Ultrafast structural and vibrational dynamics of the hydration shell around DNA

Two-dimensional infrared spectroscopy in the frequency range of OH- and NH stretch excitations serves for a direct mapping of hydration dynamics around DNA. A moderate slowing down of structural dynamics and resonant OH stretch energy transfer is observed in the DNA water shell compared to bulk water

Ultrafast spatio-temporal dynamics of terahertz generation by ionizing two-color femtosecond pulses in gases

We present a combined theoretical and experimental study of spatio-temporal propagation effects in terahertz (THz) generation in gases using two-color ionizing laser pulses. The observed strong broadening of the THz spectra with increasing gas pressure reveals the prominent role of spatio-temporal reshaping and of a plasma-induced blue-shift of the pump pulses in the generation process. Results obtained from (3+1)-dimensional simulations are in good agreement with experimental findings and clarify the mechanisms responsible for THz emission

RdgB2 is required for dim-light input into intrinsically photosensitive retinal ganglion cells.

A subset of retinal ganglion cells is intrinsically photosensitive (ipRGCs) and contributes directly to the pupillary light reflex and circadian photoentrainment under bright-light conditions. ipRGCs are also indirectly activated by light through cellular circuits initiated in rods and cones. A mammalian homologue (RdgB2) of a phosphoinositide transfer/exchange protein that functions in Drosophila phototransduction is expressed in the retinal ganglion cell layer. This raised the possibility that RdgB2 might function in the intrinsic light response in ipRGCs, which depends on a cascade reminiscent of Drosophila phototransduction. Here we found that under high light intensities, RdgB2(-/-) mutant mice showed normal pupillary light responses and circadian photoentrainment. Consistent with this behavioral phenotype, the intrinsic light responses of ipRGCs in RdgB2(-/-) were indistinguishable from wild-type. In contrast, under low-light conditions, RdgB2(-/-) mutants displayed defects in both circadian photoentrainment and the pupillary light response. The RdgB2 protein was not expressed in ipRGCs but was in GABAergic amacrine cells, which provided inhibitory feedback onto bipolar cells. We propose that RdgB2 is required in a cellular circuit that transduces light input from rods to bipolar cells that are coupled to GABAergic amacrine cells and ultimately to ipRGCs, thereby enabling ipRGCs to respond to dim light

Vibronic excitations of large molecules in solution studied by two-color pump–probe experiments on the 20 fs time scale

The ultrafast vibronic response of organic dye molecules in solution is studied in pump–probe experiments with 30 fs excitation pulses resonant to S0–Sn transitions. The molecular dynamics is probed either by pulses at the same spectral position or by 20 fs pulses overlapping with both the S0–S1 absorption and emission bands. Three contributions on distinctly different time scales are observed in the temporally and spectrally resolved two-color measurements. In the regime below 50 fs, a strong coherent coupling of the S0–Sn and the S0–S1 transitions occurs that is due to coherent vibrational motions in the electronic ground state. This signal is superimposed on the fast bleaching of the electronic ground state, resulting in a steplike increase of transmission. In the range of the S0–S1 emission band, one finds a subsequent picosecond rise of transmission that is due to stimulated emission from vibronic S1 states. The data demonstrate that the relaxation of Sn states directly populated by the pump pulses is much faster than the buildup of stimulated emission. This gives insight into different steps of intramolecular vibronic redistribution and is compared to the Sn–S1 relaxation in other molecules

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Terahertz radiative coupling and damping in multilayer graphene

The nonlinear interaction between intense terahertz (THz) pulses and epitaxial multilayer graphene is studied by field-resolved THz pump-probe spectroscopy. THz excitation results in a transient induced absorption with decay times of a few picoseconds, much faster than carrier recombination in single graphene layers. The decay times increase with decreasing temperature and increasing amplitude of the excitation. This behaviour originates from the predominant coupling of electrons to the electromagnetic field via the very strong interband dipole moment while scattering processes with phonons and impurities play a minor role. The nonlinear response at field amplitudes above 1 kV cm-1 is in the carrier-wave Rabi flopping regime with a pronounced coupling of the graphene layers via the radiation field. Theoretical calculations account for the experimental results

Photophysics of closed- and open-ring isomers of a diarylethene with a carboxylic anchor group

We study the transient photophysical properties of a diarylethene with a carboxylic anchor group by a combination of steady-state and ultrafast emission and absorption spectroscopy. After excitation of the closed-ring form, fluorescence with a quantum yield of 10−5 is observed and separated into different spectro-temporal components. The S1 state of the closed-ring form shows a lifetime of 1.3 ps and decays mainly by internal conversion to the S0 state of this isomer. This vibrationally hot ground state cools on a time scale of 10 ps

Transition from ballistic to drift motion in high-field transport in GaAs

With strong THz pulses, we measure ultrafast transport of electrons, holes, and an electron-hole plasma in GaAs. The transition from ballistic to drift-like transport is strongly influenced by electron-hole scattering