W. S. Tse

OPTICAL PROPERTIES OF COMPOSITE MATERIALS AT HIGH TEMPERATURES

The optical properties of composite materials are studied theoretically as a function of temperature via a phenomenological model for temperatures up to the melting points of the materials. Both the Maxwell-Garnett and Bruggeman models are considered and the temperature variation of the optical constants of the metallic particles is obtained with an account of the dependence of both the electron-phonon and electron-electron scattering on temperature. The results show that the extinction coefficient of the composite generally increases with temperature and that the Maxwell-Garnett and Bruggeman models can give very different results at certain optical frequency. Transmittance through a thin composite film is calculated providing a means for a simple experimental study of the various modeling results.

The surface plasmon enhancement effect on adsorbed molecules at elevated temperatures

The surface plasmon enhancement effect on adsorbed molecules at elevated substrate temperatures is studied theoretically using surface enhanced Raman scattering (SERS) as an example. The surface structure is idealized to be a monodisperse spherical particle with its nonlocal dielectric response accounted for. The temperature effects are modeled using a temperature-dependent collision frequency in the Drude model. Numerical results show that only a small decrease in the SERS enhancement ratio occurs for temperatures up to the melting point of the substrate, even for scattering close to the surface plasmon resonance frequency of the metal. More definitive results are subjected to more realistic modeling as well as systematic experimental studies. The implication of this result to other surface photochemical processes is discussed.

Optical temperature sensing based on the Goos-Hänchen effect

The possibility of constructing an optical sensor for temperature monitoring based on the Goos-Hänchen (GH) effect is explored using a theoretical model. This model considers the lateral shift of the incident beam upon reflection from a metal-dielectric interface, with the shift becoming a function of temperature due mainly to the temperature dependence of the optical properties of the metal. It is found that such a sensor can be most effective by using long wavelength p-polarized incident light at almost grazing incidence onto the metal, where significant variation of negative GH shifts can be observed as a function of the temperature.

Nonlocal electrodynamic effect on the enhancement factor for surface enhanced Raman scattering

Abstract The electromagnetic enhancement factor for surface enhanced Raman scattering is reexamined taking into account the nonlocal dielectric response of the substrate surface. Surface roughness is represented typically by a spherical island and nonlocal effect is introduced via a simple phenomenological model which makes possible a closer quantitative comparison with experiments. It is found that the nonlocal effect can lead to significaltly lower values for the enhancement factor compared to the results calculated in the literature ignoring this effect. A sample calculation shows that, for scattering frequencies much below the surface plasmon frequency of the substrate, the nonlocal correction decreases the enhancement effect by up to 10%; and for frequencies close to the surface plasmon frequency, the difference could be by orders of magnitude. Possible experimental observation of this effect is discussed.

FT-Raman, FT-IR and normal-mode analysis of carcinogenic polycyclic aromatic hydrocarbons. Part I—a density functional theory study of benzo(a)pyrene (BaP) and benzo(e)pyrene (BeP)

In the mechanistic study of the oxygenation of polycyclic aromatic hydrocarbons (PAHs) by mixed-function oxidase, we employed hemoglobin (Hb) as a model compound for cytochrome P450. We found that the relative intensities of several Raman peaks of benzo(a)pyrene (BaP) and benzo(e)pyrene (BeP) in the region between 600 and 1600 cm−1 are significantly enhanced by the oxygenated Hb but not by the deoxygenated Hb. This seems to indicate that these affected vibrations could be the modes involved in PAH epoxidation by the heme-bound oxygen. Density functional theoretical calculations were carried out using a commercially available software package. The DN** basis set appears to give the most satisfactory results. The difference in wave numbers between observed and calculated wave numbers is estimated to be <20 cm−1. Peak assignment through animation shows that all these heme-bound oxygen-affected modes are in-plane vibrations. This seems to indicate that BaP molecules, in forming the activated complexes, are more likely vibrating in-plane than out-of-plane. In other words, BaP molecules may diffuse out of the heme pocket without interaction if their vibrations do not enhance orbital overlapping with the heme-bound oxygen molecules. Copyright

Low temperature phase transition of Li0.12Na0.88NbO3 studied by Raman scattering

In this article, we will report the low temperature Raman studies of Li0.12Na0.88NbO3 ceramics at the ferroelectric–ferroelectric phase transitions. The spectral pattern indicates the phase transition occurs at 200 K in the cooling process and at 280 K on heating. The observed spectra are discussed as they relate to the internal stretching modes of the NbO6 octahedron and cation translation modes. The temperature-dependent spectra show that the phase transitions may involve the rearrangement of Na+ cation, the relative orientation of the NbO6 octahedral, and the displacement of Nb ions.

Temperature‐dependent Raman‐scattering studies of Li0.02Na0.98NbO3

Raman studies of Li0.02Na0.98NbO3 ceramics were observed in the temperature range between 10 and 300 K. The spectral pattern evolves gradually at a transition temperature (130–170 K) in the cooling process and then changes suddenly near 280 K on heating. The observed thermal hysteresis of 110 K and also the transition temperatures agree well with evidence coming from the differential scanning calorimetric, the dielectric constant, and the thermal expansion measurements. The observed spectra are discussed following the separation of modes into internal stretching modes of the NbO6 octahedron and cation translation modes. The 62‐ and 74‐cm−1 peaks are proposed due to the translational modes of Na+ cation.

The ultrafast dynamics of liquid CBrCl3 studied with optical Kerr effect and Raman scattering

The ultrafast inter- and intramolecular dynamics of liquid CBrCl3 are studied with a femtosecond laser in a pump/probe optical Kerr experiment. The prompt part of the observed Kerr signal is found to comprise not just electronic response but also coherent coupling. As reported in the literature, the weak and delayed part of the signal is attributed to three kinds of intermolecular dynamics: diffusive reorientation, polarizability distortion, and molecular libration with decay times, respectively, of 2 ps, 500 fs, and 190 fs. Included in the delayed Kerr signal are long lasting oscillations which are ascribed to quantum interference involving four normal vibrations of the molecule. By a careful fitting of both the time domain (Kerr) and frequency domain (Raman) data with existing theories the four vibrations are identified as ν2 = 422 cm−1, ν3 = 246 cm−1, ν5 = 295 cm−1, and ν6 = 191 cm−1, with dephasing times, 1.1 ps, 1.7 ps, 1 ps, and 1 ps, respectively.

FT-Raman, FT-IR and normal-mode analysis of carcinogenic polycyclic aromatic hydrocarbons. Part II—a theoretical study of the transition states of oxygenation of benzo(a)pyrene (BaP)

The first process in polycyclic aromatic hydrocarbons (PAH) metabolism is believed to be the oxygenation that leads to several structural isomeric epoxides. Most of these intermediates will be converted into non-toxic hydrophilic metabolites and be excreted. Because of this, metabolism has been considered to be primarily a detoxification mechanism. However, some may be activated to become ultimate carcinogens. Knowledge of the identities of these intermediates and their probabilities of formation will be of vital importance in mechanism elucidation and cancer prevention. Theoretically, epoxidation of benzo(a)pyrene (BaP) can take place at any one of the sites: 1,2-, 2,3-, 4,5-, 7,8-, 8,9-, 9,10- and 11,12-, leading to seven possible isomeric epoxides. The conformations of these isomers formed from direct oxygenation were simulated and their activation energies calculated. Among them, the 4,5-epoxide appears to be the most and the 8,9-complex the least energetically favorable. The 11,12-, 9,10- and 7,8-isomers are almost equally energetic. The rate of formation of the 7,8-isomer, which is believed to be the precursory carcinogen, relative to the 4,5-epoxide is estimated to be 9.1 × 10−4 : 1. On the other hand, the relative rates of 1,2- and 2,3-complexes are extremely low. When the protoporphyrin group is used for simulation, the BaP plane is seen parallel to the porphyrin plane in the 1,2- and 2,3-complexes, but angled away to various degrees in all other epoxides. The parallel orientation results in a significant lowering of the activation energy, making 1,2- and 2,3-complexes the most favorable intermediates. This is in agreement with chromatographic analyses of BaP metabolism in liver microsomes. In these experiments, 3-hydroxy-BaP is the major product. Copyright

Specific heat of magnetic fluids under a modulated magnetic field

Due to the increasing importance of thermal devices in engineering applications, the investigation of the thermal properties of magnetic fluids has become necessary. In this work, we report both the temperature and field dependence of the specific heat of a water-based magnetic fluid. The results show that the specific heat of the magnetic fluid is almost independent of temperature under lower-field strengths (<50 Oe), but decreases significantly upon increasing temperature under higher fields. To clarify this effect of the magnetic field on the specific heat capacity of the magnetic fluid, the evolution of the structure of the magnetic fluid film is examined and compared with its field-modulated heat capacity of the fluid.