Filters

20153

More filters

20223
Reset filters

Settings

End date: 2015 Γ— Start date: 2015 Γ— DDC: 540 Γ— WGL Institute: MBI Γ—

Search Results

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Excited-state relaxation of hydrated thymine and thymidine measured by liquid-jet photoelectron spectroscopy: experiment and simulation
    (Washington, DC : ACS Publications, 2015) Buchner, Franziska; Nakayama, Akira; Yamazaki, Shohei; Ritze, Hans-Hermann; LΓΌbcke, Andrea
    Time-resolved photoelectron spectroscopy is performed on thymine and thymidine in aqueous solution to study the excited-state relaxation dynamics of these molecules. We find two contributions with sub-ps lifetimes in line with recent excited-state QM/MM molecular dynamics simulations (J. Chem. Phys.2013, 139, 214304). The temporal evolution of ionization energies for the excited ππ* state along the QM/MM molecular dynamics trajectories were calculated and are compatible with experimental results, where the two contributions correspond to the relaxation paths in the ππ* state involving different conical intersections with the ground state. Theoretical calculations also show that ionization from the nΟ€* state is possible at the given photon energies, but we have not found any experimental indication for signal from the nΟ€* state. In contrast to currently accepted relaxation mechanisms, we suggest that the nΟ€* state is not involved in the relaxation process of thymine in aqueous solution.
  • Thumbnail Image
    Item
    Adaptive micro axicons for laser applications
    (Les Ulis : EDP Sciences, 2015) Wallrabe, Ulrike; Brunne, Jens; Treffer, Alexander; Grunwald, Ruediger; Bellouard, Yves
    We 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.
  • Thumbnail Image
    Item
    Ultrafast phosphate hydration dynamics in bulk H2O
    (Melville, NY : American Institute of Physics, 2015) Costard, Rene; Tyborski, Tobias; Fingerhut, Benjamin P.; Elsaesser, Thomas
    Phosphate vibrations serve as local probes of hydrogen bonding and structural fluctuations of hydration shells around ions. Interactions of H2PO4βˆ’ 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 (πœˆπ‘†(POβˆ’2)) and asymmetric (πœˆπ΄π‘†(POβˆ’2)) POβˆ’2 stretching vibrations display nearly homogeneous lineshapes and pronounced anharmonic couplings between the two modes and with the Ξ΄(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 πœˆπ‘†(POβˆ’2) and πœˆπ΄π‘†(POβˆ’2) transition frequencies with larger frequency excursions for πœˆπ΄π‘†(POβˆ’2). The calculated frequency-time correlation function is in good agreement with the experiment. The 𝜈(POβˆ’2) frequencies are mainly determined by polarization contributions induced by electrostatic phosphate-water interactions. H2PO4βˆ’/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.