Instability driven fragmentation of nanoscale fractal islands

Formation and evolution of fragmentation instabilities in fractal islands, obtained by deposition of silver clusters on graphite, are studied. The fragmentation dynamics and subsequent relaxation to the equilibrium shapes are controlled by the deposition conditions and cluster composition. Sharing common features with other materials' breakup phenomena, the fragmentation instability is governed by the length-to-width ratio of the fractal arms.     

Study of the electronic structure of rhenium and tungsten oxides on the O K-edge

Oxygen K-edge x-ray absorption spectra were studied on the electrochromic amorphous thin film a-WO₃ in the comparison with crystalline oxides having variable electronic (d⁰, d¹, d²) and atomic structure: monoclinic m-WO₃ (insulator - d⁰), cubic Na_0.6WO₃ (metal - d¹), cubic ReO₃ (metal - d¹), layered-type hexagonal h-WO₃, WO₃H₂O and with intercalated H_xReO₃ (metal - d²), H_xWO₃ oxides having a metal-isulating transition. The changes in the XANES range 10–15 eV above the absorption edge are interpreted based on the known band-structure calculations. The high-energy features are related to the multiple-scattering processes (EXAFS) on the nearest atoms. The intensity of the feature at 550–560 eV is attributed for the first time to the value of the metal-oxygen-metal bond angle. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Rotational energy transfer and anisotropic intermolecular potentials : confrontation of experiment with ab-initio calculations for CO-H2 and CO-He

A tunable diode lase in the 4.7 μm region has been used to perform measurements of pressure-broadening in infrared lines of CO(v = 1 ←0) perturbed by H₂ and He at liquid nitrogen and room temperatures. For the COH₂ system the experimental results are compared to previous double resonance data and to ab-initio quantum calculations. The COHe data make a confrontation of the two available potential energy surfaces feasible.     

Backward production of the B meson in K−p interactions at 4.2 GeVc

The backward production of the B(1235) meson is studied in the reaction K^?p→Σ^?π⁺ω. This reaction is observed in the final state Σ^?π⁺π⁺π^?π⁰. A π⁺ω mass enhancement is visible in the region of the B meson for events with small |u|K^? →Σ^?) squared four-momentum transfer. The properties of the enhancement agree with those of the B meson. The cross section for K^?p→Σ^?B⁺ at $\text{4.15}\text{GeV}\text{c}$ incident K^? momentum is (3.2 ± 0.5) μb. The backward production of the B meson is compared with similar baryon exchange productions of the A₁ and C(Q₁) axial vector mesons observed in the same experiment.     

Measurement and interpretation of the 1-0 band of 14N16O at 1900 cm−1

The wavenumbers of the vibration rotation band lines of ¹⁴N¹⁶O are reported for the ${}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{-}{}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, ${}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{-}{}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and ${}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{-}{}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ subbands of the 1-0 transition in the infrared. The full set of spectroscopic constants for this band has been determined by direct approach using the analysis of Zare, Schmeltekopf, Harrop, and Albritton. In addition to the band origin ν₀ and the B, D, H constants for the lower and upper vibrational levels, the following spin-orbit coupling constants have been derived: $\text{A}\text{?}{}_0\text{= 123.02772 ± 0.00011}$ and $\text{A}\text{?}{}_1\text{= 122.78248 ± 0.00011}$ (in cm^?1). Apparent centrifugal corrections to these constants have been determined and the values obtained for them are $\text{A}\text{?}{}_{\mathrm{<mtext>D</mtext><mtext>0</mtext>}}\text{= (0.347573 ± 0.00051) × 10}{}^{\mathrm{?3}}$ and $\text{A}\text{?}{}_{\mathrm{<mtext>D</mtext><mtext>1</mtext>}}\text{= (0.337135 ± 0.00050) × 10}{}^{\mathrm{?3}}\text{cm}{}^{\mathrm{?1}}$. Λ-Type doubling constants evaluated by using both grating and tunable laser data are also reported.     

Une méthode polarimétrique simple pour l'étude des métaux

A simple polarimetric method for the study of metals is discussed. Modulation is obtained by means of a rotating retardation plate and information is extracted from the phase of the signal. Polarizer and analyser have fixed positions, and the system requires no optical compensator and no calibration. Systematic error due to polariser-analyser reference settings is eliminated. Also scanning of the wavelengths is possible.     

Collisional effects upon optical coherences: Experimental study in a xenon three-level system

Laser induced nonlinear absorption profiles in a three-level system have been recorded to study the effect of elastic collisions upon optical coherences. In our particular system, the signal is due to a pure double-quantum term, where we have separated the effect of phase-interrupting collisions from the effect of velocity-changing collisions. Experimental results show that the effect of velocity-changing collisions is very small, and that broadening of the signal is due to phase-interrupting collisions mainly. We are able to determine the increase of the decay rate of the coherent superposition (the “coherence”) of two levels of same parity [Dγ ₁₃/dp=(6.5±1) MHz/Torr] and an upper limit for the mean velocity change (ũ≲5m/s).     

Band parameters of nickel deduced from optical measurements

The gap Δ between the top of the d↑ band and the Fermi level is deduced from optical measurements on a nickel single crystal and found to be ≈50 meV, in agreement with previous estimates. The interpretation of the optical spectrum also suggests that the spin-splitting ΔE_d ? 0.40 eV and that a spin- splitting still exists at temperatures higher than T_C.     

Conversion de frequences optiques dans le seleniure de gallium (GaSe)

Two experimental arrangements of an optical frequency up-converter using GaSe are described. Two wavelength regions have been covered by series of monochromatique rays (one ranging from 3.25 μ to 3.85 μ, the other one from 0.954 μ to 0.968 μ).     

Time resolved study of superelastic collisions in laser excited strontium vapor

We present a time-resolved experimental study of ionization by superelastic collisions in excited strontium vapor. Using short duration laser pulses for resonant excitation, we clearly observe that a delay time is necessary to reach runaway collisional ionization after the end of laser excitation.