The first PLEED measurements of an ordered two component system (Cu(,3)Au(001)) are reported. Using this data an attempt is made to determine the long range order parameter as a function of the crystal temperature. A polarized electron source with an electron spin polarization of 27% and delivering a current of 0.3(mu)A to the crystal surface is described in the first section. This source uses photoemission from negative electron affinity GaAs(001) to produce an electron beam whose polarization can be easily modulated without changing other characteristics of the beam by changing the helicity of the photoemission light. Once operational, the source is used in an attempt to determine the long range order parameter of the (001) face of Cu(,3)Au. Polarization vs. Energy (P-V) curves were taken at room temperature at a number of angles ((theta)) to obtain usable P-V curves for theoretical modeling. Next, P-V curves were measured for the 00 beam at 13 degrees and the 01 beam at normal incidence as a function of temperature. As the crystal was heated through the order-disorder tranition temperature no large change in the P-V curves was noted.

A new method, described herein, has been developed for probing magnetic and other spin-dependent surface properties. The technique utilizes a He 2 ('3)S(,1) beam in which a plurality of atoms is in one m(,J) level. This electron spin polarized beam has been used to probe the surface magnetic properties of a clean and adsorbate-covered Ni (110) crystal. The electrons ejected from the sample when the metastables deexcite depend upon metastable polarization and sample magnetization. By examining the ejected electrons, the sample magnetization has been measured as a function of electron energy, crystal temperature, and adsorbate (CO) coverage. The results demonstrate that He 2 ('3)S(,1) metastables constitute a method for probing surface magnetism with unparalleled surface specificity. In addition to discussing the physics of the metastable/surface interaction and the results obtained, the theoretical background and a perspective of surface spectroscopies are provided in sections on magnetism and other surface spectroscopies.

Muon spin relaxation ((mu)SR) has been used to study the magnetic ordering in PdMn and AgMn dilute alloys representing disordered ferromagnetism, reentrant ferromagnetism, and spin glass order. A theory of static linewidths for both transverse applied field and zero applied field (mu)SR is described for all types of spin order. The temperature dependence of the static linewidth in the spin glass and ferromagnetic phases is consistent with both a mean field theory of disordered magnetism and an inhomogeneous percolation model. Linewidths in paramagnetic AgMn privde the first direct observation of a conduction electron mediated RKKY component to the muon-local moment interaction. In PdMn, muons are excluded from occupying octahedral interstitial sites directly adjacent to Mn impurities. A theory relating muon spin-lattice relaxation to impurity spin correlation times in the paramagnetic and ordered phases is described. All types of magnetic ordering show a rapid slowing down of impurity spin fluctuations just above the ordering temperature. Muon spin-lattice relaxation in both the ferromagnetic and spin glass state is expected to be insensitive to both long wavelength spin waves and to higher frequency localized spin wave-like excitations. The observed spin glass state relaxation is consistent with a phenomenological theory of low-frequency barrier mode excitations.

The microscopic structure and dynamics of cytoplasmic water in the cells of organs and tissues are not well-understood. Much work has been done using various experimental techniques to study the properties of water in living systems, yet there is no generally accepted model describing the interaction of water with cellular constituents. Quasi-elastic neutron scattering (QNS) is a technique capable of a spatial resolution of 1-10 (ANGSTROM) and a frequency resolution of 10('9) to 10('13) sec('-1) which is suitable for the study of the diffusive motion of water in biological systems. A monochromatic beam of 0.95 THz neutrons was used to obtain QNS spectra within an energy window of (+OR-)0.2 THz for momentum transfer values in the ranges of 0.5 (ANGSTROM)('-1) to 1.9 (ANGSTROM)('-1). We have obtained QNS spectra for water in sartorius and gracilis major muscles of green leopard frogs (Rana pipiens pipiens) at 30(DEGREES)C and comparison spectra for a .15 molar solution of KCl at 3(DEGREES)C. The spectra were interpreted with a jump-diffusion model for translational water motion in both systems and a bound-free model for water in the muscle. The measured diffusion parameters of these two systems indicate that the water motion is more restricted in the frog muscle than in the aqueous KCl solution. We estimate the bound fraction of water in muscle to be 15.0 (+OR-) 4.1%. Our results for the bound water fraction in muscle and diffusion coefficients and correlation times of water in muscle and in a .15 m KCl solution agree well with the QNS and NMR results of others.

Metastable Atom De-excitation Spectroscopy (MDS) provides a powerful technique with which to investigate surface electronic structure with unparalleled surface specificity. In this technique a thermal energy beam of noble-gas metastable atoms is directed at the surface under study and the kinetic energy distribution of ejected electrons that result from metastable atom de-excitation is measured. Although the measured distribution contains information about the electronic structure of the outermost surface layer, its detailed analysis requires knowledge of the dynamics of the metastable atom-surface interaction. In the present work, these dynamics have been investigated directly, for the first time, by use of spin-labeling techniques. The electron spins on the incident metastable atoms are polarized and the spin-polarization of the ejected electrons is measured with a Mott polarimeter. Results are reported for Cu(100) and Ni(111) surfaces under a variety of surface conditions. The data indicate that metastable atom-surface interactions may be more complex than is generally assumed and thus suggest that assumptions inherent in earlier analyses of MDS spectra merit additional investigation. The present work further demonstrates the power of spin-resolved measurements in the study of particle-surface interactions and in surface spectroscopies in general.

There is a possibility that an isomer can be found which could be used to obtain a sufficient population inversion density to bring about nuclear lasing. If it can be realized, a gamma ray laser ("graser") will be a new source of radiation in the $\sim$ 1A region of unexcelled spectral brightness and coherence, and can be expected to have revolutionary consequences for physics, biology, and chemistry. The result of our research is that the effects of crystallinity are of crucial importance in considerations regarding gamma-ray lasers, bringing in new elements to laser theory which could very well prove decisive in determining whether grasing can or cannot be achieved. Certain multi-beam Borrmann eigenmodes of the radiation field in a perfect crystal have much stronger coupling to the emitting nuclei, and much less photoabsorption, than modes in the amorphous case. These modes may be selectively fed by nuclear transitions of the proper multipolarity. A crystal containing such emitters would require orders of magnitude less population inversion density in order to lase than would an amorphous sample of similar material. Multi-beam Borrmann modes also have several applications to single-photon optics, particularly the problem of anomalous emission of a $\gamma$-ray by an internal source of multipolarity M1 or higher. While this effect can occur for a 2-beam Borrmann mode, even stronger anomalous emission is possible into multi-beam Borrmann modes. In addition, certain multi-beam Borrmann modes can anomalously transmit resonant radiation through a Mossbauer crystal. (This cannot occur for the 2-beam modes, except for the case of an E1 transition, since the Borrmann mode couples strongly to the nuclei and thus will be strongly absorbed at resonance.)

Presented here are experimental/theoretical studies of the geometric structure of a Cu(001) surface and the near-surface layer magnetization of a ferromagnetic Ni(111) surface, both using spin-polarized low energy electron diffraction (SPLEED). This probe shows great promise both as a high quality structural probe, and also in a variety of magnetic studies. Significant advances have been made over the past several years in both experimental and calculational LEED techniques. The introduction r-factor analysis and streamlining of calculational codes now allows the evaluation of increasingly complex structures by LEED I-V analysis. In addition, experimental advances now allow the investigation of not only the intensities, but also the spin-dependence of scattering from many surfaces. Spin dependence in low-energy electron diffraction is found for scattering from all surfaces, and is due to two effects--the spin-orbit interaction and the exchange interaction. The former is sensitive to the positioning of ions that make up the surface and provides complementary probe of surface geometric structure in addition to LEED intensity analysis. Here we introduce a simple r-factor analysis to evaluate the structure of Cu(001). This surface was chosen because it is well studied, and a highly precise structural determination of the surface based on LEED I-V analysis is available for comparison. Experimental SPLEED A-V profiles are compared with calculated profiles to obtain the best near-surface structure. In this study, the comparison between calculated and experimental spin-asymmetry profiles obtained from this surface is unusually good, and further, the best fit is for a structure that agrees well with the previous LEED study. The present study is the first use of r-factor in a SPLEED structural determinations. Several things are readily noted from the present SPLEED r-factor analysis. First the precision of structural conclusions obtainable r-factors is much greater than possible using the visual inspection method found elsewhere. Further, because A-V's are self-normalizing, the r-factors are cleaner, requiring no arbitrary scaling factor. Also the spin asymmetries show more sensitivity to structural variations than do the comparable LEED intensities. The exchange interaction is sensitive to the electron spin-population density in the surface, an thus to surface magnetism. Our preliminary study of the ferromagnetic Ni(111) surface shows very small exchange asymmetry effects. The results are consistent with some enhancement of the sample magnetization at the surface, but a precise layer magnetization determination is beyond the limits of present technique.

Cold semiconductor clusters of Si, Ge, and GaAs have been produced by laser vaporization followed by supersonic expansion in a helium carrier. The neutral clusters were characterized by F$\sb 2$ (7.89 eV) and ArF (6.4 eV) excimer laser ionization accompanied by time-of-flight mass analysis. A semiconductor cluster ion apparatus has been developed for characterizing ionic cluster beams and mass selecting a particular ion for subsequent studies with various photolysis methods such as photodetachment, photofragmentation, and photoelectron spectroscopy. Ga$\sb{\rm x}$As$\sb{\rm y}$ clusters with x + y $>$ 10 thus generated have compositions ranging from Ga$\sb{\rm x+y}$ through Ga$\sb{\rm x}$As$\sb{\rm y}$ to As$\sb{\rm x+y}$ which closely follow a statistical binomial distribution. In the smaller clusters, strong variations were observed from this binomial distribution as a result of kinetic effects in the cluster formation process. An even/odd alternation in the ionization potential (IP) and the electron affinity (EA) of GaAs clusters was observed, which only depended upon the total number of atoms in cluster. GaAs clusters with an even total number of atoms had higher IP but lower EA than their neighboring odd ones. This suggests that the even clusters have fully paired singlet ground states with no dangling bonds. Electron affinities of Si, Ge, and GaAs clusters have been roughly determined as a function of cluster size by detecting the photoelectron intensity dependence upon the probing laser fluence. In general, the EA of cluster increases monotonically with increasing cluster size extrapolating towards the EA of the corresponding bulk semiconductor material. The fragmentation patterns of Si and Ge negative ion clusters observed were remarkably similar, implying that they have almost the same structures. In contrast with GaAs and metal clusters, Si and Ge anions fragmented by fission into mainly 5-10 atom size range, and the six and ten atom anions were the most favorite daughters. Photofragmentation processes were found to be competitive with photodetachment. For Si and Ge, one-photon detachment and one-photon fragmentation occurred simultaneously in certain laser wavelength ranges. Detachment became dominant at high photon energies.

The surface reactivity of germanium is of interest because of novel Si$\sb{x}$Ge$\sb{1-x}$ heterostructure applications and the insight into semiconductor surface chemistry attainable through comparative studies on silicon and germanium. The adsorption of the inorganic hydrides H$\sb2$S, H$\sb2$O, NH$\sb3$, and HX (X = Cl, Br) on Ge(100) was investigated by temperature-programmed desorption (TPD) for the first time. The initial sticking probability $S\sb0$ for H$\sb2$S exhibits at most only a minor temperature effect, remaining roughly constant at $\sim$0.23 within the temperature range 173 K $\le$ T $\le$ 373 K. Adsorbed H$\sb2$S decomposes upon heating into H$\sb2$ and GeS, which desorb at 570 K and 660 K, respectively. The initial sticking probability for H$\sb2$O depends strongly upon substrate temperature, dropping from 0.28 at 173 K to $\sim$0.02 at 273 K. Adsorbed water decomposes to yield H$\sb2$ and GeO, which desorb at peak temperatures of 570 K and 660 K, respectively. Both HCl and HBr adsorb on Ge(100), and desorb upon heating to 570 K as H$\sb2$ and to 580 K as HX in roughly equal proportions. The remaining X(a) desorbs as GeX$\sb2$ at 680 $\sim$ 760 K. The initial sticking probability for HBr is 0.77 at 273 K, falling slightly to 0.55 at 373 K, whereas S$\sb0$ for HCl falls from 0.30 at 273 K to 0.066 at 373 K. The peak shapes and independence of peak temperature with coverage for H$\sb2$ and HX for all adsorbates indicate approximately first-order desorption kinetics, but GeX$\sb2$ desorption follows second-order desorption kinetics. Adsorption of NH$\sb3$ was not observed, implying $S\sb0$ $\le$ 6 $\times$ 10$\sp{-5}$. The adsorption and desorption behavior of these molecules can be understood by regarding the Ge(100) dimer atoms as being linked by a strained double bond and examining analogous addition and elimination reactions of molecular germanium compounds.

The effect of spin fluctuations on the resistivities of 14 different $Fe sb{x}Ni sb{1-x}Zr sb2$ metallic glasses were analyzed in the temperature range from 4.2K to 80K. The corrections to the resistivity due to the superconductivity and quantum interference effects at very low temperatures were performed to obtain the spin fluctuation resistivity, $ rho sb{sf}$, as a function of temperature and composition. It is found that, $ rho sb{sf}$ varies as $T sp2$ at very low temperatures, i.e. below around 20K, and as T at higher temperatures. This confirms the predictions of the two-band model of Kaiser and Doniach and the one-band model of Rivier and Zlatic. The spin fluctuation temperatures for all the compositions, were determined from both models. $T sb{sf}$ calculated from the one-band model were about 30% higher than those calculated from the two-band model. The spin fluctuation temperature is lowest when the system is closest to the magnetic transition and it increases when the amount of magnetic species is reduced in the alloy, i.e. for x = 1, $T sb{sf}$ = 10K (15K) and for x = 0.4, $T sb{sf}$ = 64K (96K) for the two-band (one-band) model.