Matias Bargheer

Coherent Atomic Motions in a Nanostructure Studied by Femtosecond X-ray Diffraction

Reversible structural changes of a nanostructure were measured nondestructively with subpicometer spatial and subpicosecond temporal resolution via x-ray diffraction (XRD). The spatially periodic femtosecond excitation of a gallium arsenide/aluminum gallium arsenide superlattice results in coherent lattice motions with a 3.5-picosecond period, which was directly monitored by femtosecond x-ray pulses at a 1-kilohertz repetition rate. Small changes (ΔR/R = 0.01) of weak Bragg reflexes (R = 0.005) were detected. The phase and amplitude of the oscillatory XRD signal around a new equilibrium demonstrate that displacive excitation of the zone-folded acoustic phonons is the dominant mechanism for strong excitation.

Microfocus Cu K ? source for femtosecond x-ray science

We demonstrate a subpicosecond 1 kHz femtosecond x-ray source with a well-accessible quasi-point size (10??m diameter) providing Cu K? emission with a maximum flux of 6.8×1010? photons?s for continuous operation of 10 h. A new geometry that essentially facilitates the adjustment and diminishes the temporal jitter between the x-ray probe and the laser pump pulse is implemented for time-resolved diffraction experiments.

Localized excited charge carriers generate ultrafast inhomogeneous strain in the multiferroic BiFeO3.

We apply ultrafast x-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic BiFeO3 after above-band-gap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in BiFeO3: the relevant excited charge carriers must remain localized to be consistent with the data.

Photodissociation and recombination of F2 molecule in Ar54 cluster: Nonadiabatic molecular dynamics simulations

Photodissociation and recombination of an F2 molecule embedded in an Ar cluster is investigated. The electronic states involved are described by the valence bond approach for the F(2P)+F(2P) interaction, with spin–orbit coupling included and the anisotropic interactions between F and Ar atoms described by the diatomics-in-molecules (DIM) approach. The potential energy surfaces for 36 electronic states and the nonadiabatic couplings between them are constructed in this basis. The surface hopping method is used for dynamical simulations. The main results are: (i) Spin nonconserving transitions play a crucial role both in the dissociation and in the recombination dynamics. (ii) The ratio between the population of the triplet states and the population of the singlet states reaches the statistical equilibrium value of 3:1 60 fs after the photoexcitation, but the population of specific singlet and triplet states remains nonstatistical for at least 1.5 ps. (iii) Recombination on the only bound excited state (3Πu...

Generation of coherent zone boundary phonons by impulsive excitation of molecules

A coherent zone boundary phonon (ZBP) of solid Kr (f(m)=1.54 THz) is observed in ultrafast pump-probe spectra of I2 guest molecules in a Kr crystal. Its phase is stable for at least 10 ps. The femtosecond pump pulse induces an electronic transition in I2. The resulting expansion of the guests electronic cloud impulsively excites phonons in the host. Detection at the impurity after some picoseconds selects the ZBP due to its low group velocity. A ZBP amplitude of 0.02 A is estimated.

Extraction of potentials and dynamics from condensed phase pump–probe spectra: Application to I2 in Kr matrices

The many-body dynamics in the condensed phase often leads to structureless absorption spectra while vibrational recurrences in localized coordinates are preserved. A scheme is presented to directly extract anharmonicities and energy relaxation from the varying vibrational periods in pump–probe spectra. For the B state of I2 a constant anharmonicity and a vibrational energy relaxation rate decreasing by one order of magnitude in the wavelength range from 540 to 583 nm are obtained. An analytical expression for compensating the wave packet dispersion due to the anharmonicity is derived and it is applied to the control of wave packets by chirped pulses. Focusing of the wave packet is experimentally observed in accordance with the anharmonicity and even after significant energy relaxation. Predissociation rates are determined from the envelopes of the pump–probe spectra.

Structural characterization of a spin-assisted colloid-polyelectrolyte assembly: stratified multilayer thin films.

The assembly of polyelectrolytes and gold nanoparticles yields stratified multilayers with very low roughness and high structural perfection. The films are prepared by spin-assisted layer-by-layer self-assembly (LbL) and are characterized by X-ray reflectivity (XRR), UV-vis spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Typical structures have four repeat units, each of which consists of eight double layers (DL) of poly(sodium 4-styrenesulfonate)/poly(allylamine hydrochloride), one monolayer of gold nanoparticles (10 nm diameter), and another layer of poly(allylamine hydrochloride). XRR scans show small-angle Bragg peaks up to seventh order, evidencing the highly stratified structure. Pronounced Kiessig fringes indicate a low global roughness, which is confirmed by local AFM measurements. TEM images corroborate the layered structure in the growth direction and nicely show the distinct separation of the individual particle layers. An AFM study reveals the lateral gold particle distribution within one individual particle layer. Interestingly, the spin-assisted deposition of polyelectrolytes reduces the roughness induced by the particle layers, leading to self-healing of roughness defects and a rather perfect stratification.

Fabrication of Au@Pt Multibranched Nanoparticles and Their Application to In Situ SERS Monitoring

Here, we present an Au@Pt core-shell multibranched nanoparticle as a new substrate capable of in situ surface-enhanced Raman scattering (SERS), thereby enabling monitoring of the catalytic reaction on the active surface. By careful control of the amount of Pt deposited bimetallic Au@Pt, nanoparticles with moderate performance both for SERS and catalytic activity were obtained. The Pt-catalyzed reduction of 4-nitrothiophenol by borohydride was chosen as the model reaction. The intermediate during the reaction was captured and clearly identified via SERS spectroscopy. We established in situ SERS spectroscopy as a promising and powerful technique to investigate in situ reactions taking place in heterogeneous catalysis.

Fabrication of Bifunctional Gold/Gelatin Hybrid Nanocomposites and Their Application

Herein, a facile method is presented to integrate large gold nanoflowers (∼80 nm) and small gold nanoparticles (2-4 nm) into a single entity, exhibiting both surface-enhanced Raman scattering (SERS) and catalytic activity. The as-prepared gold nanoflowers were coated by a gelatin layer, in which the gold precursor was adsorbed and in situ reduced into small gold nanoparticles. The thickness of the gelatin shell is controlled to less than 10 nm, ensuring that the small gold nanoparticles are still in a SERS-active range of the inner Au core. Therefore, the reaction catalyzed by these nanocomposites can be monitored in situ using label-free SERS spectroscopy. In addition, these bifunctional nanocomposites are also attractive candidates for application in SERS monitoring of bioreactions because of their excellent biocompatibility.

UDKM1DSIM—A simulation toolkit for 1D ultrafast dynamics in condensed matter

The udkm1Dsim toolbox is a collection of matlab (MathWorks Inc.) classes and routines to simulate the structural dynamics and the according X-ray diffraction response in one-dimensional crystalline sample structures upon an arbitrary time-dependent external stimulus, e.g. an ultrashort laser pulse. The toolbox provides the capabilities to define arbitrary layered structures on the atomic level including a rich database of corresponding element-specific physical properties. The excitation of ultrafast dynamics is represented by an N-temperature model which is commonly applied for ultrafast optical excitations. Structural dynamics due to thermal stress are calculated by a linear-chain model of masses and springs. The resulting X-ray diffraction response is computed by dynamical X-ray theory. The udkm1Dsim toolbox is highly modular and allows for introducing user-defined results at any step in the simulation procedure.