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.     

Radii of single-proton states in isotopes of tin

Sub-Coulomb (t, α) reactions have been employed in the determination of the rms radii of the $\text{1}\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$, $\text{2}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{2}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ proton orbits in ^{112,116,118,120,124}Sn. The experimental values are compared to the results of Hartree-Fock calculations.     

Fluorescence determination of atmospheric NO2

The concentrations of nitrogen dioxide (NO₂) in the atmosphere have been determined in real time and with a sensitivity of one part per billion. Laser excitation of NO₂ was at 4416 and 4880 Å and fluorescence was monitored at 0.7 to 0.8 m. Results obtained on typical smoggy days in Los Angeles are presented.     

An action principle and canonical formalism for the Neveu Schwarz-Ramond string

A Lagrangian giving the constraints and boundary conditions of the Neveu Schwarz-Ramond model is presented.     

Classical and quantum mechanics of free relativistic membranes

Motivated by the theory of relativistic strings, the theory of a two-dimensional relativistic membrane whose action is proportional to the three-dimensional area it traces out in space-time is investigated both in Lagrangian and Hamiltonian formalisms. The quantum theory is developed using Dirac's method for constrained systems and the question of gauge choices is considered in some detail.     

The Dirac equation in exterior form

Using the correspondence between the Clifford and exterior algebras we write the Dirac equation in terms of differential forms. The covariances of the theory are then examined. We show in detail the correspondence with usual matrix methods.     

Magnetic structure of MnO at 10 K from total neutron scattering data

Total neutron scattering data from a powdered sample of MnO collected at 10 K have been analyzed using the reverse Monte Carlo method to refine the nuclear and magnetic structure. The results give the first unambiguous assignment of the average magnetic structure. The magnetic moments are aligned ferromagnetically within (111) sheets with the magnetization vectors of alternate sheets along axes parallel and antiparallel to the <112> directions, albeit with a small modulated out-of-plane component. Small displacements of Mn and O (modulated with the same periodicity) accompany the magnetic ordering and both atomic and magnetic structures may be described in the monoclinic space group C2.