We present results on surface freezing of Ga-based alloys, GaBi, GaPb and GaTl, above the liquidus line between the Ga-rich eutectic and the monotectic point. Spectroscopic ellipsometry (0.8 eV ≤hν≤4.2 eV) and kinetic single wavelength ellipsometry (2.75 eV) have been employed to probe the changes of the interfacial electronic structures on surface freezing. To minimize thermal gradients across the sample a heatable cap that covers the sample and crucible was developed. The surface freezing temperature, TSF, for the spontaneous formation of a solid-like film on top of the Ga-rich liquid on cooling the sample from the homogeneous phase region was found to be independent of the temperature difference between the upper and lower furnace (ΔT: +10 to −10 K) and only weakly dependent on the cooling rate (: 2.5–20 K h−1). In the case of GaPb the solid film consists of solid Pb with a thickness h≥400 Å. Comparing with GaBi we draw analogous conclusions for GaPb and GaTl and suggest that the surface freezing transition precedes the bulk phase transition along the liquidus line as the alloy is cooled.

Oscillatory wetting-dewetting instabilities at the liquid/vapour interface of Ga-Pb and, for the first time, of Ga-Bi alloys have been studied employing second harmonic generation and kinetic ellipsometry. Alloy samples prepared at x-T conditions inside the respective miscibility gap (Ga0.8Bi0.2: T = 520 K; Ga0.95Pb0.05: T = 595 K) show oscillations on the timescale of typically hours. The surface transforms back and forth from a Ga-rich non-wetting state to a complete wetting state where the Ga-rich bulk is covered with a Bi-rich (or Pb-rich) film. In the case of Ga-Bi the change of the wetting film thickness was characterized quantitatively with the aid of the ellipsornetric measurements. It is consistent with a wetting-dewetting mechanism whereby the wetting film varies periodically between macroscopic and microscopic values. The phenomenon can be understood taking different emissivities of the wet and the nonwet state into account. They cause at otherwise constant conditions different temperatures of the sample, which has to adopt its phase compositions according to the respective phase diagram. This results in mass transport to and away from the surface. A model calculation on Ga-Bi taking these effects into account recovers in good quantitative agreement the oscillation period and the oscillations of the temperature as well as of the film thickness. Similarities and differences between Ga-Bi and Ga-Pb are finally considered.

We have studied the Raman spectra of (NdCl2)-(NdCl3) and Ce-(CeCl3) mixtures in eutectic (LiCl)(0.58)(KCl)(0.42) as solvent in the temperature range up to 600 degreesC. For the highly corrosive samples a windowless cell in connection with a Raman microscope was utilised. To our knowledge the Raman spectra of the solutions of NdCl2, CeCl3 and (most likely) CeCl2 in eut. - LiCl - KCl are shown here for the first time. In accordance with other rare earth halides described in the literature the Raman spectra of the pure trivalent systems are dominated by octahedral LnCl(6)(3-) species which show a typical broad polarized band centred at 245 cm(-1) for NdCl3 and 240 cm(-1) for CeCl3, respectively. NdCl2 in (LiCl - KCl)(eu) shows a complex Raman spectrum consisting of depolarized bands. In mixtures of divalent and trivalent neodymium chlorides both spectra can be observed in parallel and no additional Raman bands appear. Solutions of cerium in CeCl3 -( eut. - LiCl - KCl) show temporarily new Raman bands which are presumably due to divalent cerium chloride. These bands disappear in our samples after about 1 h since CeCl2 is not stable under the experimental conditions. Our findings are discussed in the light of the strongly different electronic transport properties in the neodymium and cerium systems.

Complete wetting transitions at the liquid-vapor interface of ${\mathrm{Ga}}_{x}{\mathrm{Bi}}_{1\ensuremath{-}x}$ alloys (${x}_{\mathrm{Ga}}=0.915$, 0.88, 0.8, 0.67, and 0.57) have been investigated by ellipsometry. For this purpose we have developed a UHV apparatus equipped with an in situ phase modulation ellipsometer. The setup allows cleaning of the alloy surface under UHV conditions. Spectroscopic ellipsometry $(0.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}\ensuremath{\leqslant}h\ensuremath{\nu}\ensuremath{\leqslant}4.65\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ at various constant temperatures and time-dependent measurements at a constant energy of $2.75\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, while the sample was continuously cooled down, are utilized. These complementary approaches promise access to the functional form of the wetting film thickness with temperature as complete wetting is approached. A multistage analysis is introduced to extract the film thickness from single-wavelength ellipsometry. For a careful analysis of the spectra, it was necessary to reinvestigate the complex dielectric functions of the pure components bismuth and gallium. The spectra of the alloys have been modeled using an effective medium approximation for the liquid $\mathrm{Ga}\ensuremath{-}\mathrm{Bi}$ bulk phase covered by a film of liquid bismuth. In our particular sample geometry, the wetting film thickness grows from one or two atomic layers to about $30\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}50\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ within a few K as one approaches the demixing regime. The data show the proposed logarithmic divergence of $d$, typical for short range interactions. In the temperature and composition range studied they do not agree with the functional form proposed for long range intermolecular interactions.

We present ellipsometric spectra of liquid gallium–bismuth alloys in the energy range 0.8 eV⩽ℏω⩽4.65 eV and in the temperature range from 277 °C down to 258 °C. Using a phase modulation ellipsometer, high-precision spectra have been recorded approaching the liquid–liquid miscibility gap of this system with decreasing temperature from the homogeneous liquid phase. The spectra have been modelled using a Ga–Bi effective medium as substrate covered by a film of liquid Bi. In the temperature range 277 °C⩽T⩽264 °C, the Bi film thickness is of the order of one to two monolayers. At 264 °C, i.e. 6 K above the miscibility gap, the character of the spectra changes noticeably and the Bi film thickness increases continuously. As one reaches the demixing regime the film of our particular sample geometry has a gravitationally thinned thickness of approximately 20 Å. This phenomenon can be interpreted by a complete wetting transition in a metallic system.

Spectroscopic ellipsometry and reflectometry were employed to study the wetting behaviour of the metal-molten salt system K-x(KCl)(1-x) at the interface to an inert sapphire wall. We present new results on the wetting film spectra and on the film thickness at solid-liquid coexistence and in the homogeneous liquid phase. The film spectra clearly show the typical absorption of the liquid state F-centre. The wetting film thickness strongly increases approaching the monotectic temperature along the liquidus line. In the homogeneous regime the film thickness decreases at constant composition with increasing temperature and at constant temperature with decreasing salt concentration in agreement with complete wetting. For the interpretation within the framework of tetra point wetting we have developed a quantitative description of the excess Gibbs energy of the system.

Abstract Spectroscopic ellipsometry and reflectometry were employed to study the wetting behaviour of the metal–molten salt system K x (KCl)1− x at the interface to an inert sapphire wall. We present new results on the wetting film spectra and on the film thickness at solid–liquid coexistence and in the homogeneous liquid phase. The film spectra clearly show the typical absorption of the liquid state F-centre. The wetting film thickness strongly increases approaching the monotectic temperature along the liquidus line. In the homogeneous regime the film thickness decreases at constant composition with increasing temperature and at constant temperature with decreasing salt concentration in agreement with complete wetting. For the interpretation within the framework of tetra point wetting we have developed a quantitative description of the excess Gibbs energy of the system.

Pump-probe studies of localized excess electrons in liquid KCl have been performed at probe wavelengths between 600 and 1530 nm at elevated temperatures. After photoexcitation with 50 fs pulse centred at 800 nm a transient bleach is observed at wavelengths between 600 and 1245 nm while at wavelengths longer than 1300 nm only a transient absorption is observed. The transient bleach recovery can be assigned to the lifetime of strongly localized electrons (F-centres) and represents ground state dynamics with a time constant of (200 +/- 50) fs, in reasonable agreement with theoretical calculations for polaron-like states in liquid K-KCl. The transient absorption at longer wavelengths, that decays with the same time constant, might be due to weakly localized electrons. Consequently, the ionic motion of the alkali halide melt determines the dynamics in this system. These investigations represent the first direct ultrafast study with fs time resolution of localized electrons in the molten K-KCl sytems.

The phenomenon of surface freezing has been studied by second harmonic and plasma generation measurements in the entire composition range of liquid Ga-Bi alloys. Surface freezing was observed for the first time for all alloys with compositions between the eutectic (x(Bi) = 0.0022, T-eut = 29.48degreesC) and monotectic point (x(Bi) = 0.085, T-mono = 222degreesC). On cooling of such alloys a Bi-rich solid-like film forms on top of the bulk liquid phase at temperatures well above the liquidus line. On melting of these films a clear hysteresis behaviour is found, characterizing this type of transition as a first order phase transition. The maximum difference of 20 K between the liquidus temperature and the melting temperature of the surface freezing films was observed for the eutectic alloy. The line of surface freezing temperatures merges with the liquidus approaching the monotectic point. An estimate of the thickness of the Bi-rich surface freezing films from the interfacial free energies yields a value of the order of 10 nm. The correlation of the surface freezing and wetting transition occurring in the Ga-Bi system as well as the thermodynamics of surface freezing are qualitatively discussed.

In this paper recent results on surface phase transitions at the interface of Ga–Bi alloys are reviewed. The wetting transition in the liquid–liquid demixing region is briefly summarized. Then, the surface freezing transition, which occurs in the concentration range between the eutectic (xBi=0.0022) and the monotectic (xBi=0.085) compositions, is described. This was discovered recently using second harmonic generation and plasma generation methods. The surface freezing line was found approximately 20 K above the eutectic point; it approaches the liquidus line with increasing Bi mole fraction and presumably merges at the monotectic point. Arguments for the thermodynamic stability of the surface freezing Bi-rich films and a possible correlation between the wetting transition and the surface freezing transition are discussed. Finally, we present preliminary experimental results on a study of surface freezing in Ga–Pb.

We report the first optical spectra in molten Ndx(NdCl3)1−x and NdCl2–NdCl3(LiCl, KCl)eut, which have been obtained by reflectivity and absorption measurements in the NIR–VIS–UV range at temperatures up to 760 °C. In addition, we have measured the temperature coefficient of the electrical conductivity of Ndx(NdCl3)1−x melts at various compositions, x, and temperatures up to 902 °C. The spectroscopic results together with the apparent activation energies for the conductivity support the model of intervalence charge transfer in these systems.

Using spectroscopic ellipsometry we have measured the complex dielectric function ϵ=ϵ1−iϵ2 in the energy range 0.8<ℏω<2.7 eV of liquid K–KCl in the homogeneous phase region at 800 °C and at compostions of xK=0.22 and 0.31. In comparison to previous experiments the accuracy was improved. The spectra can be modelled using the simple Drude formalism for nearly free electrons and a Lorentzian oscillator. This clearly supports the two component model which for the transition from metallic to non-metallic states of these solutions considers both localized and mobile electronic states. The imaginary part of the dielectric function is found to be in good agreement with recent Car–Parrinello MD simulations.

We have studied the optical absorption spectra and the electrical conductivity of highly corrosive liquid NdI3-NdI2 mixtures at various temperatures up to 957degreesC. The conductivity as a function of composition shows a clear maximum near the 1: 1 composition, indicative of a considerable partial electronic conductivity of 1 Omega(-1) cm(-1) and an activation energy E-a = 0.4 eV. Optical spectra have been obtained for the first time in NdI2-rich solutions in the range 0.5 less than or equal to <(h)overbar>omega/eV less than or equal to 3. They are characterized by the typical f-f transitions in the visible range, by an absorption edge in the ultraviolet range that exhibits Urbach behaviour and, in particular, by a strong and broad absorption band with a peak near 1.5 eV. The latter shows the main characteristic features of an intervalence charge transfer band which is shown by fitting the spectra with a two-site polaron model.

Surface freezing in a liquid eutectic Ga–Bi alloy has been studied by second harmonic generation and plasma excitation in several cooling and heating cycles at different temperature rates. An ordered solid-like film is observed on top of the bulk liquid which forms at a temperature well above the eutectic temperature. The hysteresis behaviour of crystallisation and melting of this film indicates that this surface phase transition is of first order, which is reported for the first time.

The wetting and prewetting transitions at the metal-rich K–KCl melt–sapphire interface have been investigated by spectroscopic ellipsometry in combination with normal incidence reflectivity in the spectral range 0.8⩽ℏω⩽2.2 eV at temperatures up to 730 °C. Along the coexistence curve a salt-rich liquid wetting film is observed which is identified by the spectral features of the liquid state F-center. Unusually thick wetting films are found ranging from 30 nm near 540 °C to 300 nm approaching the monotectic temperature of 751 °C. Their composition has been determined from the absorption coefficients of the F-center band and it corresponds to about 90 mole % salt. At conditions off coexistence and near the prewetting line, similar mesoscopically thick wetting films exist. Crossing the prewetting line towards metal-rich solutions, the optical properties at the interface agree with those of the nearly free electron metal. The high thickness of the prewetting films is qualitatively explainable by charging and double layer formation at the interface. The occurrence of liquid F-center-like states up to 200 K below the monotectic temperature gives evidence of a strong undercooling of the wetting films with respect to the bulk phase. These characteristics of the wetting transition in a metal–molten salt solution can be described by the tetra point wetting scenario for binary fluid mixtures.

Spectroscopic ellipsometry and simultaneous reflectivity measurements of liquid KxKCl1-x solutions clearly exhibit a first order wetting transition in metal-rich melts. At the sample/substrate interface, salt-rich wetting films of mesoscopic thickness ( approximately 100 nm) are observed at and off of coexistence. They are uniquely characterized by the liquid F-center absorption band. However, crossing the prewetting line towards metal-rich concentrations, the F bands disappear. From the observation of the liquid F-center band, it is concluded that a strong undercooling of the wetting films of about 200 K may occur in binary metallic fluids, which is demonstrated here for the first time.

Optical second harmonic generation (SHG) as a particularly surface sensitive technique was employed for the first time to investigate the surface phase behaviour of a liquid alloy, BixGa1-x, in different heating and cooling cycles up to 280 °C. The aim of these experiments is to establish a relatively simple experimental access to the surface phase diagrams of liquid alloys. Measurements of the characteristic changes of the SH signal have been performed on pure Bi and different Ga-rich alloys (xBi⩽0.367). In pure bismuth the melting and freezing of the surface is indicated by a distinct polarization dependent variation of the SH intensities. Of particular interest is the characterization of the wetting transition found in Ga-Bi recently at the monotectic phase transition (xm = 0.085, Tm = 222 °C). Surprisingly, on first heating of the Ga-rich alloys up to 280 °C the SHG signals give no indication of the dramatic compositional change at the surface induced by the wetting transition. From these observations we conclude that the main source for the nonlinear polarization is the outermost layer of the alloy which in the wet and the non-wet state consists of an adsorbed Bi-rich monolayer. It is shown that SHG is very sensitive to structural changes at the surface. Most interestingly, on cooling of the Ga-rich alloys a Bi-rich film crystallizes on top of the bulk liquid alloy.

Spectroscopic ellipsometry in the energy range 0.8 eV⩽hν⩽2.7 eV was employed at temperatures up to 730°C to study the dielectric function ε=ε1+iε2 of a wetting layer in a metal rich K0.95KCl0.05 solution. The ε2-spectra measured at temperatures above the wetting transition at coexistence (T=590°C) and near coexistence (T=660°C) are similar to the spectrum of a KCl-rich bulk phase which shows a characteristic F-centre absorption band in the near infrared region. At a temperature off coexistence and above the prewetting transition (T=730°C) the dispersion of the ε2-spectrum in the near infrared is reduced compared to the lower temperatures. These findings are consistent with previous experiments which indicate that a salt rich wetting phase intrudes between the metal rich phase and a sapphire substrate above the wetting temperature Tw∼500°C at coexistence.

We report optical absorption spectra of Kx(KI)1−x, Csx(CsI)1−x, and Csx(CsCl)1−x solutions at temperatures around 800 °C and in the saltrich concentration range up to metal mole fractions of xM⩽0.04 for energies 0.5⩽ℏω⩽5 eV. Employing a high-temperature electrochemical cell, we were able to change and determine the alkalimetal activity and the alkalimetal mole fraction in situ simultaneously with the optical absorption spectra. The high quality of the absorption spectra allows to distinguish spectral contributions to localized electronic states and mobile electrons. Together with previously measured spectra of Nax(NaI)1−x melts a systematic investigation of the alkali iodide melts reveals differences in the nature of the strongly localized electronic states with varying cation. Interpretation of the spectroscopic results with the aid of a chemical defect model shows that the formation of localized dimeric electron states (bipolarons) is pronounced in melts of smaller cations. The analysis of the optical contribution due to mobile electrons has been performed with a simple Drude model for nearly free electrons and the results are in good agreement with independent measurements of electronic transport properties.