Ultrafast Vibrations Research Articles

Ultrafast Vibrational Energy Transfer between Protein and Cofactor in a Flavoenzyme.

Protein motions and enzyme catalysis are often linked. It is hypothesized that ultrafast vibrations (femtosecond–picosecond) enhance the rate of hydride transfer catalyzed by members of the old yellow enzyme (OYE) family of ene-reductases. Here, we use time-resolved infrared (TRIR) spectroscopy in combination with stable “heavy” isotopic labeling (2H, 13C, 15N) of protein and/or cofactor to probe the vibrational energy transfer (VET) between pentaerythritol tetranitrate reductase (a member of the OYE family) and its noncovalently bound flavin mononucleotide (FMN) cofactor. We show that when the FMN cofactor is photoexcited with visible light, vibrational energy is transferred from the flavin to the surrounding protein environment on the picosecond timescale. This finding expands the scope of VET investigation in proteins, which are limited by suitable intrinsic probes, and may have implications in the understanding of the mechanism of recently discovered photoactive flavoenzymes.

Crystal structure dependence of the breathing vibration of individual gold nanodisks induced by the ultrafast laser.

The ultrafast laser-excited breathing vibrations of gold nanodisks with different crystal structures have been studied via atomistic simulations. The vibrational periods and damping rates of nanodisks are obtained by the analysis of the simulated transient responses of nanodisks. It is shown that the breathing period of nanodisks is considerably dependent on their crystal structure, which is contrary to the cases for the breathing vibration of metal nanospheres and nanorods. Furthermore, single-crystal nanodisks exhibit much lower intrinsic damping as compared with polycrystalline nanodisks, for which the additional energy dissipation by the grain boundaries in the polycrystalline nanodisks could be one major factor.

Intrinsic damping for ultrafast laser-excited acoustic vibrations of single gold nanorods

The intrinsic damping for the acoustic vibrations of single gold nanorods excited by ultrafast laser has been studied through the atomistic simulations. It is shown that the intrinsic damping for the breathing mode is strongly sensitive to the nanorod sizes, which is very likely due to the different energy redistributions between the vibrational modes of nanorods and could play a non-negligible role in the broad distribution of the experimentally measured breathing-mode quality factors. In comparison, the intrinsic damping for the extensional vibration of gold nanorods appears much less influenced by the variations of nanorod dimensions. Moreover, we also find that the intrinsic mechanism is a significant source for the vibrational damping of gold nanorods, particularly for the breathing mode.

Do H-bonds explain strong ion aggregation in ethylammonium nitrate + acetonitrile mixtures?

Binary mixtures of the protic ionic liquid ethylammonium nitrate (EAN) and acetonitrile (AN) were studied at 25 °C over the entire composition range by means of broadband dielectric spectroscopy covering 0.2 ≤ ν/GHz ≤ 89. The dielectric spectra could be decomposed into two relaxation processes, both of which proved to be composite modes. For dilute solutions the higher-frequency Debye relaxation centered at ∼60 GHz is associated with the rotational diffusion of AN molecules, whereas at higher salt concentrations ultra-fast intermolecular vibrations and librations of EAN dominate the process. For EAN-rich solutions the lower-frequency relaxation is mainly due to jump reorientation of the ethylammonium cation, whereas contact ion pairs (CIPs) dominate this mode for dilute solutions. From the relaxation amplitudes effective solvation numbers and ion-pair concentrations were determined. For vanishing EAN mole fraction, xEAN → 0, an effective cation solvation number of ∼7 was found which steeply drops until xEAN ≈ 0.2 but shows only moderate decrease later on. The obtained association constant for EAN, K0(A) = 970 L mol(-1), exceeds that of other 1 : 1 electrolytes in AN by a factor of ∼30-50. This observation, as well as the fact that CIPs are formed despite strong cation solvation, indicates that ion pairing is mainly driven by the formation of strong hydrogen bonds between anions and cations.

Structure and Dynamics of 1-N-Alkyl-3-N-Methylimidazolium Tetrafluoroborate + Acetonitrile Mixtures

A detailed investigation of the binary mixtures of the ionic liquids (ILs) 1-N-R-3-N-methylimidazolium tetrafluoroborate (R = ethyl, n-butyl, n-hexyl) with the important molecular solvent acetonitrile (AN) over the entire composition range has been made at 25 °C using broadband dielectric spectroscopy. All spectra showed two modes: a Cole-Cole (CC) mode centered at ~2 GHz and a Debye mode centered at ~50 GHz. However, detailed analysis indicated both relaxations were composites. The Debye mode arises from the rotational diffusion of free AN molecules with contributions from ultrafast vibrations and librations of the ILs. The CC mode corresponds to the jump rotation of the imidazolium cations and the hindered rotational diffusion of "slow" AN molecules solvating them. At very low IL concentrations 1:1 contact ion pairs are dominant. Overall, these IL + AN mixtures can be divided into two broad regions: at IL mole fraction (x(IL)) ≲ 0.2 the IL behaves as a rather weakly associated conventional electrolyte while at x(IL) ≳ 0.2 it takes on its IL characteristics, "lubricated" by the AN.

Frequency-Tunable Multigigawatt Sub-Half-Cycle Light Pulses from Coupled-State Dynamics of Optical Solitons and Impulsively Driven Molecular Vibrations

Coupling ultrashort optical field waveforms to ultrafast molecular vibrations in an impulsively excited Raman medium is shown to enable the generation of frequency-tunable sub-half-cycle multigigawatt light pulses. In a gas-filled hollow waveguide, this coupled-state dynamics is strongly assisted by soliton effects, which help to suppress temporal stretching of subcycle optical pulses, providing efficient Raman-type impulsive excitation of ultrafast molecular vibrations over large propagation paths.

Temporally Two-Dimensional Fifth-Order Raman Scattering on Intermolecular Vibrations in Cs2/Pentane Binary Mixtures

Third- and fifth-order impulsive stimulated Raman scattering experiments are performed to study low-frequency intermolecular vibrations in CS2/pentane binary mixtures. The changes of the optical Kerr effect response cannot be assigned unequivocally to a single microscopic process. In particular, the homogeneous and inhomogeneous line broadening mechanisms cannot be determined. Temporally two-dimensional Raman scattering experiments indicate that the correlation time of fluctuations of the intermolecular potential decreases upon dilution. The experimental results give evidence of strong coupling between the ultrafast coherent vibrations and the slow reorientational diffusion.

Ultrafast stress pulse detection by laser beam deflection

We describe a direct method for the detection of optically excited ultrashort stress pulses in thin films using a time-resolved pump and probe scheme. Changes in the surface profile are monitored by the angular deflection of a probe beam. Application to the detection of interfacial layers is also demonstrated. The time-resolved detection of high frequency phonons excited by picosecond or femtosecond optical pulses can be achieved by a variety of methods. Terahertz optical phonons, for example, have been detected through the electrooptic effect,[ll whereas acoustic phonons up to the 100-GHz range have been detected through the photoelastic effect.[2] Here we describe a n ultrafast method for the detection of stress pulses associated with longitudinal acoustic phonons, which i s based on laser beam deflection from ultrafast surface vibrations.[3] Such a detection scheme has been widely used by the photoacoustics community in lower frequency photothermal deflection or photothermal displacement experiments.E4,51 Optical pump pulses of duration 3 ps (FWHM), repetition rate 76 MHz, wavelength A = 630 nm and energy 0.6 n J are used to excite the stress pulses. Light absorbed within the optical absorption depth (-10 nm for the metal samples used here) is converted to heat, and, through the resulting thermal expansion, longitudinal stress pulses in the 100-GHz region are generated. Changes in the surface slope of the opaque films, induced by the stress pulses bouncing back and forth inside the films, are interrogated by measuring the angular deflection (68-1 prad) of a probe laser beam derived from the same laser (see Fig. 1). This beam is focused to a 20-pm diam. spot size to partially overlap with the pump spot of similar size. The angular deflection is monitored with a dual-element photodiode. The displacement of the surface, typically -0.001 nm, can therefore be measured. By scanning the delay line, time-resolved detection on a picosecond time scale is achieved. This angular deflection scheme was previously used to measure the transient thermal expansion of bulk crystalline silicon a t lower frequencies of order 2 GHz, but not for the detection of stress pulses.[61 ( 1 ) Present address: Consiglio Nazionale delle Ricerche (CNR), Istituto di Acustica O.M. Corbino, Via Cassia 12 16,OO 1 X9 Roma, Italy Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19947165 JOURNAL DE PHYSIQUE IV

Ultrafast nonequilibrium stress generation in gold and silver.

The dynamics of coherent phonon generation by femtosecond optical pulses in thin gold and silver films is studied using a pump and probe scheme. Detection is achieved by monitoring ultrafast surface vibrations in real time using laser-beam deflection. The phonon strain pulse shapes can be explained through the nonequilibrium coupling of the electron and phonon distributions, suggesting a new method for measuring the electron-phonon coupling constant.

Ultrasonic Detection from Picosecond Surface Vibrations: Application to Interfacial Layer Detection

A new optical technique is used to detect picosecond ultrasonic pulses in thin opaque films by direct time-resolved detection of ultrafast vibrations of the film surface. For thin films of tungsten we show how the strain pulse shape in the film can be measured without significant distortion at frequencies around 100 GHz. The potential for characterization of thin film bonding and adhesion is demonstrated by the detection of an ultrathin interfacial amorphous silicon layer, of nanometre order in thickness, which is sandwiched between two different materials.

Coherent phonon detection from ultrafast surface vibrations.

Using an optical technique ultrafast vibrations of the surface of thin opaque films in the 0.1-THz range have been directly observed in the time domain. We propose a new type of phonon detection by monitoring the surface velocity, and demonstrate an exceptional sensitivity to atomic-scale inhomogeneities by observing an anomalous reflection from a buried film of monolayer order in thickness.

Thickness and sound velocity measurement in thin transparent films with laser picosecond acoustics

A new thin-film optical testing method is described for both thickness and sound velocity of transparent films on opaque substrates with laser picosecond acoustics using the optical pump-probe technique. The theory of excitation and detection of ultrasonic stress pulses for this geometry is presented in detail together with experimental results for sputtered thin films of silica of thickness 200 nm–2 μm on amorphous germanium substrates. Reflectance variations, measured as a function of pump-probe delay time, are characterized by echoes and beating oscillations superimposed on periodic steplike changes. These effects are modeled as a sum of an echo contribution from the stress-induced modulation of the substrate reflectance, an interference contribution from the light reflected by the moving stress pulse in the transparent film, a contribution from the modulation of the light on transmission through this stress pulse, and a contribution from the stress-induced ultrafast vibrations of the film interfaces of order 10−3 nm. The latter contribution arises from thin-film interference effects that represent a novel detection mechanism for surface vibrations in the picosecond regime. Sound velocity and thickness are derived from the data to an accuracy of a few percent, and the photoelastic constant of the transparent film is determined.