Enhancement of Above Threshold Ionization in Resonantly Excited Helium Nanodroplets.

Clusters and nanodroplets hold the promise of enhancing high-order nonlinear optical effects due to their high local density. However, only moderate enhancement has been demonstrated to date. Here, we report the observation of energetic electrons generated by above-threshold ionization (ATI) of helium (He) nanodroplets which are resonantly excited by ultrashort extreme ultraviolet (XUV) free-electron laser pulses and subsequently ionized by near-infrared (NIR) or near-ultraviolet (UV) pulses. The electron emission due to high-order ATI is enhanced by several orders of magnitude compared with He atoms. The crucial dependence of the ATI intensities with the number of excitations in the droplets suggests a local collective enhancement effect.

Characterisation of microbunching instability with 2D Fourier analysis.

The optimal performance of high-brightness free-electron lasers (FELs) is limited by the microbunching instability, which can cause variations in both the slice energy spread and longitudinal profile of electron beams. In this paper, we perform 2D Fourier analysis of the full bunch longitudinal phase space, such that modulations in both planes can be studied simultaneously. Unlike the standard 1D analysis, this method is able to reveal modulations in a folded phase space, which would otherwise remain uncovered. Additionally, the plasma oscillation between energy and density modulations is also revealed by this method. The damping of the microbunching instability, through the use of a laser heater, is also analysed with this technique. We confirm a mitigation of the amplitude of modulation and a red-shift of the microbunching frequency as the energy spread added increases. As an outcome of this work, a systematic experimental comparison of the development of the instability in the presence of different compression schemes is here presented for the first time.

Coherent THz Emission Enhanced by Coherent Synchrotron Radiation Wakefield.

We demonstrate that emission of coherent transition radiation by a ∼1 GeV energy-electron beam passing through an Al foil is enhanced in intensity and extended in frequency spectral range, by the energy correlation established along the beam by coherent synchrotron radiation wakefield, in the presence of a proper electron optics in the beam delivery system. Analytical and numerical models, based on experimental electron beam parameters collected at the FERMI free electron laser (FEL), predict transition radiation with two intensity peaks at ∼0.3 THz and ∼1.5 THz, and extending up to 8.5 THz with intensity above 20 dB w.r.t. the main peak. Up to 80-µJ pulse energy integrated over the full bandwidth is expected at the source, and in agreement with experimental pulse energy measurements. By virtue of its implementation in an FEL beam dump line, this work promises dissemination of user-oriented multi-THz beamlines parasitic and self-synchronized to EUV and x-ray FELs.

The FERMI free-electron lasers.

FERMI is a seeded free-electron laser (FEL) facility located at the Elettra laboratory in Trieste, Italy, and is now in user operation with its first FEL line, FEL-1, covering the wavelength range between 100 and 20 nm. The second FEL line, FEL-2, a high-gain harmonic generation double-stage cascade covering the wavelength range 20-4 nm, has also completed commissioning and the first user call has been recently opened. An overview of the typical operating modes of the facility is presented.

Screening of 64 tryptamines at NMDA, 5-HT1A, and 5-HT2A receptors: a comparative binding and modeling study.

Tryptamine (T) and several T derivatives (Ts) inhibit in a voltage-dependent manner the NMDA receptor (NR). This effect is influenced by substituents at various positions, but has not yet been subjected to a detailed SAR study. Here, 64 Ts have been tested as inhibitors of [3H]MK-801 binding to NRs on rat brain membranes. For comparison, they were also tested as inhibitors of [3H]8-OHDPAT binding to 5-HT1A and of [3H]ketanserin binding to 5-HT2A receptors. Since most of these Ts have not been tested before at any of these receptors, we start with a review of the effects of Ts on 5-HT1A and 5-HT2A binding sites. NRs were inhibited with IC50s from 2 to 7 µM by Ts with alkyl or halogen at positions 2, 5, and/or 7. Inhibition by some Ts was attenuated more than 10-fold by 30 µM spermine. The most potent inhibitors at 5-HT1A receptors were 5-carboxamido-T (IC50 0.00015 µM) and serotonin (0.0016 µM), at 5-HT2A receptors 2-Me-4,7-Cl2-T (1.2 µM) and 2,7-Me2-4-Cl-T (2.0 µM). Fujita-Ban modified Free-Wilson analyses pointed to the individual significance of particular substituents. Also QSARs based on molecular operating environment descriptors resulted in sound correlations at all 4 targets. No similarities between the NR and 5-HT receptors could be found. At the NR, only L-Trp-NH2 bound 10 times better than at both 5-HT receptors studied. L-Trp-NH2 may be a structural lead to endogenous non-competitive NR antagonists.

Zinc and ifenprodil allosterically inhibit two separate polyamine-sensitive sites at N-methyl-D-aspartate receptor complex.

In this study, we investigated the hypothesis that inhibition of the N-methyl-D-aspartate (NMDA) receptor complex by zinc involves a polyamine-sensitive regulatory site. We found that the specific binding of the open channel ligand [3H]MK-801 to rat hippocampal membranes 1) was inhibited by low concentrations of Zn2+ (IC50 = 5.5 microM) by 65%. 2) This high-affinity component of inhibition was reversed by the polyamine spermine to an extent that could be reconciled with competitive interaction between Zn2+ and spermine. 3) Partial inhibition by Zn2+ was additive with partial inhibition by ifenprodil, an inhibitor of the NMDA receptor complex supposed to act at a polyamine-sensitive regulatory site, and 4) in membranes prepared from several other brain regions, inhibition of [3H]MK-801 binding by Zn2+ and by ifenprodil was either less than additive, or superadditive. Our observation that ifenprodil, at concentrations saturating its high-affinity component of inhibition, prevented spermine from reversing the inhibition by Zn2+ indicates that spermine did not increase [3H]MK-801 binding by competition with Zn2+ but rather via another polyamine regulatory site not sensitive to zinc but sensitive to ifenprodil. We conclude that Zn2+ reduces channel opening of the NMDA receptor complex by allosteric inhibition of a polyamine-sensitive regulatory site different from that inhibited by ifenprodil and that these two allosteric sites influence each other in a manner dependent on the brain region investigated. The different proportions of zinc/ifenprodil inhibition in different regions could reflect different percentages of various NMDA receptor subtypes.

Aminooxy-analogues of spermidine: new partial agonists and antagonists at the polyamine site of the rat hippocampal NMDA receptor complex.

The amino-1-oxy- and amino-8-oxy-analogues of spermidine (1-O-SPD and 8-O-SPD) were tested in vitro with rat hippocampal membranes as potential modulators of the N-methyl-D-aspartate (NMDA) receptor complex via the polyamine regulatory site. In the presence of 1 microM glutamate and glycine, the binding of the NMDA channel ligand [3H]MK-801 was stimulated by 8-O-SPD (EC50 = 50 microM); 1-O-SPD was without significant influence at concentrations up to 1 mM. Addition of 2 and 4 microM of the polyamine agonist spermine eliminated the stimulatory property of 8-O-SPD, whereas 1-O-SPD was inhibitory under these conditions (IC50 = 274 and 481 microM, respectively). At higher concentrations of spermine, both compounds were inhibitory. Inhibition of [3H]MK-801 binding by the inverse polyamine agonists 1,10-diaminodecane, 1,12-diaminododecane, and arcaine was attenuated by 1 mM 1-O-SPD. The data are compatible with the notion that 8-O-SPD is a partial polyamine agonist and that 1-O-SPD is an antagonist without intrinsic activity.