Vacuum-ultraviolet frequency-modulation spectroscopy

Frequency-modulation (FM) spectroscopy has been extended to the vacuum-ultraviolet (VUV) range of the electromagnetic spectrum. Coherent VUV laser radiation is produced by resonance-enhanced sum-frequency mixing ($\nu_{\mathrm{VUV}}=2\nu_{\mathrm{UV}}+\nu_2$) in Kr and Xe using two near-Fourier-transform-limited laser pulses of frequencies $\nu_{\mathrm{UV}}$ and $\nu_2$. Sidebands generated in the output of the second laser ($\nu_2$) using an electro-optical modulator operating at the frequency $\nu_{\mathrm{mod}}$ are directly transfered to the VUV and used to record FM spectra. Demodulation is demonstrated both at $\nu_{\mathrm{mod}}$ and $2\nu_{\mathrm{mod}}$. The main advantages of the method are that its sensitivity is not reduced by pulse-to-pulse fluctuations of the VUV laser intensity, compared to VUV absorption spectroscopy is its background-free nature, the fact that its implementation using table-top laser equipment is straightforward and that it can be used to record VUV absorption spectra of cold samples in skimmed supersonic beams simultaneously with laser-induced-fluorescence and photoionization spectra. To illustrate these advantages we present VUV FM spectra of Ar, Kr, and N$_2$ in selected regions between 105000cm$^{-1}$ and 122000cm$^{-1}$.Comment: 23 pages, 10 figure

CGPS time-series and trajectories of crustal motion along the West Hellenic Arc

Western Greece is one of the seismotectonically most active regions in Europe. The main tectonic structures are the West Hellenic Arc (WHA) and the Kephalonia Fault Zone. In order to monitor and understand the crustal movements in space and time, a continuous GPS network was installed. In this paper we present results of 6 yr (1995-2001) of measurements. To ensure a consistent reference frame, 54 mainly European IGS and EUREF sites were included in the processing. A selected subset was used to estimate an Euler pole for the rotation of Eurasia. In order to obtain coordinate time-series of high precision that are representative for crustal deformation, special emphasis was given to the elimination of non-tectonic effects. Four steps of improvement were pursued, including a reprocessing after exclusion of poor data, the removal of remaining outliers, the correction of unknown phase centre offsets after antenna changes and weighted common-mode filtering. With this procedure, non-tectonic irregularities were reduced significantly, and the precision was improved by an average of 40 per cent. The final time-series are used as a base for depicting trajectories of crustal motion, interpreting the temporal behaviour of the sites and for estimating velocities. For the first time, height changes in the WHA area were detected and quantified by GPS. Sites that are located near the epicentres of the 1997 Strofades (Mw = 6.6) and the 1999 Athens (Mw = 6.0) earthquakes are particularly considere

Challenges for creating magnetic fields by cosmic defects

We analyse the possibility that topological defects can act as a source of magnetic fields through the Harrison mechanism in the radiation era. We give a detailed relativistic derivation of the Harrison mechanism at first order in cosmological perturbations, and show that it is only efficient for temperatures above T ~ 0.2 keV. Our main result is that the vector metric perturbations generated by the defects cannot induce vorticity in the matter fluids at linear order, thereby excluding the production of currents and magnetic fields. We show that anisotropic stress in the matter fluids is required to source vorticity and magnetic fields. Our analysis is relevant for any mechanism whereby vorticity is meant to be transferred purely by gravitational interactions, and thus would also apply to dark matter or neutrinos.Comment: 9 pages, 1 figure; minor corrections and additions; accepted for publication in Physical Review

Decoding Corticotropin-Releasing Factor Receptor Type 1 Crystal Structures.

The structural analysis of class B G protein-coupled receptors (GPCR), cell surface proteins responding to peptide hormones, has until recently been restricted to the extracellular domain (ECD). Corticotropin-releasing factor receptor type 1 (CRF1R) is a class B receptor mediating stress response and also considered a drug target for depression and anxiety. Here we report the crystal structure of the transmembrane domain of human CRF1R in complex with the small-molecule antagonist CP-376395 in a hexagonal setting with translational non-crystallographic symmetry. Molecular dynamics and metadynamics simulations on this novel structure and the existing TMD structure for CRF1R provides insight as to how the small molecule ligand gains access to the induced-fit allosteric binding site with implications for the observed selectivity against CRF2R. Furthermore, molecular dynamics simulations performed using a full-length receptor model point to key interactions between the ECD and extracellular loop 3 of the TMD providing insight into the full inactive state of multidomain class B GPCRs.This is the accepted manuscript. It is currently embargoed pending publication

Probe measurements of plasma potential nonuniformity due to edge asymmetry in large-area radio-frequency reactors: the telegraph effect

In large-area radio-frequency (rf) capacitive reactors, the redistribution of rf current to maintain current continuity near asymmetric sidewalls causes a perturbation in rf plasma potential to propagate along the resistive plasma between capacitive sheaths. The damping length of the perturbation can be determined by a telegraph equation. Experiments are described using a surface array of unbiased electrostatic probes in the ground electrode to verify the theoretical model of the telegraph effect in Howling [J. Appl. Phys. 96, 5429 (2004)]. The measured spatial dependence of the plasma potential rf amplitude and circulating nonambipolar current agree well with two-dimensional numerical solutions of the telegraph equation. The rf plasma potential can be made uniform by using symmetric reactor sidewalls

Imperfect Dark Energy from Kinetic Gravity Braiding

We introduce a large class of scalar-tensor models with interactions containing the second derivatives of the scalar field but not leading to additional degrees of freedom. These models exhibit peculiar features, such as an essential mixing of scalar and tensor kinetic terms, which we have named kinetic braiding. This braiding causes the scalar stress tensor to deviate from the perfect-fluid form. Cosmology in these models possesses a rich phenomenology, even in the limit where the scalar is an exact Goldstone boson. Generically, there are attractor solutions where the scalar monitors the behaviour of external matter. Because of the kinetic braiding, the position of the attractor depends both on the form of the Lagrangian and on the external energy density. The late-time asymptotic of these cosmologies is a de Sitter state. The scalar can exhibit phantom behaviour and is able to cross the phantom divide with neither ghosts nor gradient instabilities. These features provide a new class of models for Dark Energy. As an example, we study in detail a simple one-parameter model. The possible observational signatures of this model include a sizeable Early Dark Energy and a specific equation of state evolving into the final de-Sitter state from a healthy phantom regime.Comment: 41 pages, 7 figures. References and some clarifying language added. This version was accepted for publication in JCA