Surface enhanced Raman scattering of organic sample powders spread over vacuum-evaporated silver thin film

Surface enhanced Raman scattering (SERS) from samples prepared by spreading para-nitrobenzoic acid (PNBA) and adenosine powders over silver thin films was achieved. The SERS intensities of the ionized PNBA on the silver film increase with increased applied pressure through a cover-glass and reach a maximum at 0.6 MPa. In contrast, the signals caused by adenosine remain nearly unchanged under applied pressures of 0-0.6 MPa. Beyond 0.6 MPa, the signals attributable to samples decrease in intensity. Atomic force microprobe images reveal that nanometer-scale surface morphology is changed by 0.8 MPa pressure, suggesting that the decrease in SERS intensity is related to pressure-induced morphological changes. Results obtained in this study indicate that SERS spectra are obtainable easily, without solvents, under ambient conditions using dispersion of the sample powder.     

Structural, electrical and magnetic characterization of the double perovskites Sr2CrMO6 (M=Mo, W): B′ 4d-5d system

We report on the preparation and characterization of the variation of B′-site transition metal in Sr₂CrMO₆ (M=Mo, W) with double perovskites structure. The magnetic susceptibility shows that Sr₂CrMoO₆ and Sr₂CrWO₆ are antiferromagnets with T_N=40 and 30 K at H=1 T, respectively. In addition, a large magnetoresistance ratio (MR) of ∼38% (H=3 T) at 5 K was observed in the Sr₂CrWO₆ compound. However, the Sr₂CrMoO₆ compound does not show any significant MR even at high fields (MR∼4%; H=3 T and 5 K). The measured O K-edge X-ray absorption is in agreement with the calculated O p-density of states for both compounds.     

Hydrogen atom transfer in indole clusters: formation dynamics of , n = 1-6, fragments

The H atom transfer reaction in electronically excited indole(NH ₃ ) _n clusters is studied in pump-probe experiments with femtosecond laser pulses. By applying different probe photon energies we are able to detect the dissociation products (NH ₃ ) _{n - 1} NH ₄ for n = 1-6. Furthermore we show that the analysis of the corresponding ion signals is not distorted by contributions from larger cluster ions due to evaporation of NH ₃ molecules. The formation times of the products are ca. 140ps for n = 2-4 and about 80ps for n = 5, 6. Received 30 April 2002 / Received in final form 29 May 2002 Published online 13 September 2002     

Langmuir-Blodgett and Langmuir-Schaefer films of poly(5-amino-1-naphthol) conjugated polymer

Langmuir-type films of poly(5-amino-1-naphthol) conductive polymer were formed at the air/water interface. The surface pressure versus molecular area isotherms of the polymeric layers showed a high tendency to aggregate and non-monomolecular behavior on the water surface. Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) procedures have been successfully used to transfer the Langmuir films of this polymer onto hydrophilic silicon substrates. Atomic force microscopy images showed that the films obtained by the LS method have much better quality than those obtained by the LB one. In particular, we have obtained uniform and smooth LS films covering practically all the substrate, while the LB films showed rather aggregated material, only partially covering the substrate.     

Submicron foaming in gelatine by nanosecond and femtosecond pulsed laser irradiation

We compare the foaming characteristics induced by irradiation with single ns and fs laser pulses of UV, VIS and IR wavelengths on gelatines differing in gel strength (bloom values 75 and 225) and in crosslinking degree. We have observed that while laser irradiation with nanoseconds leads to the formation of a microfoam layer at 266 nm, and melting and crater formation at longer wavelengths (532 and 1064 nm), fs pulse irradiation leads to submicron foaming at all wavelengths studied (266, 400 and 800 nm). These results show the possibility of controlling the submicrometric foam structure in this biomaterial and can shed light into the working mechanisms of fs laser nanoprocessing in biomaterials, where increase of temperature, thermoelastic stress generation, and stress-induced bubble formation are mediated by the generated plasma.     

Temperature dependence of positron annihilation lifetime spectra for polyethylene: positron irradiation effects

Lifetime spectra for linear and branched polyethylene have been measured as a function of temperature. The longest lifetime, τ₃, and its intensity, I₃, are traced over a temperature range of 105–370 K. The lifetime decreases with decreasing temperature, and upon reheating the plot is retraced. I₃ has a minimum near 265 K with considerable hysteresis in the cooling/heating cycle. At the lower temperatures the increase in I₃ is attributed to source irradiation.     

Mixture of ultracold lithium and cesium atoms in an optical dipole trap

We present the first simultaneous trapping of two different ultracold atomic species in a conservative trap. Lithium and cesium atoms are stored in an optical dipole trap formed by the focus of a CO₂ laser. Techniques for loading both species of atoms are discussed and observations of elastic and inelastic collisions between the two species are presented. A model for sympathetic cooling of two species with strongly different mass in the presence of slow evaporation is developed. From the observed Cs-induced evaporation of Li atoms we estimate a cross-section for cold elastic Li-Cs collisions. Received: 1 August 2001 / Published online: 23 November 2001     

Evolution of Ge and SiGe Quantum Dots under Excimer Laser Annealing

We present different relaxation mechanisms of Ge and SiGe quantum dots under excimer laser annealing. Investigation of the coarsening and relaxation of the dots shows that the strain in Ge dots on Ge films is relaxed by dislocation since there is no interface between the Ge dots and the Ge layer, while the SiGe dots on Si0.77Ge0.23 film relax by lattice distortion to coherent dots, which results from the obvious interface between the SiGe dots and the Si0.77Ge0.23 film. The results are suggested and sustained by Vanderbilt and Wickham＇s theory, and also demonstrate that no bulk diffusion occurs during the excimer laser annealing.     

Vibrational relaxation of KrF* and XeCl* by rare gases

A steady-state, chemiluminescence technique has been used to measure effective rate constants for vibrational relaxation as a function of vibrational level for KrF^* in collisions with He, Ne, and Ar and XeCl^* with Ar. The effective rate constants reported include contributions to relaxation due to intersystem crossing between theB andC states, in addition to direct relaxation within theB state. The relevance of these results to the understanding of previous measurements in KrF and XeCl lasers is discussed.     

Large area X-ray preionizer for electric discharge lasers

A pulsed X-ray source with a pulse duration of 250 ns (FWHM) which uses plasma cathodes to supply sufficient electrons for emission is described. It is simple to construct and requires low energy input. Less than 14 J of energy can produce up to 10⁷ electron-ion pairs/cm³ in a 10l volume of neon at 1 atm.ENEA Guest on leave from the Shanghai Institute of Optics and Fine Mechanics the Chinese Academy of Sciences, China