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- ItemExcited-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, AndreaTime-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 ΟΟ* 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 ΟΟ* 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.
- ItemFemtosecond stimulated Raman spectroscopy of the cyclobutane thymine dimer repair mechanism: A computational study(Washington, DC : American Chemical Society, 2014) Ando, H.; Fingerhut, B.P.; Dorfman, K.E.; Biggs, J.D.; Mukamel, S.Cyclobutane thymine dimer, one of the major lesions in DNA formed by exposure to UV sunlight, is repaired in a photoreactivation process, which is essential to maintain life. The molecular mechanism of the central step, i.e., intradimer C-C bond splitting, still remains an open question. In a simulation study, we demonstrate how the time evolution of characteristic marker bands (C=O and C=C/C-C stretch vibrations) of cyclobutane thymine dimer and thymine dinucleotide radical anion, thymidylyl(3β²β5β²)-thymidine, can be directly probed with femtosecond stimulated Raman spectroscopy (FSRS). We construct a DFT(M05-2X) potential energy surface with two minor barriers for the intradimer C5-Cβ²5 splitting and a main barrier for the C6-Cβ²6 splitting, and identify the appearance of two C5=C6 stretch vibrations due to the C6-Cβ²6 splitting as a spectroscopic signature of the underlying bond splitting mechanism. The sequential mechanism shows only absorptive features in the simulated FSRS signals, whereas the fast concerted mechanism shows characteristic dispersive line shapes. (Figure Presented).
- ItemCompetition between excited state proton and OH- transport via a short water wire: Solvent effects open the gate(London [u.a.] : Royal Society of Chemistry, 2014) BekΓ§ioΗ§lu, G.; Allolio, C.; Ekimova, M.; Nibbering, E.T.J.; Sebastiani, D.We investigate the acid-base proton exchange reaction in a microsolvated bifunctional chromophore by means of quantum chemical calculations. The UV/vis spectroscopy shows that equilibrium of the keto-and enol-forms in the electronic ground state is shifted to the keto conformation in the excited state. A previously unknown mechanism involving a hydroxide ion transport along a short water wire is characterized energetically, which turns out to be competitive with the commonly assumed proton transport. Both mechanisms are shown to have a concerted character, as opposed to a step-wise mechanism. The alternative mechanism of a hydrogen atom transport is critically examined, and evidence for strong solvent dependence is presented. Specifically, we observe electrostatic destabilization of the corresponding ΟΟ* state by the aqueous solvent. As a consequence, no conical intersections are found along the reaction pathway.
- ItemProbing the dynamics of plasmon-excited hexanethiol-capped gold nanoparticles by picosecond X-ray absorption spectroscopy(London [u.a.] : Royal Society of Chemistry, 2014) Zamponi, F.; Penfold, T.J.; Nachtegaal, M.; LΓΌbcke, A.; Rittmann, J.; Milne, C.J.; Chergui, M.; van Bokhoven, J.A.Picosecond X-ray absorption spectroscopy (XAS) is used to investigate the electronic and structural dynamics initiated by plasmon excitation of 1.8 nm diameter Au nanoparticles (NPs) functionalised with 1-hexanethiol. We show that 100 ps after photoexcitation the transient XAS spectrum is consistent with an 8% expansion of the Au-Au bond length and a large increase in disorder associated with melting of the NPs. Recovery of the ground state occurs with a time constant of βΌ1.8 ns, arising from thermalisation with the environment. Simulations reveal that the transient spectrum exhibits no signature of charge separation at 100 ps and allows us to estimate an upper limit for the quantum yield (QY) of this process to be <0.1.
- ItemX-ray imaging of chemically active valence electrons during a pericyclic reaction(London : Nature Publishing Group, 2014) Bredtmann, T.; Ivanov, M.; Dixit, G.Time-resolved imaging of chemically active valence electron densities is a long-sought goal, as these electrons dictate the course of chemical reactions. However, X-ray scattering is always dominated by the core and inert valence electrons, making time-resolved X-ray imaging of chemically active valence electron densities extremely challenging. Here we demonstrate an effective and robust method, which emphasizes the information encoded in weakly scattered photons, to image chemically active valence electron densities. The degenerate Cope rearrangement of semibullvalene, a pericyclic reaction, is used as an example to visually illustrate our approach. Our work also provides experimental access to the long-standing problem of synchronous versus asynchronous bond formation and breaking during pericyclic reactions.
- 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.; RouzΓ©e, 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.; HΓΆmke, A.; Holmegaard, L.; Johnsson, P.; Kienitz, J.S.; Kierspel, T.; Krasniqi, F.; KΓΌhnel, K.-U.; Maurer, J.; Messerschmidt, M.; Moshammer, R.; MΓΌller, 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.; ThΓΈgersen, J.; Trippel, S.; Viefhaus, J.; Vrakking, M.; Stapelfeldt, H.; KΓΌpper, 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.
- 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 H2PO4β 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 (ππ(POβ2)) and asymmetric (ππ΄π(POβ2)) POβ2 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 ππ(POβ2) and ππ΄π(POβ2) transition frequencies with larger frequency excursions for ππ΄π(POβ2). The calculated frequency-time correlation function is in good agreement with the experiment. The π(POβ2) frequencies are mainly determined by polarization contributions induced by electrostatic phosphate-water interactions. H2PO4β/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.