The effects of electron beams on the composition of ion bombarded calcium fluoride surfaces have been studied with Auger electron spectroscopy (AES), electron loss spectroscopy (ELS), and x‐ray photoelectron spectroscopy (XPS). It was found with AES that when using typical AES excitation electron‐beam parameters (5 keV, 5 μA), the calcium fluoride surface decomposed, resulting in the desorption of fluorine. The ELS and XPS measurements suggest that simultaneous with the desorption of fluorine from the surface, is the accumulation of calcium in a metalliclike state. Threshold measurements for the onset of fluorine desorption showed that AES analysis could be made without significant electron‐beam effects by rastering a 2.5‐keV, 0.05‐μA electron beam over an area of 6×10−4 cm2, corresponding to a current density of 0.082 mA/cm2.

Electron‐beam‐induced decomposition of ion bombarded calcium fluoride surfaces

The characteristic energy losses suffered by a 20 keV electron beam through thin films of some transition metals (Sc, Y, Zr, Hf) and of their oxides are measured using a magnetic spectrograph up to 50 eV. The energy loss function — Im 1/e and the complex dielectric constant are calculated. It is shown that the electron excitation spectra of metals contain two main contributions in this energy range: one due to collective excitations of the conduction electrons, the other to the excitation of inner p-electrons. In the case of oxides the spectra display three strong structures: there are probably two collective excitations of valence electrons followed by the excitation of inner p-electrons of the metal.



Les pertes caracteristiques d'energie subies par un faisceau d'electrons de 20 keV a la traversee de films minces de quelques metaux de transitions places en debut de serie (Sc, Y, Zr, Hf) et de leurs oxydes ont ete mesurees avec un spectrographe magnetique jusqu'a 50 eV. La fonction —Im 1/e et la constante dielectrique complexe sont calculees. On montre que le spectre des excitations electroniques des metaux contient deux contributions principales dans le domaine d'energie considere: l'une due a l'excitation collective du gaz d'electrons de conduction, l'autre a l'excitation des electrons p. Dans le cas des oxydes, les spectres mettent en evidence trois structures importantes: deux excitations collectives des electrons de valence et l'excitation des electrons p du metal.

Electronic Excitations in Some Transition Metals and Their Oxides. Characteristic Energy Loss Measurements up to 50 eV

We present the results of a first-principles study on BaF2 in its stable (cubic) and high-pressure phases. A linear combination of atomic orbitals approach in the framework of density functional theory is employed for total energy calculations in cubic, orthorhombic and hexagonal phases of BaF2. A fitting of the energy surface to the equation of state yields the lattice constant and the bulk modulus of these phases at zero pressure which are in good agreement with the corresponding experimental values. Analysis of band structure determines the high-pressure phases to be direct-gap materials and no metallization of BaF2 is predicted to occur for pressures up to 50 GPa. Furthermore, several peaks observed in the spectroscopic experiments have been identified with interband transitions in the cubic BaF2. The calculated mean value of the refractive index is found to increase in going from the cubic to orthorhombic to hexagonal phases of BaF2.

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First-principles study of structural, electronic and optical properties of BaF2 in its cubic, orthorhombic and hexagonal phases

The Raman scattering efficiencies for Г phonons have been measured for Gap, ZnTe, ZnSe, ZnS, CAF2, SrF2and BAF2, using the Brillouin-Raman method. The results are used to calibrate resonance Raman curves previously published for Gap, ZnTe, ZnSe and ZnS. In this case a theoretical treatment of the data based on a parabolic-band model yields values of the deformation potential d o which are compared with other data. For the fluorite-type materials the scattering efficiency is attributed to electron-phonon coupling to the edge exciton. This analysis yields a deformation potential of the Г15valence band, also abelled d o. The results are compared with pseudopotential and LCAO calculations.

Absolute Raman scattering efficiencies of some zincblende and fluorite-type materials

A theoretical study of structural, elastic, electronic and optical 
properties of CaF2, SrF2 and BaF2 is presented, using the 
full-potential linearized augmented plane-wave (FPLAPW) method as implemented in 
the Wien97 code. In this approach the generalized gradient approximation 
(GGA) was used for the exchange-correlation (XC) potential. Results are 
given for lattice constant, bulk modulus, its pressure derivative and 
elastic constants. Band structure, density of states, pressure coefficients 
of energy gaps and refractive indices are also given. The results are 
compared with previous calculations and experimental data.

Structural, electronic and optical properties of fluorite-type compounds

The characteristic energy losses suffered by a 20 keV electron beam through CaF2, SrF2, and BaF2 films have been measured with a magnetic spectrograph. From the energy loss function — Im e−1, optical constants are calculated by means of Kramers-Kronig analysis in the 8 to 40 eV range and are compared with the available reflectance data, especially those obtained with synchrotron radiation. The observed structure is tentatively explained for the three compounds, following a band model based upon symmetry considerations. The origin of the energy losses corresponding to the soft X-ray range is also briefly discussed.



Les pertes d'energie caracteristiques subies par un faisceau d'electrons de 20 keV a la traversee de films de CaF2, SrF2, et BaF2 ont ete mesurees avec un spectrographe magnetique. Les fonctions optiques ont ete calculees jusqu'e 40 eV par analyse de Kramers-Kronig a partir de la fonction — Im e−1, et sont comparees aux mesures de reflectivite trouvees dans la litterature, particulieerement a celles obtenues avec le rayonnement synchrotron. A partir d'un modele de structure de bande base sur des considerations de symetrie, les spectres relatifs aux trois composes sont interpretes. l'origine des pertes d'energie mises en evidence dans le domaine des rayons × mous est sommairement discutee.

B. Lahaye

Papers

Spectra of Electronic Excitations in CaF2, SrF2, and BaF2 in the 8 to 150 eV Range