ALBERT

Notes: A model is presented for the electrostatic component of solvatochromic shifts in vertical electronic excitation energies. The model, which makes use of the mean-field approximation, combines quantum mechanics (QM) in the description of the solute molecule and molecular mechanics (MM) in the description of the solvent. The method is implemented at the multiconfigurational self-consistent field level. We present illustrative applications to the (n→π*)1 transitions of formaldehyde, acetaldehyde, and acetone in liquid water. The solvent shifts obtained compare well with other ab initio QM/MM calculations and when the electron correlation components are included with the experimental solvent shift, but differ from the results obtained with semiempirical QM/MM and continuum models. © 2000 American Institute of Physics.

Notes: In this paper, we elaborate on our previous communication of high coverages of oxygen on Rh(111) [J. Chem. Phys. 110, 2757 (1999)]. When dosing with O2, half of a monolayer of O is adsorbed. Higher coverages can be achieved when exposing the surface to O atoms. As the quantity of adsorbed O increases from a half to a full monolayer, the overlayer structure undergoes several distinct phase changes. At a full monolayer, the (1×1)-O structure is stable at surface temperatures less than ∼400 K. Continued dosing with O atoms results in the rapid migration of O into the bulk. We also report on the chemical reactivity of this densely oxygen-covered surface with CO, H2, and propene. © 2000 American Institute of Physics.

Notes: In previous works, the size-consistent self-consistent matrix dressing method has been applied to single reference as well as to complete active space singles and doubles configuration interaction. The wave function of the dressed state was assumed to be dominated by a closed shell determinant. Here, the method has been extended to systems where the dressed state is a single-configurational doublet. The method allows for the accurate calculation of energies and wave functions of other electronic states of the same system, having or not the same symmetry, as well as in the case of closed shell systems. This statement has been thoroughly assessed by the calculation of vertical ionization energies corresponding to a few low lying states of HF+ that are compared to full CI results obtained with the cc-pVDZ basis set. The method has been applied, using larger basis sets, to the calculation of vertical ionization potentials (VIP) of HF, H2CO, N2, and NH3. The results are compared to experimental VIP values. The effects of selecting different CAS spaces and using pseudonatural adapted MO's obtained from the CASSCF matrix densities are shown and discussed. Mean absolute error (MAE) for the calculated states is about 0.07 eV. The difficulties to improve this precision limit are shown. However, the mean signed absolute errors, that measure accuracy, can be made smaller. Small MAE can be obtained using simultaneously VTZ basis sets and intermediate levels of calculation. The possibility of this effect to be assigned to a fortuitous cancellation of errors is pointed out. © 2000 American Institute of Physics.

Notes: The photoluminescence characteristics (peak intensity, peak wavelength and full width at half maximum of the emission band) of p-type porous silicon have been measured as a function of the etching time. Experimental data are discussed in the light of a pore nucleation and growth mechanism recently proposed by the authors. The steps of the formation of the porous layer are clearly apparent in the photoluminescence characteristic evolution. © 2002 American Institute of Physics.

Notes: In this article we present a theoretical and experimental study of the multiple scattering of a plane thermal wave between a two-layer subsurface cylinder and the material surface. The thermal wave is generated at the surface of an opaque material by a modulated extended laser beam. First we study the multiple scattering of the thermal wave produced by a single conducting cylinder with arbitrary thermal properties. Then, we present a model of the multiple scattering of the wave produced by the two-layer cylinder and the surface. We calculate the surface temperature for two extreme cases of layering: (i) a coating air layer and (ii) a highly conducting coating. Measurements performed with an infrared radiometry photothermal technique on a calibrated set of samples are presented. They validate our models for the homogeneous layered cylindrical conductors. It is demonstrated that the most direct application of this study is the quantitative analysis of the thermal contact resistance (or the characteristics of a coating layer) in a fiber reinforced composite. © 2001 American Institute of Physics.

Notes: Polarized photoluminescence as well as birefringence measurements have been performed in different films of side-chain liquid crystal polyacrylates prepared by in situ photopolymerization. The polymeric systems contain a cyanoterphenyl chromophore covalently bound as a side group. The stability of the luminescence has been studied as a function of the composition and crosslinking of the polymer films. Although both the thermal quenching and the optical bleaching of the luminescence depend on the composition of the polymer matrix they seem to be independent of the crosslinking degree. Polymer films showing a stable luminescence with a high dichroic ratio have been obtained in copolymers combining mono and diacrylates having the same mesogenic core, with a low content (10%) of the diacrylate (crosslinking agent). © 2000 American Institute of Physics.

Notes: We present a theoretical model and experimental measurements of the temperature field produced by the multiple scattering of thermal waves between two close subsurface cylinders and the material surface. The thermal waves are excited at the surface of an opaque material by an extended modulated optical beam. The cylinders are taken as thermal insulating holes and the model is run for a variety of modulation frequencies, thermal properties of the sample, and geometric properties of the cylinders (sizes, depths, mutual separation). The photothermal signal obtained using an infrared radiometry technique on calibrated samples validate our model predictions. The model is used to address the question of the spatial resolving power in photothermal wave experiments and introduce a novel resolution criterion. © 2000 American Institute of Physics.

Notes: Over the last decade, x-ray magnetic circular dichroism (XMCD) and x-ray resonant magnetic scattering (XRMS) have become important experimental techniques in the study of magnetic materials. In order to reduce systematic errors in these measurements, typical XMCD and XRMS experiments are carried out using magnetization switching, although there are many situations where polarization switching is clearly desirable. Recently, fast polarization modulation techniques have been developed using either polarization conversion optical elements, such as phase plate, or special insertion devices. At the National Synchrotron Light Source (NSLS), an elliptically polarized wiggler (EPW), jointly developed by NSLS, Advanced Photon Source, and BINP of Novosibirsk, for this purpose. The EPW consists of a permanent magnet vertical wiggler and an electromagnet horizontal wiggler. The polarization of the device can be switched up to 100 Hz by switching the electromagnet. To take advantage of the fast switching capability, a phase sensitive detection system was also implemented. The sensitivity of using fast polarization modulation is demonstrated by measuring the element specific hysteresis loop and MCD spectrum of Cu induced moment at the interface of a Co/Cu multilayer. By comparing with results obtained using conventional measurements from similar samples, it clearly shows the advantages of using polarization modulation for small MCD effects. The sensitivity of this technique and the possibility of performing magnetic field dependent measurements of using polarization modulation have been applied to a number of magnetic systems. First, the spatial distribution of Cr induced moment in an ideal exchange-biased Fe/Cr multilayer was measured using soft-x-ray XRMS. Specular reflectivity was measured as a function of both angle and energy near Cr and Fe L3 edges. The Cr induced moment was clearly observed. Moreover, the spatial distribution of the induced Cr moment was determined by combining the measurements with detailed simulation of the energy and angle dependent specular reflectivity. Second, the magnetic behavior and spatial distribution of the interfacial Ni spins in an exchange-biased Co/NiO bilayer were studied using angle and energy dependent specular reflectivity as well as total electron yield. These results will be discussed in relation to several existing models of interface spin structures for exchange bias. Other applications of polarization modulation will also be discussed. © 2002 American Institute of Physics.