Phonon driven charge dynamics in polycrystalline acetylsalicylic acid mapped by ultrafast x-ray diffraction.

The coupled lattice and charge dynamics induced by phonon excitation in polycrystalline acetylsalicylic acid (aspirin) are mapped by femtosecond x-ray powder diffraction. The hybrid-mode character of the 0.9 ± 0.1 THz methyl rotation in the aspirin molecules is evident from collective charge relocations over distances of some 100 pm, much larger than the sub-picometer nuclear displacements. Oscillatory charge relocations around the methyl group generate a torque on the latter, thus coupling electronic and nuclear motions.

Soft-mode driven polarity reversal in ferroelectrics mapped by ultrafast x-ray diffraction.

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.

Towards shot-noise limited diffraction experiments with table-top femtosecond hard x-ray sources.

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 K α 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.

Theoretical analysis of hard x-ray generation by nonperturbative interaction of ultrashort light pulses with a metal.

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.

Femtosecond X-ray diffraction maps field-driven charge dynamics in ionic crystals.

X-Ray diffraction provides insight into the distribution of electronic charge in crystals. Equilibrium electron distributions have been determined with high spatial resolution by recording and analysing a large number of diffraction peaks under stationary conditions. In contrast, transient electron densities during and after structure-changing processes are mainly unknown. Recently, we have introduced femtosecond X-ray powder diffraction from polycrystalline samples to determine transient electron density maps with a spatial resolution of 0.03 nm and a temporal resolution of 100 fs. In a pump-probe approach with a laser-driven tabletop hard X-ray source, optically induced structure changes are resolved in time by diffracting the hard X-ray probe pulses at different time delays from the excited powder sample and recording up to several tens of reflections simultaneously. Time-dependent changes of the atomic arrangement in the crystal lattice as well as modified electron densities are derived from the diffraction data. As a prototypical field-driven process, we address here quasi-instantaneous changes of electron density in LiBH(4), LiH and NaBH4 in response to a non-resonant strong optical field. The light-induced charge relocation in LiBH(4) and NaBH(4) exhibits an electron transfer from the anion (BH) to the respective cation. The distorted geometry of the BH4 tetrahedron in LiBH(4) leads to different contributions of the H atoms to electron transfer. LiH displays a charge transfer from Li to H, i.e., an increase of the ionicity of LiH in the presence of the strong electric field. This unexpected behavior originates from strong electron correlations in LiH as is evident from a comparison with quasi-particle bandstructures calculated within the Coulomb-hole-plus-screened-exchange (COHSEX) formalism.

Field-driven dynamics of correlated electrons in LiH and NaBH4 revealed by femtosecond x-ray diffraction.

We study the quasi-instantaneous change of electron density in the unit cells of LiH and NaBH4 in response to a nonresonant strong optical field. We determine for the first time the related transient electron density maps, applying femtosecond x-ray powder diffraction as a structure probe. The light-induced charge relocation in NaBH4 exhibits an electron transfer from the anion (BH(4)(-)) to the Na(+) cation. In contrast, LiH displays the opposite behavior, i.e., an increase of the ionicity of LiH in the presence of the strong electric field. This behavior originates from strong electron correlations in LiH, as is evident from a comparison with quasiparticle band structures calculated within the Coulomb-hole-plus-screened-exchange formalism.

Deletion of protein kinase C-epsilon signaling pathway induces glomerulosclerosis and tubulointerstitial fibrosis in vivo.

Protein kinase C (PKC), a family of 12 distinct serine-threonine kinases, is an important intracellular signaling pathway involved in various cellular functions, such as proliferation, hypertrophy, apoptosis, and adhesion. PKC-epsilon, a novel PKC isoform that is activated in the diabetic kidney, has been demonstrated to have a central role in the underlying signaling infrastructure of myocardial ischemia and hypertrophy. The renal phenotype of PKC-epsilon(-/-) mice was studied with regard to renal hypertrophy and fibrosis. PKC-epsilon(-/-) deficient knockout mice were generated and then killed after 6, 16, and 26 wk of life. Kidney/body weight ratio did not show any significant group difference compared with appropriate wild-type controls. Urinary albumin/creatinine ratio remained normal in wild-type mice, whereas PKC-epsilon(-/-) mice after 6 and 16 wk showed elevated albuminuria. Masson-Goldner staining revealed that tubulointerstitial fibrosis and mesangial expansion were significantly increased in PKC-epsilon(-/-) mice. However, this profibrotic phenotype was not observed in other organs, such as liver and lung. Immunohistochemistry of the kidneys from PKC-epsilon(-/-) mice showed increased renal fibronectin and collagen IV expression that was further aggravated in the streptozotocin-induced diabetic stress model. Furthermore, TGF-beta(1), phospho-Smad2, and phospho-p38 mitogen-activate protein kinase expression was increased in PKC-epsilon(-/-) mice, suggesting a regulatory role of PKC-epsilon in TGF-beta(1) and its signaling pathway in the kidney. These results indicate that deletion of PKC-epsilon mediates renal fibrosis and that TGF-beta1 and its signaling pathway might be involved. Furthermore, these data suggest that activation of PKC-epsilon in the diabetic state may rather represent a protective response to injury than be a mediator of renal injury.