The adsorption of water on the (110) surface of rutile is investigated by recording Thermal Desorption spectra, and characterizing the substrate using Low Energy Electron Diffraction. The sample is sputtered and annealed according to a previous flattening study of the same surface. After cooling to T $\approx$ 110K, the surface is dosed with H$\sb2$O. The adsorbate/substrate system is heated at 1K/s, and the H$\sb2$O partial pressure is measured with a shrouded quadrupole mass spectrometer. Three desorption peaks are observed below room temperature, two monolayer peaks: one at 310-260K (coverage dependence) and another at 175K, and one multilayer peak at 150K. A fourth peak, representing a coverage of the order of a few percent of a monolayer, is observed at T $\approx$ 500K; this peak is associated with an adsorption state that is more strongly bound than the other three. Analysis of the first three adsorption states provides information on the kinetics of the system, and the bond energies at these adsorption sites.

The (110) surface of TiO$\sb2$ is examined using a 9.92 keV oxygen ion beam. The positions of the peaks in the emitted ion energy spectrum are consistent with those predicted by ion-surface scattering theory for O$\sp{+}$-O and O$\sp{+}$-Ti collisions. Upon examination of the peaks present in the ion energy spectrum, information about the surface crystallographic structure can be determined. This includes studying the shadowing/blocking effects as the azimuthal and elevation angles are varied. Using this information the dimensions of the surface unit cell are found to be consistent with the bulk dimensions. The height of the oxygen rows that are present above the surface can also be determined and is found to be lower than predicted by the bulk.

A study of the temperature dependence of the resistivity of gated SiGe quantum well structures has revealed a metal-insulator transition as a function of carrier density at zero magnetic field. Although early scaling theories (Abrahams et al., 1979) have argued against the existence of a metal-insulator transition at zero temperature in infinite 2D and 1D systems, more recent theoretical results using a random set of two-dimensional point potentials have shown that such a transition is allowed in two dimensions (Az'bel, 1992). Mounting experimental evidence for such a transition in 2D systems with short range scattering has accumulated in both semiconducting and superconducting structures (Kravchenko et al., 1995, and others). Pseudomorphic, CVD-grown p-type Si/Si$\sb{0.87}$Ge$\sb{0.13}$/Si quantum wells of various widths (65-200 A) have been studied. The samples were gated using a Ti-Au Schottky gate to allow for carrier density variation. Measurement of the transport to quantum lifetime ratio indicates that the transport is dominated by short range scattering. In the temperature range from 400 mK - 4.2 K, the temperature dependence shows a transition from a metallic phase in the high density regime to an insulating phase in the low density regime with a transition boundary close to 2.2 $\times$ 10$\sp $ cm$\sp{-2}$. The scaling properties of the observed metal-insulator transition will be discussed, and compared to previous scaling results from silicon MOSFETs. Below 400 mK, the onset of another transition is accompanied by a sharp drop in resistivity with temperature followed by a monotonic decrease in resistivity below 115 mK. The phase diagram was explored using temperature and density dependences of the current-voltage characteristics.

The optical properties of the chromium doped spinel type semiconductor $\alpha$-$\rm ZnAl\sb2S\sb4$ have been examined over the temperature range 2-540K. From extrinsic absorption and photoluminescence spectra the transitions between the ground state $\rm\sp4A\sb{2g}$ and the excited levels $\rm\sp2E\sb{g}\ (R\sb1$ and R$\rm\sb2),\ \sp2T\sb{2g},\ \sp4T\sb{2g}$ and $\rm\sp4T\sb{1g}$ of the Cr$\sp{3+}$ ions were observed. A fit of the measured temperature dependence of the PL decay, using a four level model, yielded the lifetimes of the $\rm\sp2E\sb{g}\ (R\sb1$ and R$\sb2)$ and $\rm\sp4T\sb{2g}$ levels and the energy gaps between the $\rm\sp2E\sb{g}\sp{(1)}$-$\rm\sp2E\sb{g}\sp{(2)}$ and $\rm\sp2E\sb{g}\sp{(1)}$-$\rm\sp4T\sb{2g}$ states which were found to be very close to the values obtained from steady-state measurements. A configurational coordinate diagram for Cr$\sp{3+}$ ions in a $\alpha$-$\rm ZnAl\sb2S\sb4$:Cr spinel host has been constructed. Optical gain measurements at 198 K, 298 K and 380 K displayed two separate spectral regions associated to the Cr$\sp{3+}$ ions and possibly to Cr$\sp{4+}$ impurity centers. Gain values of up to 25 cm$\sp{-1}$ (730 nm) at 198 K were obtained. Finally the intrinsic photoluminescence measurements revealed an intense UV peak around 380 nm from 20 K to 296 K which corresponds to the indirect band gap of the host crystal, $\alpha$-$\rm ZnAl\sb2S\sb4.$

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