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- ItemUltrafast phosphate hydration dynamics in bulk H2O(Melville, NY : American Institute of Physics, 2015) Costard, Rene; Tyborski, Tobias; Fingerhut, Benjamin P.; Elsaesser, ThomasPhosphate vibrations serve as local probes of hydrogen bonding and structural fluctuations of hydration shells around ions. Interactions of H2PO4− ions and their aqueous environment are studied combining femtosecond 2D infrared spectroscopy, ab-initio calculations, and hybrid quantum-classical molecular dynamics (MD) simulations. Two-dimensional infrared spectra of the symmetric (𝜈𝑆(PO−2)) and asymmetric (𝜈𝐴𝑆(PO−2)) PO−2 stretching vibrations display nearly homogeneous lineshapes and pronounced anharmonic couplings between the two modes and with the δ(P-(OH)2) bending modes. The frequency-time correlation function derived from the 2D spectra consists of a predominant 50 fs decay and a weak constant component accounting for a residual inhomogeneous broadening. MD simulations show that the fluctuating electric field of the aqueous environment induces strong fluctuations of the 𝜈𝑆(PO−2) and 𝜈𝐴𝑆(PO−2) transition frequencies with larger frequency excursions for 𝜈𝐴𝑆(PO−2). The calculated frequency-time correlation function is in good agreement with the experiment. The 𝜈(PO−2) frequencies are mainly determined by polarization contributions induced by electrostatic phosphate-water interactions. H2PO4−/H2O cluster calculations reveal substantial frequency shifts and mode mixing with increasing hydration. Predicted phosphate-water hydrogen bond (HB) lifetimes have values on the order of 10 ps, substantially longer than water-water HB lifetimes. The ultrafast phosphate-water interactions observed here are in marked contrast to hydration dynamics of phospholipids where a quasi-static inhomogeneous broadening of phosphate vibrations suggests minor structural fluctuations of interfacial water.
- ItemA Mechanistic Perspective on Plastically Flexible Coordination Polymers(Weinheim : Wiley-VCH, 2019) Bhattacharya, Biswajit; Michalchuk, Adam A.L.; Silbernagl, Dorothee; Rautenberg, Max; Schmid, Thomas; Feiler, Torvid; Reimann, Klaus; Ghalgaoui, Ahmed; Sturm, Heinz; Paulus, Beate; Emmerling, FranziskaMechanical flexibility in single crystals of covalently bound materials is a fascinating and poorly understood phenomenon. We present here the first example of a plastically flexible one-dimensional (1D) coordination polymer. The compound [Zn(μ-Cl)2(3,5-dichloropyridine)2]n is flexible over two crystallographic faces. Remarkably, the single crystal remains intact when bent to 180°. A combination of microscopy, diffraction, and spectroscopic studies have been used to probe the structural response of the crystal lattice to mechanical bending. Deformation of the covalent polymer chains does not appear to be responsible for the observed macroscopic bending. Instead, our results suggest that mechanical bending occurs by displacement of the coordination polymer chains. Based on experimental and theoretical evidence, we propose a new model for mechanical flexibility in 1D coordination polymers. Moreover, our calculations propose a cause of the different mechanical properties of this compound and a structurally similar elastic material. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemCovalency-Driven Preservation of Local Charge Densities in a Metal-to-Ligand Charge-Transfer Excited Iron Photosensitizer(Weinheim : Wiley-VCH, 2019) Jay, Raphael M.; Eckert, Sebastian; Vaz da Cruz, Vinicius; Fondell, Mattis; Mitzner, Rolf; Föhlisch, AlexanderCovalency is found to even out charge separation after photo-oxidation of the metal center in the metal-to-ligand charge-transfer state of an iron photosensitizer. The σ-donation ability of the ligands compensates for the loss of iron 3d electronic charge, thereby upholding the initial metal charge density and preserving the local noble-gas configuration. These findings are enabled through element-specific and orbital-selective time-resolved X-ray absorption spectroscopy at the iron L-edge. Thus, valence orbital populations around the central metal are directly accessible. In conjunction with density functional theory we conclude that the picture of a localized charge-separation is inadequate. However, the unpaired spin density provides a suitable representation of the electron–hole pair associated with the electron-transfer process. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemVibrational sum-frequency generation spectroscopy of lipid bilayers at repetition rates up to 100 kHz(Melville, NY : American Institute of Physics, 2018) Yesudas, Freeda; Mero, Mark; Kneipp, Janina; Heiner, ZsuzsannaBroadband vibrational sum-frequency generation (BB-VSFG) spectroscopy has become a well-established surface analytical tool capable of identifying the orientation and structure of molecular layers. A straightforward way to boost the sensitivity of the technique could be to increase the laser repetition rate beyond that of standard BB-VSFG spectrometers, which rely on Ti:sapphire lasers operating at repetition rates of 1-5 kHz. Nevertheless, possible thermally induced artifacts in the vibrational spectra due to higher laser average powers are unexplored. Here, we discuss laser power induced temperature accumulation effects that distort the BB-VSFG spectra of 1,2-diacyl-sn-glycero-3-phosphocholine at an interface between two transparent phases at repetition rates of 5, 10, 50, and 100 kHz at constant pulse energy. No heat-induced distortions were found in the spectra, suggesting that the increase in the laser repetition rate provides a feasible route to an improved signal-to-noise ratio or shorter data acquisition times in BB-VSFG spectroscopy for thin films on transparent substrates. The results have implications for future BB-VSFG spectrometers pushing the detection limit for molecular layers with low surface coverage.
- ItemProperties of LiGa0.5In0.5Se2: A Quaternary Chalcogenide Crystal for Nonlinear Optical Applications in the Mid-IR(Basel : MDPI, 2016) Isaenko, Ludmila; Yelisseyev, Alexander; Lobanov, Sergei; Vedenyapin, Vitaliy; Krinitsyn, Pavel; Petrov, ValentinLiGaSe2 (LGSe) and LiInSe2 (LISe) are wide band-gap nonlinear crystals transparent in the mid-IR spectral range. LiGa0.5In0.5Se2 (LGISe) is a new mixed crystal, a solid solution in the system LGSe–LISe, which exhibits the same orthorhombic structure (mm2) as the parent compounds in the same time being more technological with regard to the growth process. In comparison with LGSe and LISe its homogeneity range is broader in the phase diagram. About 10% of the Li ions in LGISe occupy octahedral positions (octapores) with coordination number of 3. The band-gap of LGISe is estimated to be 2.94 eV at room temperature and 3.04 eV at 80 K. The transparency at the 0-level extends from 0.47 to 13 µm. LGISe crystals exhibit luminescence in broad bands centered near 1.7 and 1.25 eV which is excited most effectively by band-to-band transition. From the measured principal refractive indices and the fitted Sellmeier equations second-harmonic generation from 1.75 to 11.8 μm (fundamental wavelength) is predicted. The nonlinear coefficients of LGISe have values between those of LGSe and LISe. 6LGISe crystals are considered promising also for detection of thermal neutrons.
- ItemWater Dynamics in the Hydration Shells of Biomolecules(Washington, DC : ACS Publ., 2017) Laage, Damien; Elsaesser, Thomas; Hynes, James T.The structure and function of biomolecules are strongly influenced by their hydration shells. Structural fluctuations and molecular excitations of hydrating water molecules cover a broad range in space and time, from individual water molecules to larger pools and from femtosecond to microsecond time scales. Recent progress in theory and molecular dynamics simulations as well as in ultrafast vibrational spectroscopy has led to new and detailed insight into fluctuations of water structure, elementary water motions, electric fields at hydrated biointerfaces, and processes of vibrational relaxation and energy dissipation. Here, we review recent advances in both theory and experiment, focusing on hydrated DNA, proteins, and phospholipids, and compare dynamics in the hydration shells to bulk water.
- ItemAdaptive micro axicons for laser applications(Les Ulis : EDP Sciences, 2015) Wallrabe, Ulrike; Brunne, Jens; Treffer, Alexander; Grunwald, Ruediger; Bellouard, YvesWe report on the design, fabrication and testing of novel types of low-dispersion axicons for the adaptive shaping of ultrashort laser pulses. An overview is given on the basic geometries and operating principles of our purely reflective adaptive MEMS-type devices based on thermal or piezoelectric actuation. The flexible formation of nondiffracting beams at pulse durations down to a few oscillations of the optical field enables new applications in optical communication, pulse diagnostics, laser-matter interaction and particle manipulation. As an example, we show first promising results of adaptive autocorrelation. The combination of excellent pulse transfer, self-reconstruction properties and propagation invariance of nondiffracting beams with an adaptive approach promises to extend the field of practical applications significantly.
- ItemImaging molecular structure through femtosecond photoelectron diffraction on aligned and oriented gas-phase molecules(Cambridge [u.a.] : Royal Society of Chemistry, 2014) Boll, R.; Rouzée, A.; Adolph, M.; Anielski, D.; Aquila, A.; Bari, S.; Bomme, C.; Bostedt, C.; Bozek, J.D.; Chapman, H.N.; Christensen, L.; Coffee, R.; Coppola, N.; De, S.; Decleva, P.; Epp, S.W.; Erk, B.; Filsinger, F.; Foucar, L.; Gorkhover, T.; Gumprecht, L.; Hömke, A.; Holmegaard, L.; Johnsson, P.; Kienitz, J.S.; Kierspel, T.; Krasniqi, F.; Kühnel, K.-U.; Maurer, J.; Messerschmidt, M.; Moshammer, R.; Müller, N.L.M.; Rudek, B.; Savelyev, E.; Schlichting, I.; Schmidt, C.; Scholz, F.; Schorb, S.; Schulz, J.; Seltmann, J.; Stener, M.; Stern, S.; Techert, S.; Thøgersen, J.; Trippel, S.; Viefhaus, J.; Vrakking, M.; Stapelfeldt, H.; Küpper, J.; Ullrich, J.; Rudenko, A.; Rolles, D.This paper gives an account of our progress towards performing femtosecond time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe setup combining optical lasers and an X-ray free-electron laser. We present results of two experiments aimed at measuring photoelectron angular distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C8H5F) and dissociating, laser-aligned 1,4-dibromobenzene (C6H4Br2) molecules and discuss them in the larger context of photoelectron diffraction on gas-phase molecules. We also show how the strong nanosecond laser pulse used for adiabatically laser-aligning the molecules influences the measured electron and ion spectra and angular distributions, and discuss how this may affect the outcome of future time-resolved photoelectron diffraction experiments.
- ItemOriented zinc oxide nanorods: A novel saturable absorber for lasers in the near-infrared(Frankfurt, M. : Beilstein-Institut zur Förderung der Chemischen Wissenschaften, 2018-10-23) Loiko, Pavel; Bora, Tanujjal; Serres, Josep Maria; Yu, Haohai; Aguiló, Magdalena; Díaz, Francesc; Griebner, Uwe; Petrov, Valentin; Mateos, Xavier; Dutta, JoydeepZinc oxide (ZnO) nanorods (NRs) oriented along the crystallographic [001] axis are grown by the hydrothermal method on glass substrates. The ZnO NRs exhibit a broadband (1–2 µm) near-IR absorption ascribed to the singly charged zinc vacancy VZn−1. The saturable absorption of the ZnO NRs is studied at ≈1 µm under picosecond excitation, revealing a low saturation intensity, ≈10 kW/cm2, and high fraction of the saturable losses. The ZnO NRs are applied as saturable absorbers in diode-pumped Yb (≈1.03 µm) and Tm (≈1.94 µm) lasers generating nanosecond pulses. The ZnO NRs grown on various optical surfaces are promising broadband saturable absorbers for nanosecond near-IR lasers in bulk and waveguide geometries.
- ItemXUV double-pulses with femtosecond to 650 ps separation from a multilayer-mirror-based split-and-delay unit at FLASH(Chester : IUCr, 2018-8-3) Sauppe, Mario; Rompotis, Dimitrios; Erk, Benjamin; Bari, Sadia; Bischoff, Tobias; Boll, Rebecca; Bomme, Cédric; Bostedt, Christoph; Dörner, Simon; Düsterer, Stefan; Feigl, Torsten; Flückiger, Leonie; Gorkhover, Tais; Kolatzki, Katharina; Langbehn, Bruno; Monserud, Nils; Müller, Erland; Müller, Jan P.; Passow, Christopher; Ramm, Daniel; Rolles, Daniel; Schubert, Kaja; Schwob, Lucas; Senfftleben, Björn; Treusch, Rolf; Ulmer, Anatoli; Weigelt, Holger; Zimbalski, Jannis; Zimmermann, Julian; Möller, Thomas; Rupp, DanielaExtreme ultraviolet (XUV) and X-ray free-electron lasers enable new scientific opportunities. Their ultra-intense coherent femtosecond pulses give unprecedented access to the structure of undepositable nanoscale objects and to transient states of highly excited matter. In order to probe the ultrafast complex light-induced dynamics on the relevant time scales, the multi-purpose end-station CAMP at the free-electron laser FLASH has been complemented by the novel multilayer-mirror-based split-and-delay unit DESC (DElay Stage for CAMP) for time-resolved experiments. XUV double-pulses with delays adjustable from zero femtoseconds up to 650 picoseconds are generated by reflecting under near-normal incidence, exceeding the time range accessible with existing XUV split-and-delay units. Procedures to establish temporal and spatial overlap of the two pulses in CAMP are presented, with emphasis on the optimization of the spatial overlap at long time-delays via time-dependent features, for example in ion spectra of atomic clusters.