbase energy transfer and promotes formation of the collective excited states. This feature of the local excitation that is subsequently followed by rapid energy delocalization into nearby bases may occur in many base multimer systems. Our results provide an important new contribution to better understanding DNA photophysics.

Vibrational Dynamics of Ions in Glass from Fifth-Order Two-Dimensional Infrared Spectroscopy

The vibrational dynamics of the first four antisymmetric stretch vibrational levels of azide in an ionic glass have been measured and correlated using a heterodyned fifth-order two-dimensional infrared pulse sequence. By rephasing a two-quantum coherence, a process not possible with third-order spectroscopy, solvent effects on the frequencies and anharmonicities of the potential energy surface are measured. Fifth-order pulse sequences are another step towards precisely controlling vibrational coherence in analogy to the manipulation of spins in NMR but with ultrafast time resolution.

Interrogating conformationally dependent electron-transfer dynamics via ultrafast visible pump/IR probe spectroscopy.

We demonstrate for the first time the utility of time-resolved visible pump/mid-infrared (IR) probe spectroscopy to interrogate directly, and provide unique infomation regarding, conformationally dependent photoinduced ET dynamics and the subsequent structural evolution of the resulting charge-separated (CS) state. Exemplary polarized visible pump/IR probe experiments involving N-[5-(10,20-diphenylporphinato)zinc(II)]-N'(octyl)pyromellitic diimide (PZn-PI) and [5-[4'-(N-(N'-octyl)pyromellitic diimide)phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) (PZn()PI) show that it is possible to assess the mean PZn-to-PI interplanar torsional angle of electronically excited structural conformers that undergo ET within the sub-ps time domain for both of these donor-spacer-acceptor (D-Sp-A) systems. Further transient specroscopic experiments carried out on PZn()PI determine how this angle evolves with time. Because vibrational transition moments are often known and typically localized, this work underscores that polarized visible pump/IR probe spectroscopy defines a valuable tool to interrogate structrure in both electronically excited and CS states; this fact, coupled with the ultrafast time resolution and high senseitivity offered by this technique, make it ideally suited to probe a range of mechanistic issues relevant to charge-transfer reactions.

Coherent coupling dynamics in a quantum-dot microdisk laser

Luminescence intensity autocorrelation (LIA) is employed to investigate coupling dynamics between (In,Ga)As QDs and a high-Q (~7000) resonator with ultrafast time resolution (150 fs), below and above the lasing threshold at T = 5 K. For QDs resonant and non-resonant with the cavity we observe both a six-fold enhancement and a 0.77 times reduction of the spontaneous emission rate, respectively. In addition, LIA spectroscopy reveals the onset of coherent coupling at the lasing threshold through qualitative changes in the dynamic behavior and a tripling of the resonant QD emission rate.