R. Garrigos

Solid-liquid phase transitions optically investigated for distributions of metallic aggregates. absence of hysteresis for the smallest sizes

Melting and solidification temperatures for distributions of lead aggregates of very small sizes (mean radius = 42 A, the minimum detectable radius being r = 23 A) are reported. These new results show the existence of a limit temperature TL = 392K below which all the aggregates are solid. A discussion of the results for the melting point depression is given on the basis of a phenomenological approach. For the case of solidification, TL is compared to the stability limit temperature of the supercooled liquid predicted by a simple adiabatic nucleation model.

Étude expérimentale et théorique de l'électroréflexion de l'or en incidence oblique

Abstract Electroreflectance spectra of gold have been investigated at oblique incidence, the light wave being polarized either perpendicular or parallel to the plane of incidence. The experimental procedure is described and the electrochemical behaviour of the metal-electrolyte junction is examined in great detail. A phenomenological interpretation of the electro-reflectance at the metal-electrolyte interface is given, in which this effect is due to a surface current related to a change in the electron density at the surface of the metal. The agreement between the theoretical and experimental results is quite good.

Metallic electroreflectance; anisotropic aspects

Abstract New metallic electroreflectance (MER) experiments have been done at normal incidence for the (100), (111) and (110) faces of gold single crystals in contact with an electrolyte. For every face, a mapping of the fractional change in reflectance has been drawn up in the photon energy range 1.5 to 5.0eV, and in the metallic surface charge interval −15 μC/cm 2 to +20 μC/cm 2 . These results, obtained by a non-traditional potentiodynamic method, are seen to be free of any drift during the experiments. Furthermore, the fractional change in reflectance is found to be different with (110) face when light is polarized parallel to the 〈001〉 direction and parallel to the 〈110〉 direction, while no anisotropy is seen for the (100) and (111) faces. A new theoretical calculation based upon a non-local analysis of the dielectric function of the metal is given, which allows to find the shape, the magnitude and the good sign of the experimental electroreflectance spectrum.

Optically monitored melting, supercooling and solidification of lead aggregates embedded in a dielectric matrix

The solid liquid transitions of lead aggregates embedded in a silicon matrix are investigated through optical measurements. A phenomenological model linking optics and thermodynamics of melting is proposed.

Optical response of a charged metal-aqueous electrolyte interphase

Abstract The logarithmic derivative of the reflectance of an ideally polarizable metal-aqueous electrolyte interphase with respect to the surface charge is expressed in terms of the optical surface impedances on the metal side as well as the electrolyte side. The theoretical spectra for gold are compared with the experimental spectra for the three low index planes of the single crystal and the results are discussed.

Optical investigation of the solid-liquid transition in gallium

Reflectance of thin gallium films in the spectral range 0.3-0.9 mu m is measured for several temperatures around the melting point. When melting occurs, the reflectance increases considerably in the whole spectral range (about 20% at 0.6 mu m). A free-electron model is assumed; this is found to be in good agreement with experiments on liquid-phase systems. A qualitative interpretation is proposed and is found to explain the considerable change occurring during melting.

Modulation spectroscopy of gold: Comparative studies

Abstract An apparatus has been constructed to investigate the modulated optical properties of thin films. We report here the piezo- (PR), thermo- (TR) and electroreflectance (ER) spectra for a thin gold film, 800 A thick. The experimental results show that the PR and TR spectra are similar to each other but quite different from the ER one. A new interpretation of ER is given in which this phenomenon is assumed to be due to a surface current related to a change in the electron density. The agreement between the theoretical and experimental results is encouraging.

Thermoreflectance as a practical tool for the extension of the optical constant data in a wide domain of temperature. Application to lead between 298 K and 538 K

Abstract Thermoreflectance measurements allow the extension of the optical constant data of a material, known for one or a few temperatures, in a wider domain of temperature. This extrapolation method is easier to use than the conventional produres on both the experimental and theoretical grounds.

Static Electric Field Profile in a Metal and Electroreflectance

Numerical resolution of Poisson-Boltzmann’s equation has been carried out to determine the charge distribution induced by a strong static electric field just within the surface of a metal. This distribution has been fitted by an analytical model and then introduced in the wave equation of the light-electric field. The resolution of this last equation with the help of the Fourier transform gives the optical surface impedance of the metal as a function of the excess surface charge σ. The logarithmic derivative of the reflectance with respect to σ is then calculated and successfully compared to the experimental electroreflectance spectrum for the (111) face of a gold single crystal at the potential of zero charge.

Investigation of metallic surfaces through electroreflectance

Abstract Electroreflectance spectra obtained at normal incidence on (100) and (110) faces of gold single crystals in contact with an electrolyte are given for the spectral range 0.2 – 0.7 μm. The fractional change in the reflectance of the (110) face is different when the light is polarized parallel to the [001] direction than when it is polarized parallel to the [ 1 10] direction; no anisotropy is seen on the (100) face. This suggests that electroreflectance is a powerful tool to investigate metal surfaces where the optical electrons are sensitive to the distribution of the surface atoms. A new theoretical approach to metallic electroreflectance using a non-local analysis of the dielectric function of the metal is proposed.