M. Prutton

The (100) surfaces of alkali halides: II. Electron stimulated dissociation

Abstract This is the second in a set of three papers reporting observations by low energy electron diffraction, Auger emission spectroscopy and quadrupole mass spectrometry of the structure of the (100) surfaces of the alkali halides LiF, KCl, NaCl, NaF and also of MgO. This paper reports observations of the electron stimulated dissociation of these materials as a function of primary electron beam energy and intensity. After removal from the UHV system optical absorption measurements reveal the presence of M and F bands in the damaged surfaces. The amount of absorption in these bands depends upon the material and the temperature during electron bombardment.

The (100) surfaces of alkali halides: I. The air and vacuum cleaved surfaces

Abstract This paper is the first of a series of three which report the results of studies on the (100) surfaces of LiF, NaF and KCl. The air cleaved and vacuum cleaved surfaces were examined by LEED and Auger emission spectroscopy and the partial pressures of the constituent elements were measured on cleavage of the specimen. Auger analysis of the air cleaved surfaces of LiF and NaF showed the presence of carbon and oxygen, and oxygen was detected on the air cleaved surface of KCl. These impurities could not be detected on the vacuum cleaved surfaces of NaF and KCl but were still observed on the vacuum cleaved surface of LiF. Mass spectrometer studies showed dissociation occurred on cleavage for all three alkali halides although different in character in each case. The form of the partial pressure curves suggests that this dissociation may be non-stoichiometric. LEED patterns obtained from the vacuum cleaved surfaces were of much better quality than those from the air cleaved surfaces except for the case of KCl which gave a poor LEED pattern directly after cleavage but which improved with time.

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

Abstract 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.

Quasi-atomic fine structure in the Auger spectra of solid silver and indium

Abstract High resolution M 4 , 5 N 4,5 N 4,5 Auger spectra of Ag and In reveal fine structure, which may be interpreted in terms of spin-orbit splitting of the initial state and multiplet structure in the final state. Interaction between d holes in Ag is an important factor in determining the shape of the Auger spectrum, and so the Auger profile is not related in any simple way to the one-particle density of states.

The (100) surfaces of alkali halides: III. Electron stimulated desorption and overall discussion

Abstract The electron stimulated desorption of common gaseous materials from the (100) surfaces of LiF, KCl, NaF and MgO is reported. Measurements are given of the desorption rate as a function of primary electron beam energy and intensity. Most of the results are interpreted in terms of a simple model in which damage of the surface by the incident electrons allows gaseous impurities trapped in the bulk of the crystal to diffuse to the surface. The results of this set of three papers on the surfaces of alkali halides are discussed in relation to the mechanisms of epitaxial overgrowth. It is concluded that simple experiments on alkali halides are often unreliable in that the surface structure and stoichiometry depend upon a number of generally uncontrolled parameters. M and F centres are suggested as a pair of potential nucleation sites for condensing metal vapour.

A comparison of reliability (R) factors in a LEED structural analysis of the copper (111) surface

A large experimental data set of I(V) curves was collected over an energy range 50-400 eV at three diffraction geometries from the copper (111) surface. The experimental curves were compared with I(V) curves calculated from model structures. The result obtained for the relaxation of the topmost layer is -0.3%+or-1% (+or-0.02 AA) of the bulk interlayer spacing. The precision of the result comes from the large experimental data base and the application of statistics to an R-factor analysis. R-factors are presented for the formalisms both of Zanazzi and Jona and of Pendry. The two R-factor analyses give similar results for the relaxation of the copper (111) surface. However, the Pendry R-factor is found to be more sensitive to model parameters and faster to compute than the Zanazzi and Jona R-factor.

LEED calculations for the NiO (100) surface: extension to lower energies

A previous LEED theory/experiment comparison for the NiO (100) surface (Kinniburgh and Walker, 1977) has been extended by the inclusion of a new I(V) data set recorded at 120 degrees C for primary energies down to 60 eV. The possibility of a small degree of rumpling of the ions in the surface layer was explicitly tested in the course of the analysis using the CAVLEED suite of programs. The results are consistent with the earlier findings and confirm that the surface is not rumpled on a scale detectable by LEED. The necessity of including exchange effects in the theoretical calculations at low energies is also demonstrated.

LEED and electron spectroscopic observations on NiO (100)

LEED, Auger, and electron-loss spectra are reported for a UHV-cleaved NiO (100) surface. Data are given for the clean surface and a surface contaminated mostly with about half a monolayer of chlorine. Large systematic intensity variations are found between different LEED beams, and the possibility of anisotropic electron mean free paths is suggested. The loss spectra are interpreted as containing features due to charge transfer excitons.

A LEED study of the structure of the (100) surface of CaO

The structure of the (100) surface of UHV cleaved CaO has been determined at room temperature by LEED. The experimental data set consists of 39 beams obtained between 120 and 400 eV in seven diffraction geometries. The calculations were based upon rumpled ionic models and used the CAVLEED library of LEED programs. Up to 57 beams and nine phase shifts were used. Very good agreement was obtained between theory and experiment, the best fit yielding a rumple of less than 2% and a surface layer contraction of 1% of an interlayer spacing. The R factor computed for this solution was 0.156. A shell-model calculation of the surface structure is in general agreement with this result in that it predicts a rumple of 5% and a surface contraction of 3% of an interlayer spacing.

LEED observations of the (11) beam from the (100) surfaces of some NaCl-type crystals

Abstract Low energy electron diffraction patterns from the (100) surfaces of crystals of RbBr, KCl, NaF, KBr, NaCl, LiF and MgO have been observed as a function of electron energies between 30 eV and 1000 eV. It was found that (11) reflections occur at low voltages, even in cases of crystals in which the atomic scattering amplitudes are such that (11) reflections should be too weak to be observed. The (11) reflections were found to become very weak above 59 eV for RbBr, 100 eV for KC1 and 195 eV for NaF. Published work on NaCl suggests that the (11) spot persists to above 260 eV. It is shown that kinematical scattering theory is most suitable when the ratio of cation to anion diameters is near to unity, because then the transparency will be the greatest.