Molecular layering and local order in thin films of 1-alkyl-3-methylimidazolium ionic liquids using X-ray reflectivity

X-ray reflectivity measurements in air of thin films of 1-alkyl-3-methylimidazolium salts in the liquid, liquid crystalline and solid states supported on Si( 111) are described. The films show Bragg features in both liquid crystalline and solid phases, but only after an initial annealing cycle. Kiessig fringes are observed only for the 1-octadecyl-3-methyl-imidazolium hexafluorophosphate films and, following analysis using Parratt32, a bi-layer model is proposed whereby the molecules are orientated with ionic groups at both salt-air and salt-silicon interfaces.

Diagnostic of laser contrast using target reflectivity

Using three different laser systems, we demonstrate a convenient and simple plasma based diagnostic of the contrast of high-power short-pulse lasers. The technique is based on measuring the specular reflectivity from a solid target. The reflectivity remains high even at relativistic intensities above 10(19) W/cm(2) in the case of a high-contrast prepulse-free laser. On the contrary, the specular reflectivity drops with increasing intensities in the case of systems with insufficient contrast due to beam breakup and increased absorption caused by preplasma.

Polarisation analysis of VUV synchrotron radiation emitted from a bending magnet source in the energy range 20-50 eV: A comparison between measurements and theoretical predictions

The results of a study to characterise the polarisation properties of the photon beam emerging from beamline 5D, mounted on a bending magnet source at the Synchrotron Radiation Source, Daresbury Laboratory, are presented. The expectation values for the Stokes parameters corresponding to the light transmitted by the beamline have been calculated by combining ray-tracing and optical methods. The polarisation of the light at the source is modified both by the beamline geometry and by the reflections at the optical components. Although it is often assumed that the polarising properties of grazing incidence optics are negligible, this assumption leads to rather inaccurate results in the VUV region. A study of the reflectivity shows that even at incidence angles (theta(i) = 80-85degrees) which are far from the Brewster angle (theta(B) similar to 45degrees for VUV and soft X-ray radiation) the residual changes in the amplitudes of the reflected light can result in non-negligible polarisation effects. Furthermore, reflection at grazing incidence gives rise to a substantial change in the phase, and this has the effect of rotating the elliptically polarised state. Theoretical Stokes parameters have been compared with full polarisation measurements obtained using a reflection polarimeter in the energy range 20-40 eV. (C) 2003 Elsevier B.V. All rights reserved.

William Herschel Telescope observations of distant comets

We present results from broad-band V- and R-filter observations obtained at the 4.2-m William Herschel Telescope on La Palma on 2002 July 12-14. A total of six comets were imaged, and their heliocentric distances ranged from 2.8 to 6.1 au. The comets observed were 43P/Wolf-Harrington, 129P/Shoemaker-Levy 3, 133P/Elst-Pizarro, 143P/Kowal-Mrkos, P/1998 U4 (Spahr) and P/2001 H5 (NEAT). A detailed surface brightness profile analysis indicates that three of the targeted comets (43P/Wolf-Harrington, 129P/Shoemaker-Levy 3 and P/1998 U4) were visibly active, and the remaining three comets were stellar in appearance. Further analysis shows that for the three `stellar-like' comets the possible coma contribution to the observed flux does not exceed 12.2 per cent, and in the case of comet 143P/Kowal-Mrkos the coma contribution is expected to be as low as 1 per cent, and so the resulting photometry most likely represents that of the projected nucleus surface. Effective radii for the inactive comets range from 1.02 to 4.56 km, and the effective radius upper limits for the active comets range from 1.94 to 4.15 km. We assume an albedo and phase coefficient of 0.04 and 0.035 mag deg-1, respectively, with the exception of comets 133P/Elst-Pizarro and 143P/Kowal-Mrkos for which phase coefficients were previously measured. These values are compared with previous measurements, and for comet 43P/Wolf-Harrington we find that the nucleus axial ratio a/b could be as large as 2.44. For the active comets we measured dust production levels in terms of the Af? quantity. Spectral gradients were extracted for two of the inactive comets from their measured broad-band colour indices, and compared with the rest of the comet population for which (V-R) colour and spectral gradient values exist. We find a spectral gradient for 143P/Kowal-Mrkos of 9.9 +/- 8.1 per cent/100 nm, which is very typical of Jupiter-family comets, the majority of which have reflectivity gradients in the range 0-13 per cent (100 nm)-1. The spectral gradient for comet 133P/Elst-Pizarro is amongst the bluest yet measured. We measure a (V-R) colour index value of 0.14 +/- 0.11 for the nucleus of 133P/Elst-Pizarro which is considerably lower than previous measurements. A possible explanation for this difference is considered.

Ultra-thin tunable microwave absorber using liquid crystals

A dynamically adaptive radar absorber is described which is based on a periodic array of microstrip patches that are printed on a 500 mu m-thick liquid crystal substrate. The measured reflectivity of the structure is less than -38 dB with a 200 MHz -10 dB bandwidth at 10.19 GHz when a +4 DC bias is applied. It is shown that a 34 dB reduction in signal loss occurs when the bias voltage is increased to 20 V.

Small-angle scattering from long-chain alkylimidazolium-based ionic liquids

This paper compares the structure of 1-alkyl-3-methylim ridazolium salts using SAXS and X-ray reflectivity. A range of anions have been investigated namely chloride, bromide, trifluoromethanesulfonate (OTf), bis(trifluoromethanesulfonyl)imide (TFI) and tetrachloropalladate(II) with cation alkyl chains ranging from n = 12-20. In general, the salts show liquid crystalline behaviour whose structure is still observed on melting into an isotropic liquid.

Structure and bonding in ionic liquids

The influence of peripheral substitution on the physical properties of 1-alkyl-3-methylimidazolium based ionic liquids is described. Studies into the molecular structure of ionic liquids using X-ray crystallography, XAFS, recoil mass spectrometry and reflectivity measurements are described with particular reference to the interactions between ionic liquids and solutes; the example of an ionic liquid-organic co-crystal is given.

Soft x-ray free electron laser microfocus for exploring matter under extreme conditions

We have focused a beam (BL3) of FLASH (Free-electron LASer in Hamburg: lambda = 13.5 nm, pulse length 15 fs, pulse energy 10-40 mu J, 5Hz) using a fine polished off-axis parabola having a focal length of 270 mm and coated with a Mo/Si multilayer with an initial reflectivity of 67% at 13.5 nm. The OAP was mounted and aligned with a picomotor controlled six-axis gimbal. Beam imprints on poly(methyl methacrylate) -PMMA were used to measure focus and the focused beam was used to create isochoric heating of various slab targets. Results show the focal spot has a diameter of

Design of a salisbury screen absorber using frequency selective surfaces to improve bandwidth and angular stability performance

A new design method that greatly enhances the reflectivity bandwidth and angular stability beyond what is possible with a simple Salisbury screen is described. The performance improvement is obtained from a frequency selective surface (FSS) which is sandwiched between the outermost 377 Ω/square resistive sheet and the ground plane. This is designed to generate additional reflection nulls at two predetermined frequencies by selecting the size of the two unequal length printed dipoles in each unit cell. A multiband Salisbury screen is realised by adjusting the reflection phase of the FSS to position one null above and the other below the inherent absorption band of the structure. Alternatively by incorporating resistive elements midway on the dipoles, it is shown that the three absorption bands can be merged to create a structure with a −10 dB reflectivity bandwidth which is 52% larger and relatively insensitive to incident angle compared to a classical Salisbury screen having the same thickness. CST Microwave Studio was used to optimise the reflectivity performance and simulate the radar backscatter from the structure. The numerical results are shown to be in close agreement with bistatic measurements for incident angles up to 40° over the frequency range 5.4−18 GHz.

Characterization of a picosecond laser generated 4.5 keV TiK-alpha source for pulsed radiography

K alpha radiation generated by interaction of an ultrashort (1 ps) laser with thin (25 mu m) Ti foils at high intensity (2x10(16) W/cm(2)) is analyzed using data from a spherical Bragg crystal imager and a single hit charge-coupled device spectrometer together with Monte Carlo simulations of K alpha brightness. Laser to K alpha and electron conversion efficiencies have been determined. We have also measured an effective crystal reflectivity of 3.75 +/- 2%. Comparison of imager data with data from the relatively broadband single hit spectrometer has revealed a reduction in crystal collection efficiency for high K alpha yield. This is attributed to a shift in the K-shell spectrum due to Ti ionization. (c) 2005 American Institute of Physics.

Atmospheric pressure chemical vapour deposition of titanium dioxide coatings on glass

Atmospheric pressure chemical vapour deposition of titanium dioxide coatings on glass substrates was achieved by the reaction of TiCl4 and a co-oxygen source (MeOH, EtOH, (PrOH)-Pr-i or H2O) at 500-650degreesC. The coatings show excellent uniformity, surface coverage and adherence. Growth rates were of the order of 0.3 mum min(-1) at 500degreesC. All films are crystalline and single phase with XRD showing the anatase TiO2 diffraction pattern; a = 3.78(1), c = 9.51(1) Angstrom. Optically, the films show minimal reflectivity from 300-1600 nm and 50-80% total transmission from 300-800 nm. Contact angles are in the range 20-40degrees for as-prepared films and 1-10degrees after 30 min irradiation at 254 nm. All of the films show significant photocatalyic activity as regards the destruction of an overlayer of stearic acid.

High contrast plasma mirror: spatial filtering and second harmonic generation at 10(19)Wcm(-2)

Recently, the use of plasma optics to improve temporal pulse contrast has had a remarkable impact on the field of high- power laser-solid density interaction physics. Opening an avenue to previously unachievable plasma density gradients in the high intensity focus, this advance has enabled researchers to investigate new regimes of harmonic generation and ion acceleration. Until now, however, plasma optics for fundamental laser reflection have been used in the sub-relativistic intensity regime (10(15) - 10(16)Wcm(-2)) showing high reflectivity (similar to 70%) and good focusability. Therefore, the question remains as to whether plasma optics can be used for such applications in the relativistic intensity regime (> 10(18)Wcm(-2)). Previous studies of plasma mirrors (PMs) indicate that, for 40 fs laser pulses, the reflectivity fluctuates by an order of magnitude and that focusability of the beam is lost as the intensity is increased above 5 x 10(16)Wcm(-2). However, these experiments were performed using laser pulses with a contrast ratio of similar to 10(7) to generate the reflecting surface. Here, we present results for PM operation using high contrast laser pulses resulting in a new regime of operation - the high contrast plasma mirror (HCPM). In this regime, pulses with contrast ratio > 10(10) are used to form the PM surface at > 10(19)Wcm(-2), displaying excellent spatial filtering, reflected near- field beam profile of the fundamental beam and reflectivities of 60 +/- 5%. Efficient second harmonic generation is also observed with exceptional beam quality suggesting that this may be a route to achieving the highest focusable harmonic intensities. Plasma optics therefore offer the opportunity to manipulate ultra-intense laser beams both spatially and temporally. They also allow for ultrafast frequency up-shifting without detrimental effects due to group velocity dispersion (GVD) or reduced focusability which frequently occur when nonlinear crystals are used for frequency conversion.

The plasma mirror - A subpicosecond optical switch for ultrahigh power lasers

Plasma mirrors are devices capable of switching very high laser powers on subpicosecond time scales with a dynamic range of 20–30 dB. A detailed study of their performance in the near-field of the laser beam is presented, a setup relevant to improving the pulse contrast of modern ultrahigh power lasers ~TW–PW!. The conditions under which high reflectivity can be achieved and focusability of the reflected beam retained are identified. At higher intensities a region of high specular reflectivity with rapidly decreasing focusability was observed, suggesting that specular reflectivity alone is not an adequate guide to the ideal range of plasma mirror operation. It was found that to achieve high reflectivity with negligible phasefront distortion of the reflected beam the inequality csDt,lLaser must be met (cs : sound speed, Dt: time from plasma formation to the peak of the pulse!. The achievable contrast enhancement is given by the ratio of plasma mirror reflectivity to cold reflectivity.