Low energy Auger and loss electron spectra from magnesium and its oxide

Data have been obtained from Auger and energy loss processes in clean metallic Mg, Mg during stages of oxidation, and UHV cleaved MgO(100) surfaces. Particular attention has been paid to twenty features below 200 eV in the Auger spectra from these surfaces. A comparison of spectra from the metal, oxidised metal surface, and single crystal MgO has enabled estimates to be made of surface charging effects, and the MgO steady state surface potential is found to be near + 10 V above ground. All the Auger features are given assignments, two of which are interfacial processes involving ionic initial states and metallic final states. Several features in the low energy Auger spectrum are attributed to diffraction of true secondary electrons.     

Early stages in the oxidation of magnesium,aluminium and magnesium/aluminium alloys: I. Exoelectron emission and long wavelength photoemission

The initial stages in the oxidation of magnesium, aluminium and magnesium/aluminium alloys have been studied using a photoelectron spectrometer (XPS and exoelectron energy spectra) and the diode technique (surface potentials and volumetric adsorption of oxygen). This paper describes and characterises the exoelectron emission and the visible wavelength photoemission which occurs during oxidation. At low oxygen exposures (< 10 L), exoelectrons with a 4 eV energy spread are emitted in the dark from magnesium and magnesium/aluminium alloy. Aluminium shows no such emission. Another, dark pressure-dependent emission of exoelectrons occurs only on magnesium/aluminium alloy and aluminium at high oxygen exposures. The onset of this emission lies 3 eV higher and it again has a 4 eV spread. When the surface is illuminated, it is possible to distinguish photo/exoelectron emission, which occurs during oxygen uptake, and true photoemission which occurs in vacuo. Both these types of emission, which were only recorded on magnesium, show a resolved triplet. Photoelectric work functions as low as 0.3 eV were observed. A patchy surface evidently develops during oxidation. This process, which may correspond to the nucleation of oxide islands, is slower than the chemisorption of of oxygen. The nature of the low work function patches, which we associate with the electron emissions, is therefore very dependent on the oxygen ambient pressure. A tentative energy scheme is given for such “exopatches” which are neither metal nor oxide and contain a curious energy level that must be situated above the vacuum level of the patch, as well as above the metal Fermi level. The energy with which exoelectrons are emitted derives from the heat of adsorption.     

Effect of surface charge on laser-induced neutral atom desorption

When an ionic metal oxide crystal is cleaved, inhomogeneous electrical charging of the surface can be a result. Such an effect has been well documented in magnesium oxide (100). For example, recent rigorous AFM studies indicate that nanoscale charged clusters of MgO are created during cleavage, with high concentrations often located at terrace step edges (Barth and Henry in J. Phys. Chem. C 113:247, 2009). In addition, ablation processes of freshly cleaved magnesium oxide crystals may be effected by remnant surface charging and microstructures (Stoneham et al. in Appl. Phys. A 69:S81, 1999). We report here that such surface charging strongly impacts neutral atom desorption, even under conditions of extremely mild excitation of surface terrace features. In our experiments, single-crystal MgO (100) is cleaved in air and placed in an ultra-high vacuum chamber (UHV). We irradiate the crystal at 6.4 eV, photon energy resonant with five-coordinated (5-C) terrace sites and probe desorbing neutral oxygen atoms. It is found that a significant fraction of desorbed neutral oxygen atoms from the charged surface possess kinetic energies in excess of 0.7 eV. This is in contrast to uncharged samples (discharged in vacuo over 24 hours) that display a near-thermal oxygen atom distribution.     

Effect of oxygen on the hydrogenation properties of magnesium films

The effect of magnesium oxide on the magnesium and hydrogen desorption properties of magnesium films have been investigated. We find that by capping metallic magnesium films with oxide overlayers the apparent desorption energy of magnesium is increased from 146 kJ/mol to 314 kJ/mol. The results are discussed in light of previous investigations of ball-milled magnesium powders.     

Electronic relaxation processes in the KLL′ auger spectra of the free magnesium atom,solid magnesium and MgO

The K-Auger spectrum of the free magnesium atom and the magnesium metal is calculated. The Friedel model is used to account for extra-atomic relaxation effects. Theoretical Auger electron energies are compared with the corresponding experimental values. In general the agreement between theory and experiment is good.The evolution of the extra-atomic relaxation energy in going from the metal to the oxide is presented.     

Sapphire surface polariton splitting due to resonance with magnesium oxide film phonon

Surface polariton spectra of magnesium oxide films (of thickness ranging from 10 to 300 nm) prepared by chemical vapor deposition technique on sapphire substrates have been studied. The splitting of the dispersion curve of the sapphire surface polariton appears near 700 cm⁻¹ due to the resonance interaction of the sapphire substrate surface polariton with the film optical phonon. This splitting is proportional to the square root of the film thickness. Optical constants of ultrathin magnesium oxide films have been obtained from surface polariton spectra measurements. For the thinnest film (10 nm) they appear to be close to the bulk crystal values.     

Lattice statics calculations for a vacancy in magnesium

Using the Green function method of lattice statics, the lattice distortion in the neighborhood of a vacancy in magnesium has been calculated in three different axially-symmetric (AS) host lattice models and also in a model derived from an empirical interatomic pair potential. The variations in the lattice distortion and the relaxation energy of the vacancy are studied as the size of the defect space is allowed to vary from two to four surrounding neighbors. The perfect static lattice Green functions are computed up to 19 neighbors in the 4 models and the values obtained are shown to be not very sensitive to the model chosen. The lattice relaxation is found to be negligible in all models except in a four-neighbor AS model. The relaxation energies in these four models are computed to be 0.0027 eV, 0.034 eV, 0.28 eV and 0.0069 eV respectively. Results for the monovacancy formation energy, the elastic dipole tensor and the volume change of the crystal due to the vacancy are also presented.     

Passive Q-switching of erbium glass laser by magnesium aluminosilicate sitall with cobalt ions

We have studied the radiation output parameters for an erbium glass laser, lasing at a wavelength of 1.54 μm, with passive Q-switching by means of a cobalt-containing magnesium aluminosilicate sitall compared with a saturable absorber based on a magnesium aluminum spinel crystal with cobalt ions. We have shown that the output characteristics of the laser emission when using sitall are not inferior to the analogous characteristics of a laser based on a spinel crystal, and are practically independent of the temperature of the saturable absorber in the range 0°C–80°C. The duration (energy) of the output pulses was 70 nsec (∼4 mJ), the energy dispersion of the radiation pulse relative to the average value was no greater than 3%, the beam divergence was 2.8 mrad, the laser beam quality factor was M² = 1.2. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 126–131, January–February, 2007.     

Crystal growth, stability and photoluminescence studies of tetra aqua diglycine magnesium (II) hexa aqua magnesium (II) bis sulfate crystal

Single crystals of tetra aqua diglycine magnesium (II) hexa aqua magnesium (II) bis sulfate have been grown from saturated aqueous solution by slow evaporation solution growth technique. The solubility of the title compound in water at various temperatures has been determined. Single-crystal X-ray diffraction analyses reveal that the title compound crystallizes in triclinic system with space group P1?. Fourier transform infrared spectral analyses confirm the presence of functional groups in the grown crystal. The thermal stability of the grown crystal has been investigated by thermogravimetric and differential scanning calorimetric analysis. It indicates that the material is stable upto 100 °C. The crystalline perfection of the grown crystal has been evaluated by high-resolution X-ray diffraction technique. Vickers microhardness measurements indicate the mechanical strength of the grown crystal. Photoluminescence of the grown crystal has been investigated and it reveals that the crystal has blue-violet fluorescence emission.     

Effect of fillers on magnesium–poly(ethylene oxide) solid polymer electrolyte

A solid polymer electrolyte (SPE) film consisting of poly(ethylene oxide) (PEO) with magnesium chloride as electrolytic salt and B₂O₃ as the filler has been prepared by solution casting technique. The polymeric film was flexible and self-standing with proper mechanical strength and studied for application in a solid-state rechargeable magnesium battery. The interactions between the filler and PEO chains are studied by differential scanning calorimeter and Fourier transform infrared techniques. Composition of SPE is optimized, and maximum conductivity is obtained at 2 wt% B₂O₃. Filler seems to increase the number of free magnesium cations by decoordinating the bond between magnesium cations and ether oxygen of PEO. Cyclic voltammetry results show the reversible capability of magnesium electrode. Solid-state magnesium cell employing magnesium anode, SPE, and manganese oxide was assembled, and its open circuit voltage is found to be 1.9 V.     

Anomalous electron emission spectra and polarization phenomena in a lead magnesium niobate single crystal

The time dependences of the intensity and the energy positions of fine-structure of the spectra of anomalous electron emission from a polarized lead magnesium niobate (PMN) single crystal irradiated with soft x-rays are investigated experimentally. It is shown that the relaxation time of the electret charge can be determined from the graphs of these dependences, and the depth distribution of the potential in the surface layer of the sample can be determined from the profile of the anomalous electron emission spectrum. Estimates of the fields in the surface layers of a polarized PMN single crystal, obtained from the characteristics of the anomalous electron emission spectra, agree with data obtained by electrophysical research techniques. Fiz. Tverd. Tela (St. Petersburg) 39, 1446–1451 (August 1997)     

Near-threshold electron-impact ionization of magnesium

The effective electron-impact ionization cross section of the magnesium atom is studied in detail in the energy range from the ionization threshold to 16 eV. A large number of particular features caused both by atomic autoionization states and by the formation and decay of a short-lived state of the negative ion of the magnesium atom are revealed. These particular features are identified using experimental and theoretical data on the photoionization (photoabsorption) and on the spectra of ejected electrons.     

The influence of surface microchemistry in protective film formation on multi-phase magnesium alloys

The high strength:weight ratio of magnesium alloys makes them an ideal metal for automotive and aerospace applications where weight reduction is of significant concern. Unfortunately, magnesium alloys are highly susceptible to corrosion particularly in salt-spray conditions. This has limited their use in the automotive and aerospace industries, where exposure to harsh service conditions is unavoidable. The simplest way to avoid corrosion is to coat the magnesium-based substrate by a process such as electroless plating, which is a low-cost, non line of sight process.Magnesium is classified as a difficult to plate metal due to its high reactivity. This means that in the presence of air magnesium very quickly forms a passive oxide layer that must be removed prior to plating. Furthermore, high aluminium content alloys are especially difficult to plate due to the formation of intermetallic species at the grain boundaries, resulting in a non-uniform surface potential across the substrate and thereby further complicating the plating process.The objective of this study is to understand how the magnesium alloy microstructure influences the surface chemistry of the alloy during both pretreatment and immersion copper coating of the substrate.A combination of scanning electron microscopy, energy dispersive spectroscopy and scanning Auger microscopy has been used to study the surface chemistry at the various stages of the coating process. Our results indicate that the surface chemistry of the alloy is different on the aluminum rich β phase of the material compared to the magnesium matrix which leads to preferential deposition of the metal on the aluminum rich phase of the alloy.     

Trapping of positrons at vacancies in magnesium

Trapping of positrons at vacancy-type defects in magnesium was studied by positron lifetime and Doppler-broadening measurements. Vacancy defects were produced by quenching, electron irradiation and deformation at low temperatures as well as by thermal agitation at elevated temperatures. In the first three cases we observed trapping at multiple vacancies, which anneal out between 77...400 K. Thermal equilibrium measurements show S-shape behaviour originating from positron trapping at magnesium monovacancies. However, changes in the positron parameters were very small, which is due to the weakness of the positron-vacancy interaction. A detrapping analysis yielded a positron-vacancy binding energy of the order of 0.3...0.4 eV.     

Combustion of a single magnesium particle in water vapor

The combustion of magnesium particles in water vapor is of interest for underwater propulsion and hydrogen production. In this work, the combustion process of a single magnesium particle in water vapor is studied both experimentally and theoretically. Combustion experiments are conducted in a combustor filled with motionless water vapor. Condensation of gas-phase magnesia on the particle surface is confirmed and gas-phase combustion flame characteristics are observed. With the help of an optical filter and a neutral optical attenuator, flame structures are captured and determined. Flame temperature profiles are measured by an infrared thermometer. Combustion residue is a porous oxide shell of disordered magnesia crystal, which may impose a certain influence on the diffusivity of gas phases. A simplified one-dimensional, spherically symmetric, quasi-steady combustion model is then developed. In this model, the condensation of gas-phase magnesia on the particle surface and its influence on the combustion process are included, and the Stefan problem on the particle surface is also taken into consideration. With the combustion model, the parameters of flame temperature, flame diameter, and the burning time of the particle are solved analytically under the experimental conditions. A reasonable agreement between the experimental and modeling results is demonstrated, and several features to improve the model are identified.     

Enhancement of second harmonic generation efficiency: Growth and characterization of magnesium(II)-doped tetrakis(thiourea)nickel(II) chloride crystals

The influence of magnesium doping on the properties of tetrakis(thiourea)nickel(II) chloride crystals has been described. The reduction in the intensity observed in powder X-ray diffraction of doped specimen and slight shifts in vibrational frequencies confirm the lattice stress as a result of doping. Surface morphological changes due to doping of the alkaline earth metal are observed by scanning electron microscopy. The incorporation of Mg(II) into the crystal lattice was confirmed by energy dispersive X-ray spectroscopy. Lattice parameters are determined by single crystal XRD analysis. The thermogravimetric and differential thermal analysis studies reveal the purity of the materials and no decomposition is observed up to the melting point. The crystal is further characterized by Kurtz powder technique and dielectric studies.     

Novel fluorescent nuclear track detector technology for mixed neutron-gamma fields

The latest achievements in fluorescent nuclear track detector (FNTD) technology are described. FNTDs are aluminum oxide crystals containing aggregate oxygen vacancy defects and doped with carbon and magnesium (Al₂O₃:C,Mg). Unlike most nuclear track detectors, Al₂O₃:C,Mg is sensitive to low linear energy transfer (LET) radiation including secondary electrons resulting from interactions of photons with the crystal. A new image processing method is investigated as a technique to discriminate and measure the doses of gamma and fast neutrons in mixed field conditions. Dose dependencies for both gamma and neutron irradiated FNTDs are shown. The new image processing method increased the dynamic range of detectable neutron doses from 4 orders of magnitude for track counting method to at least 6 orders of magnitude by combining track counting with the new image processing method. The new image processing method is combined with a detector configuration utilizing three converters: Teflon^®, polyethylene, and lithium fluoride.     

The charging stability of different silica glasses studied by measuring the secondary electron emission yield

This paper reports that the charging properties of lead silica,Suprasil silica and Infrasil silica are investigated by measuring the secondary electron emission(SEE) yield.At a primary electron beam energy of 25 keV,the intrinsic SEE yields measured at very low injection dose are 0.54,0.29 and 0.35,respectively for lead silica,Suprasil and Infrasil silica glass.During the first e-beam irradiation at a high injection current density,the SEE yields of lead silica and Suprasil increase continuously and slowly from their initial values to a steady state.At the steady state,the SEE yields of lead silica and Suprasil are 0.94 and 0.93,respectively.In Infrasil,several charging and discharging processes are observed during the experiment.This shows that Infrasil does not reach its steady state.Two hours later,all samples are irradiated again in the same place as the first irradiation at a low current density and low dose.The SEE yields of lead silica,Suprasil and Infrasil are 0.69,0.76 and 0.55,respectively.Twenty hours later,the values are 0.62,0.64 and 0.33,respectively,for lead silica,Suprasil and Infrasil.These results show that Infrasil has poor charging stability.Comparatively,the charging stability of lead silica is better,and Suprasil has the best characteristics.     

A spectroscopic method of determining the isotopic composition of magnesium

The composition is determined from the isotopic shift of the Mgl 880.68 nm line. The coefficient of variation for magnesium metal is 1.8%, but 4% for magnesium oxide. Several spectroscopic analyses can be performed in the time needed for one mass-spectrometer analysis, which reduces the error of analysis by a factor two or three.We are indebted to Professor A.N. Zaidel for valuable advice and to N. A. Novozhilova for assistance with the measurements.     

Hydrogenation properties of catalyzed and non-catalyzed magnesium films

The hydrogenation properties of 400 Å thick magnesium films, grown under ultra-high vacuum conditions, have been studied under conditions relevant for hydrogen storage, i.e., elevated temperatures and pressures. The hydrogen uptake of films modified by deposition of platinum on the magnesium surface was also investigated. A realistic hydrogen storage system is modeled by studying the combined effect on hydrogen sorption by a platinum catalyst and magnesium oxide, the latter a common impurity in magnesium-based hydrogen storage systems. Magnesium and platinum are found to inter-diffuse readily at room temperature. However, a partially oxidized magnesium layer stabilizes platinum on the surface, while still being permeable to hydrogen.