Optical behaviour of melting for a thin metal film

Reflectance measurements have been performed for gallium films at normal incidence (from 0.3 to 0.9 μm) in terms of temperature (from -20°C to + 40°C). The basic results are: (i) a drastic change in reflectance when melting occurs (about 20% at 0.6 μm), and (ii) a shift in the temperature of the solid-liquid transition with the thickness of the film which only takes place on and after the second melting (about 7°C for a film 250A?thick).     

Optical absorption near the melting point of LiNO3 crystal

Temperature dependence of the absorption spectrum in the ultraviolet region below 4.2 eV is measured in the temperature range from room temperature to just below the melting point T_M of LiNO₃ crystal. Anomalous behaviors in the observed spectra are found on approaching to T_M. The low energy tails of the spectra are analyzed by using Urbach rule. The steepness parameter σ(T), derived from the experimental results, also shows anomalous temperature change near T_M.     

Molecular dynamics studies of ultrafast laser-induced nonthermal melting

Molecular Dynamics (MD) is employed to investigate nonthermal melting triggered by coherent phonon excitation in bismuth telluride, which has Peierls distortion in the lattice structure. Results showed that the structural distortion caused by coherent phonons appears as early as 80 fs, while it takes several picoseconds for the whole phonon-excited area to evolve into a liquid state. It was also found that the temperature in the phonon-excited area rises quickly within tens of femtoseconds, while the rest of the lattice remains at the initial temperature even after several picoseconds, which is separated from the high temperature region across a thin transition area. This phenomenon is analogous to the heat transfer across a solid–liquid interface, even though in our case there is no abrupt solid-liquid interface between the cold lattice and the quasiliquid.     

Optical bleaching studies of quartz

The bleaching properties of a hydrothermal, Arkansas quartz are studied in detail. The thermoluminescence (TL) and phototransferred TL (PTTL) signals are examined as a function of wavelength, illumination time and temperature at which the sample is illuminated. We have also briefly studied the effects of pre-irradiation annealing on the sample. The results of these experiments are described and their relevance to the models for TL is discussed.     

Acoustic studies of melting and crystallization of nanostructured decane

Acoustic studies of melting and crystallization of decane loaded in porous glasses (Vycor and laboratory-produced glass) have been performed. Measurements of the temperature dependences of the ultrasound velocity have revealed a decrease in the melting and crystallization temperatures of decane as compared to the melting point of bulk decane and a diffuseness of these phase transitions. The results obtained are compared with the predictions of the models describing melting of individual small particles. The specific features revealed in the acoustic properties of nanocomposites based on decane-loaded porous glasses are discussed.     

Optical studies of samarium-doped fluoride nanoparticles

Samarium-doped calcium fluoride (CaF₂) nanoparticles were synthesized by the co-precipitation method and characterized by powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), optical absorption and photoluminescence (PL) techniques. The PXRD patterns confirmed the cubic crystallinity of the synthesized nanoparticles. The average particle size estimated using Scherer's formula was ～20?nm. The purity of the synthesized nanoparticles was confirmed by the FTIR spectrum. The morphological features studied using SEM revealed that the nanoparticles were agglomerated and porous. The optical absorption spectrum showed a strong and prominent absorption peak at ～264?nm and a weak one at ～212?nm. The PL spectrum showed broad and prominent emissions with peaks at ～387 and 532?nm along with weak emissions at 573 and 605?nm.     

Optical studies of chemisorption on metals

Optical techniques, particularly ellipsometry, have been highly successful in the study of chemisorption on semiconductors. Such studies on metal surfaces are more difficult because the short screening lengths limit the chemisorption-induced perturbation to a surface-layer only a few ångströms thick. Some success has been achieved using differential reflectance spectroscopy. However this necessitates (1) two independent reflectance measurements or the use of KK analysis to obtain complex optical constants, and (2) assumed or independently-measured values for the thickness of the surface layer in order to completely specify the adsorption-induced changes. The advent of extremely precise in situ modulated ellipsometers has made the optical study of gas-metal interactions less difficult, in particular eliminating the necessity of KK analysis. We briefly describe such a system here, the automated polarization-modulated ellipsometer. When this instrument is coupled with a recently developed approximation technique for obtaining surface-layer thickness and optical constants from only two ellipsometric measurements, the complete optical characterization of many metal-absorbate systems is straightforward. Recent experimental results indicating a chemisorption-induced surface state within 2.4 eV of the Fermi level for oxygen on silver are discussed.     

Optical studies of carbon nanotubes and nanographites

Resonance Raman and photoluminescence excitation (PLE) spectroscopies are used to study the optical properties of different types of carbon nanostructures such as carbon nanotube, nanoribbons, nanographites and graphite edges. In the resonance Raman experiments of carbon nanotubes, the (n,m) assignment is obtained by comparing the experimental and theoretical diameter and chirality dependence of the optical transitions. The influence of the environment on the optical transitions of the nanotubes is also obtained in the Raman experiments. The PLE measurements in different samples of carbon nanotubes show both direct and phonon-assisted optical transitions, and the results give new evidences that the optical transitions in nanotubes have an excitonic character, which is very strong for the low energy transitions. We also analyze the Raman spectra of nanoribbons and nanographites, showing that this technique is an important tool for defect characterization in graphitic materials, and can be used to distinguish the atomic structure of the graphite edges.     

Brillouin and raman scattering studies of the melting transition in salol

Brillouin and Raman scattering studies of salol from room temperature to within 5 mK of the melting transition at T_m = 40.97°C are reported. Changes in the Brillouin shifts and linewidths were accurately determined by nonlinear least-squares fitting and deconvolution. A marked increase in the deconvoluted Brillouin linewidth (~400%) and a gradual softening (~20%) of the transverse acoustic modes were observed very close to T_m. The increase of the Brillouin linewidths was analyzed by a simple dislocation model assuming the hypersonic attenuation to be proportional to the concentration of thermally generated defects near T_m. The defect formation energy E_D(T) was computed from the temperature-dependent linewidth data, and was found to decrease significantly (~60%) near T_m, suggesting a cooperative effect producing a catastrophic growth of defects which brings about melting by destroying the long range order of the crystal. The conclusion that melting is mediated by a sudden growth of defect concentration near T_m was further strengthened by Raman scattering experiments in which 13 new Raman modes appeared close to T_m. These new modes are believed to be defect activated through breaking of the local symmetry of the crystal. A slight softening of the Raman modes (~5%) was observed close to the melting point.     

Laser melting of carbide tool surface: Model and experimental studies

Laser controlled melting is one of the methods to achieve structural integrity in the surface region of the carbide tools. In the present study, laser heating of carbide cutting tool and temperature distribution in the irradiated region are examined. The phase change process during the heating is modeled using the enthalpy–porosity method. The influence of laser pulse intensity distribution across the irradiated surface (β) on temperature distribution and melt formation is investigated. An experiment is carried out and the microstructural changes due to laser consecutive pulse heating is examined using the scanning electron microscope (SEM). It is found that melt depth predicted agrees with the experimental results. The maximum depth of the melt layer moves away from the symmetry axis with increasing β.     

Optical studies of pulsed-laser fragmentation of biliary calculi

The fragmentation of gallstones and kidney stones using pulsed visible laser radiation has recently been demonstrated; however, the fragmentation mechanism is not well understood. The temporal and spectral characteristics of the bright flash of light accompanying fragmentation of gallstones were studied using 0.8 and 360-s-long, 690-nm-wavelength, dye-laser pulses. Time-resolved visible emission spectra show a broad continuum upon which line spectra are superimposed. The continuum emission is due to free-free and free-bound electron transitions indicative of a plasma and the line spectra are due to neutral and ionized calcium. Initiation of this plasma is fluence rather than intensity dependent. A model is proposed in which laser energy is coupled to the plasma, which then impulsively expands, generating intense acoustic transients which fracture the stone.     

Acoustic studies of melting and crystallization of indium-gallium alloy in porous glass

Temperature dependences of the velocity of the longitudinal ultrasonic waves in a nanocomposite on the basis of porous glass filled with gallium-indium alloy have been measured. Acoustic anomalies due to crystallization and melting of the alloy in nanopores have been revealed for the complete and partial cooling-heating cycles. A two-step temperature hysteresis loop between the curves of the velocity change upon cooling and heating has been found, the existence of which is related to the formation in pores of two types of mixtures, with α- and β-Ga. Stabilization of β-Ga in nanopores has been observed. It was shown that the conditions of confined geometry lead to a shift to low temperature of the melting regions for both mixtures in comparison with the regions of the coexistence of the liquid and solid phases in bulk alloy.     

Optical absorption studies on cobalt doped struvite

Abstract

Optical absorption spectrum of cobalt doped MgNH₄PO₄ · 6H₂O (struvite) is investigated in UV-VIS-NIR regions. The spectrum in UV-VIS-NIR region is attributed to Co²⁺ in octahedral symmetry whereas the IR spectrum is attributed to vibrations due to PO₄ ^3-, NH₄ ⁺ and H₂O. The following crystal field (Dq) and interelectronic repulsion (B, C) parameters are evaluated: Dq = 940cm^?1, B = 870cm^?1 and C = 3970cm^?1.     

Optical absorption and EPR studies on enargite

The behaviour of transition metal ions in enargite has been studied by electron paramagnetic resonance and absorption spectroscopy in the UV–VIS and near-IR regions. The ground state of Cu(II) ions in enargite is confirmed as ²B_1g since g₁₁>g_⊥ (2.54>2.11). Three characteristic bands observed in the optical absorption spectra at 8275, 13105 and 18420 cm^?1 are assigned to the transitions, ²B_1g→²A_1g, ²B_1g→²B₂ and ²B_1g→²E_g, respectively, of Cu(II) ion in the tetragonal field. The presence of Fe(II) bands is an evidence for iron impurities in the mineral.     

Optical and resistivity studies of intercalated layer compounds

The optical absorption of three layered compounds has been studied at 4K between 0·1 and 3·0eV. The three compounds, TaS_1·6Se_0·4, TaS_1·6Se_0·4 (pyridine)_1/2, and TaS_1·6Se_0·4 (aniline)_3/4 have layer separations of 6·14, 12·09 and 18·07 A, respectively. Well-defined, free-carrier absorption edges exist near 1.0 eV in all three materials, from which values of the plasma frequency, the interband dielectric constant, and the optical scattering time were derived using the Drude theory. As the layer spacing increases both the plasma frequency and the dielectric constant decrease. The scattering time is short, 3 × 10^?15 sec, and almost independent of layer spacing. The calculated d.c. resistivity is consistent with the measured value in two of the three compounds.The variations of plasma frequency and dielectric constant with layer spacing are successfully interpreted in terms of a simple model which assumes the organic material acts as a dielectric separating metallic layers unchanged by its presence. Although a number of possibilities are put forward, no complete explanation is given for the short relaxation time in these compounds.     

Optical absorption studies of copper phthalocyanine thin films

The optical absorption of thermally evaporated copper phthalocyanine (CuPc) was studied in the UV-VIS-NIR region. The absorption spectra recorded in the UV-VIS region show two well-defined absorption bands of the phthalocyanine molecules, namely, the Soret (B) and the Q-band . The Q-band shows its characteristic splitting (Davydov splitting) of the main absorption peak in the metal phthalocyanine correlates with the relative tendencies of the metal to out-of-plane bonding. Some of the important spectral characteristics such as the molar extinction coefficient (ε_molar), the oscillator strength (f), the electric dipole strength (q²) and absorption half-bandwidth (Δλ) of the principle optical transitions were evaluated. The analysis of the spectral behavior of the absorption coefficient α in the absorption region revealed two indirect allowed transitions with corresponding energies 2.95±0.03 and 1.55±0.02 eV.The spectra of the infrared absorption allow characterization of vibration modes for the powder, as-deposited material and thin films of CuPc annealed at 423 K for two hours. Discussion of the obtained results and their comparison with the previous published data are also given.