Search Results
Ultrafast vibrational dynamics of the DNA backbone at different hydration levels mapped by two-dimensional infrared spectroscopy
2015, Guchhait, Biswajit, Liu, Yingliang, Siebert, Torsten, Elsaesser, Thomas
DNA oligomers are studied at 0% and 92% relative humidity, corresponding to N < 2 and N > 20 water molecules per base pair. Two-dimensional (2D) infrared spectroscopy of DNA backbone modes between 920 and 1120 cm(-1) maps fluctuating interactions at the DNA surface. At both hydration levels, a frequency fluctuation correlation function with a 300 fs decay and a slow decay beyond 10 ps is derived from the 2D lineshapes. The fast component reflects motions of DNA helix, counterions, and water shell. Its higher amplitude at high hydration level reveals a significant contribution of water to the fluctuating forces. The slow component reflects disorder-induced inhomogeneous broadening.
Transient magnetic gratings on the nanometer scale
2020, Weder, D., von Korff Schmising, C., Günther, C.M., Schneider, M., Engel, D., Hessing, P., Strüber, C., Weigand, M., Vodungbo, B., Jal, E., Liu, X., Merhe, A., Pedersoli, E., Capotondi, F., Lüning, J., Pfau, B., Eisebitt, S.
Laser-driven non-local electron dynamics in ultrathin magnetic samples on a sub-10 nm length scale is a key process in ultrafast magnetism. However, the experimental access has been challenging due to the nanoscopic and femtosecond nature of such transport processes. Here, we present a scattering-based experiment relying on a laser-induced electro- and magneto-optical grating in a Co/Pd ferromagnetic multilayer as a new technique to investigate non-local magnetization dynamics on nanometer length and femtosecond timescales. We induce a spatially modulated excitation pattern using tailored Al near-field masks with varying periodicities on a nanometer length scale and measure the first four diffraction orders in an x-ray scattering experiment with magnetic circular dichroism contrast at the free-electron laser facility FERMI, Trieste. The design of the periodic excitation mask leads to a strongly enhanced and characteristic transient scattering response allowing for sub-wavelength in-plane sensitivity for magnetic structures. In conjunction with scattering simulations, the experiment allows us to infer that a potential ultrafast lateral expansion of the initially excited regions of the magnetic film mediated by hot-electron transport and spin transport remains confined to below three nanometers.
Photodissociation of aligned CH3I and C6H3F2I molecules probed with time-resolved Coulomb explosion imaging by site-selective extreme ultraviolet ionization
2018, Amini, Kasra, Savelyev, Evgeny, Brauße, Felix, Berrah, Nora, Bomme, Cédric, Brouard, Mark, Burt, Michael, Christensen, Lauge, Düsterer, Stefan, Erk, Benjamin, Höppner, Hauke, Kierspel, Thomas, Krecinic, Faruk, Lauer, Alexandra, Lee, Jason W. L., Müller, Maria, Müller, Erland, Mullins, Terence, Redlin, Harald, Schirmel, Nora, Thøgersen, Jan, Techert, Simone, Toleikis, Sven, Treusch, Rolf, Trippel, Sebastian, Ulmer, Anatoli, Vallance, Claire, Wiese, Joss, Johnsson, Per, Küpper, Jochen, Rudenko, Artem, Rouzée, Arnaud, Stapelfeldt, Henrik, Rolles, Daniel, Boll, Rebecca
We explore time-resolved Coulomb explosion induced by intense, extreme ultraviolet (XUV) femtosecond pulses from a free-electron laser as a method to image photo-induced molecular dynamics in two molecules, iodomethane and 2,6-difluoroiodobenzene. At an excitation wavelength of 267 nm, the dominant reaction pathway in both molecules is neutral dissociation via cleavage of the carbon-iodine bond. This allows investigating the influence of the molecular environment on the absorption of an intense, femtosecond XUV pulse and the subsequent Coulomb explosion process. We find that the XUV probe pulse induces local inner-shell ionization of atomic iodine in dissociating iodomethane, in contrast to non-selective ionization of all photofragments in difluoroiodobenzene. The results reveal evidence of electron transfer from methyl and phenyl moieties to a multiply charged iodine ion. In addition, indications for ultrafast charge rearrangement on the phenyl radical are found, suggesting that time-resolved Coulomb explosion imaging is sensitive to the localization of charge in extended molecules.
Soft-mode driven polarity reversal in ferroelectrics mapped by ultrafast x-ray diffraction
2018, Hauf, Christoph, Hernandez Salvador, Antonio-Andres, Holtz, Marcel, Woerner, Michael, Elsaesser, Thomas
Quantum theory has linked microscopic currents and macroscopic polarizations of ferroelectrics, but the interplay of lattice excitations and charge dynamics on atomic length and time scales is an open problem. Upon phonon excitation in the prototypical ferroelectric ammonium sulfate [(NH4)2SO4], we determine transient charge density maps by femtosecond x-ray diffraction. A newly discovered low frequency-mode with a 3 ps period and sub-picometer amplitudes induces periodic charge relocations over some 100 pm, a hallmark of soft-mode behavior. The transient charge density allows for deriving the macroscopic polarization, showing a periodic reversal of polarity.
Multi-color imaging of magnetic Co/Pt heterostructures
2017, Willems, Felix, von Korff Schmising, Clemens, Weder, David, Günther, Christian M., Schneider, Michael, Pfau, Bastian, Meise, Sven, Guehrs, Erik, Geilhufe, Jan, Merhe, Alaa El Din, Jal, Emmanuelle, Vodungbo, Boris, Lüning, Jan, Mahieu, Benoit, Capotondi, Flavio, Pedersoli, Emanuele, Gauthier, David, Manfredda, Michele, Eisebitt, Stefan
We present an element specific and spatially resolved view of magnetic domainsin Co/Pt heterostructures in the extreme ultraviolet spectral range. Resonantsmall-angle scattering and coherent imaging with Fourier-transform holographyreveal nanoscale magnetic domain networks via magnetic dichroism of Co at theM2,3 edges as well as via strong dichroic signals at the O2,3 and N6,7 edges of Pt.We demonstrate for the first time simultaneous, two-color coherent imaging at afree-electron laser facility paving the way for a direct real space access toultrafast magnetization dynamics in complex multicomponent material systems.
Theoretical analysis of hard x-ray generation by nonperturbative interaction of ultrashort light pulses with a metal
2015, Weisshaupt, Jannick, Juvé, Vincent, Holtz, Marcel, Woerner, Michael, Elsaesser, Thomas
The interaction of intense femtosecond pulses with metals allows for generating ultrashort hard x-rays. In contrast to plasma theories, tunneling from the target into vacuum is introduced as electron generation step, followed by vacuum acceleration in the laser field and re-entrance into the target to generate characteristic x-rays and Bremsstrahlung. For negligible space charge in vacuum, the Kα flux is proportional to the incident intensity and the wavelength squared, suggesting a strong enhancement of the x-ray flux by mid-infrared driving pulses. This prediction is in quantitative agreement with experiments on femtosecond Cu Kα generation.
Imaging plasma formation in isolated nanoparticles with ultrafast resonant scattering
2020, Rupp, Daniela, Flückiger, Leonie, Adolph, Marcus, Colombo, Alessandro, Gorkhover, Tais, Harmand, Marion, Krikunova, Maria, Müller, Jan Philippe, Oelze, Tim, Ovcharenko, Yevheniy, Richter, Maria, Sauppe, Mario, Schorb, Sebastian, Treusch, Rolf, Wolter, David, Bostedt, Christoph, Möller, Thomas
We have recorded the diffraction patterns from individual xenon clusters irradiated with intense extreme ultraviolet pulses to investigate the influence of light-induced electronic changes on the scattering response. The clusters were irradiated with short wavelength pulses in the wavelength regime of different 4d inner-shell resonances of neutral and ionic xenon, resulting in distinctly different optical properties from areas in the clusters with lower or higher charge states. The data show the emergence of a transient structure with a spatial extension of tens of nanometers within the otherwise homogeneous sample. Simulations indicate that ionization and nanoplasma formation result in a light-induced outer shell in the cluster with a strongly altered refractive index. The presented resonant scattering approach enables imaging of ultrafast electron dynamics on their natural timescale.
Towards shot-noise limited diffraction experiments with table-top femtosecond hard x-ray sources
2017, Holtz, Marcel, Hauf, Christoph, Weisshaupt, Jannick, Salvador, Antonio-Andres Hernandez, Woerner, Michael, Elsaesser, Thomas
Table-top laser-driven hard x-ray sources with kilohertz repetition rates are an attractive alternative to large-scale accelerator-based systems and have found widespread applications in x-ray studies of ultrafast structural dynamics. Hard x-ray pulses of 100 fs duration have been generated at the Cu Kα wavelength with a photon flux of up to 109 photons per pulse into the full solid angle, perfectly synchronized to the sub- 100-fs optical pulses from the driving laser system. Based on spontaneous x-ray emission, such sources display a particular noise behavior which impacts the sensitivity of x-ray diffraction experiments. We present a detailed analysis of the photon statistics and temporal fluctuations of the x-ray flux, together with experimental strategies to optimize the sensitivity of optical pump/x-ray probe experiments. We demonstrate measurements close to the shot-noise limit of the x-ray source.
Toward ultrafast magnetic depth profiling using time-resolved x-ray resonant magnetic reflectivity
2021, Chardonnet, Valentin, Hennes, Marcel, Jarrier, Romain, Delaunay, Renaud, Jaouen, Nicolas, Kuhlmann, Marion, Ekanayake, Nagitha, Léveillé, Cyril, von Korff Schmising, Clemens, Schick, Daniel, Yao, Kelvin, Liu, Xuan, Chiuzbăian, Gheorghe S., Lüning, Jan, Vodungbo, Boris, Jal, Emmanuelle
During the last two decades, a variety of models have been developed to explain the ultrafast quenching of magnetization following femtosecond optical excitation. These models can be classified into two broad categories, relying either on a local or a non-local transfer of angular momentum. The acquisition of the magnetic depth profiles with femtosecond resolution, using time-resolved x-ray resonant magnetic reflectivity, can distinguish local and non-local effects. Here, we demonstrate the feasibility of this technique in a pump–probe geometry using a custom-built reflectometer at the FLASH2 free-electron laser (FEL). Although FLASH2 is limited to the production of photons with a fundamental wavelength of 4 nm (≃310 eV), we were able to probe close to the Fe L3 edge (706.8 eV) of a magnetic thin film employing the third harmonic of the FEL. Our approach allows us to extract structural and magnetic asymmetry signals revealing two dynamics on different time scales which underpin a non-homogeneous loss of magnetization and a significant dilation of 2 Å of the layer thickness followed by oscillations. Future analysis of the data will pave the way to a full quantitative description of the transient magnetic depth profile combining femtosecond with nanometer resolution, which will provide further insight into the microscopic mechanisms underlying ultrafast demagnetization.
A liquid flatjet system for solution phase soft-x-ray spectroscopy
2015, Ekimova, Maria, Quevedo, Wilson, Faubel, Manfred, Wernet, Philippe, Nibbering, Erik T. J.
We present a liquid flatjet system for solution phase soft-x-ray spectroscopy. The flatjet set-up utilises the phenomenon of formation of stable liquid sheets upon collision of two identical laminar jets. Colliding the two single water jets, coming out of the nozzles with 50 μm orifices, under an impact angle of 48° leads to double sheet formation, of which the first sheet is 4.6 mm long and 1.0 mm wide. The liquid flatjet operates fully functional under vacuum conditions (<10(-3) mbar), allowing soft-x-ray spectroscopy of aqueous solutions in transmission mode. We analyse the liquid water flatjet thickness under atmospheric pressure using interferomeric or mid-infrared transmission measurements and under vacuum conditions by measuring the absorbance of the O K-edge of water in transmission, and comparing our results with previously published data obtained with standing cells with Si3N4 membrane windows. The thickness of the first liquid sheet is found to vary between 1.4-3 μm, depending on the transverse and longitudinal position in the liquid sheet. We observe that the derived thickness is of similar magnitude under 1 bar and under vacuum conditions. A catcher unit facilitates the recycling of the solutions, allowing measurements on small sample volumes (∼10 ml). We demonstrate the applicability of this approach by presenting measurements on the N K-edge of aqueous NH4 (+). Our results suggest the high potential of using liquid flatjets in steady-state and time-resolved studies in the soft-x-ray regime.