Stress-induced optical dichroism of V−-centers in MgO

The stress-induced optical dichroism and optical absorption spectra of the V^?-centers in electron irradiated MgO are reported. The dichroism spectrum can be largely attributed to a paraelastic alignment of inherently-dichroic V^?-centers. A curve-fit analysis of the lower-energy portions of the dichroism and absorption spectra yields the ratio of the transition dipole strengths, $\text{D}{}_{\mathrm{\parallel }}\text{D}{}_{\mathrm{\perp }}\text{= 1.55 ± 0.2}$, and the energy splitting, E_∥ ? E_⊥ = 0.22 ± 0.07 eV, for light polarized parallel (∥) and perpendicular (⊥) to the defect symmetry axis. The spectra also have considerable intensity in their high-energy tails, and it is speculated that a portion of this intensity is due to the V-center. The implications of the results with regard to the electronic structure of the center are discussed.     

Optical transitions on GaAs [110] surface

Modulation techniques have been employed in optical reflection measurements on GaAs [110] cleaved surface. Spectral bands at 2.62 and 2.83 eV observed for the light polarization $\text{E6[}\text{1}\text{10]}$ have been assigned to the surface state transitions. The E₁ and E₁ + Δ₁ bulk transitions were found to be sensitive to the surface conditions.     

Resonance photoelectron spectroscopy from autoionization states of CH3I

Resonance photoelectron spectra of methyl iodide have been taken by exciting individual autoionizing Rydberg states, and measuring the ejected electron energies by a time of flight technique. A number of spectra were obtained from autoionization states which are members of two series, converging respectively to the ²E_{$\text{1}\text{2}$} (ν = 0) and ²E_{$\text{1}\text{2}$} (ν₂ =1) states of the ion. In addition to observing vibrational bands not previously reported, we found that the peak intensities are consistent with Bardsley's formulation of the configuration interaction theory of autoionization.     

Photoemission (XPS,UPS) studies of Pd-Si metallic glasses

The valence bands of glassy $\text{Pd}{}_{\mathrm{100?x}}\text{Si}{}_{\mathrm{x}}\text{(15?x?21)}$ and pure Pd were studied by XPS and UPS. The valence band spectra of the alloys show a strongly reduced density of states at the Fermi energy E_F compared to Pd. From the measured relative photoelectric cross sections for the different excitation energies we conclude that the electron states near E_F of the glassy alloys have mainly d-character. This is in good agreement with recent measurements of the low-temperature specific heat, the magnetic susceptibility and the optical reflectivity.     

The application of tailored modulation techniques to depth profiling with auger electron spectroscopy

Tailored modulation techniques (TMT) are applied to depth profiling to eliminate errors in signal strength measurements caused by Auger line shape changes. The application of TMT is illustrated by profiling through Al₂ O₃Al interfaces. When peak-to-peak heights in first derivative spectra $\text{n}$⁽¹⁾_m (E) are used for profiling, the measured Al KL_2,in3L_2,3 signal strength shows a large decrease near the interface. This artifact is reduced when peak heights in $\text{n}$_m (E) are used but can be eliminated only when Auger area values are used for profiling. The peak heights in $\text{n}$_m (E) and Auger area values can both be obtained in real time with TMT and plotted automatically with conventional multiplexing equipment. Some features of Auger spectra obtained using TMT are illustrated, and a comparison of depth profiles obtained using peak-to-peak heights in $\text{n}$¹_m (E), peak heights in $\text{n}$_m (E) and Auger area values demonstrates the usefulness of TMT in depth profiling.     

Multiparameter study of 1H, 3H and 4He in fast neutron induced fission of 235U

The emission probabilities per fission of α-particles, tritons and protons have been measured in fast neutron induced fission of ²³⁵U. The measurements were carried out at neutron energies of 120, 180, 230 and 550 keV. AΔE-E semiconductor detector telescope was used to identify different light charged particles and the fission fragments were detected with an ionization chamber. The three-parameter data corresponding to the pulse heights from the ΔE-E detectors and the ion-chamber were recorded event by event on magnetic tape and were analyzed off-line by computer. No significant variation in the most probable energy ($\text{E}$) and the standard deviation (σ_E) of the energy spectra of different light charged particles with incident neutron energy was observed, although $\text{E}{}_{\mathrm{\alpha }}$ was seen to have a slightly higher value beyond E_n = 230 keV. The yield of α-particles in fission induced by neutrons of E_n ～ 200 keV was found to be higher by about 20 % than that in thermal neutron induced fission. The yields of tritons and protons were found to increase significantly with neutron energy.     

Optical phase shifts from low-energy neutron cross sections

The Lorentz-weighted average of the S-matrix introduced by G.E. Brown is used in the Feshbach theory of the generalized optical potential to show that the average many-body S-matrix for elastic scattering is exactly equal to the two-body S-matrix of an optical potential. However, the optical potential S-matrix must be evaluated at the complex energy $\text{E}\text{= E + iI,}\text{where}\text{I}$ is the half-width of the lorentzian. The resulting equation for the optical phase shifts (OPS) δ_c, exp $\text{[2iδ}{}_{\mathrm{c}}\text{(}\text{E}\text{)] = 〈S}{}_{\mathrm{cc}}\text{(E)〉}$, holds even when the level spacing D forces the use of an averaging half-width I > D which is comparable to the energy E, providing that the OPS are also evaluated at the complex $\text{E}$ instead of being approximated by their values on the real energy axis at E. An appendix discusses briefly the conditions on a potential necessary for the result obtained by Brown that $\text{〈S}{}_{\mathrm{cc}}\text{(E)〉 = S}{}_{\mathrm{cc}}\text{(}\text{E}\text{)}$ when Lorentz-weighted averaging is used.     

Raman spectra of two new modifications of germanium: Allo-germanium and 4H-Ge

The Raman spectra of allogermanium and 4H germanium have been measured. Allogermanium shows a band which mimics the density of states of diamond-like Ge(Ge?I). 4H-Ge shows a peak at 287 cm^-1 which can be assigned to an E₂ phonon. This phonon corresponds to a TO-mode of Ge-I at the $\text{(}\text{π}\text{2}\text{a}\text{) (1|1)}$ point of the Brillouin zone.     

Study of Ag/Si(111) submonolayer interface: I. Electronic structure by angle-resolved UPS

Angle-resolved ultraviolet photoelectron spectra have been measured for well defined Ag/Si(111) submonolayer interfaces of (1) $\text{Si(111)(}\text{3}\text{×}\text{3}\text{)R30°-Ag}$, (2) “Si(111)(6 × 1)-Ag”, and (3) Ag/Si(111) as deposited at room temperature. Non-dispersive and very narrow (FWHM ～ 0.4–0.5 eV) Ag 4d derived peaks are found at 5.6 and 6.5 eV below the Fermi level for surface (1) and at 5.3 and 6.0 eV for surface (2). Dispersions of sp “binding” states in the energy range between E_F and Ag 4d states have been precisely determined for surface (1). Electronic structures similar to those of the Ag(111) surface, including the surface state near E_F, have been observed for surface (3).     

Electron diffraction and infrared study of the semimetallic layered compound Ti1−xVxSe2

The crystals of the system Ti_1?xV_xSe₂ (C ≤ x ≤ 0.05) undergo a second order structural phase transition. Electron diffraction studies show that the transition temperature decreases with progressive V-doping. The phase transition considerably affects the infrared reflectivity, measured at 300 and 77 K in the spectral range 40 cm^?1 to 4000 cm^?1.The presence of free carriers and the existence of optical infrared active E_u phonon modes $\text{(}\text{E}\text{⊥}\text{c}\text{)}$ confer their characteristic appearance to the spectra. At room temperature one phonon structure is measured at 143 cm^?1. At 77 K a new series of phonon peaks appears up to a V-concentration of 5 % as a direct consequence of superlattice formation.At room temperature the plasma-edge shifts towards higher frequencies as the vanadium concentration increases. This effect is caused by a large increase of $\text{N}\text{m}{}^1$, associated with the mixing of impurities. It is indicative of the small density of states at the Fermi level in semimetallic TiSe₂. Our results suggest a phase transition driven by lattice dynamical effects.     

Infrared study of lattice and free carrier effects in p-type CuInTe2 single crystals

Infrared reflectivity spectra of p-type CuInTe₂ single crystals were measured at room temperature for the polarization direction E ⊥ c and E | c in the wavenumber range from 55 to 4000 cm^?1. Fits to a dielectric function including contributions from lattice oscillators and free carriers gave the first determination of the effective hole masses of $\text{m}{}_{\mathrm{p\perp }}\text{m}{}_0\text{= 0.85}$ and $\text{m}{}_{\mathrm{p|}}\text{m}{}_0\text{= 0.66}$. Two optical phonon modes were found for both polarization directions.     

Electronic properties of tin dichalcogenides (SnS2 and SnSe2)

Using Phillips and Van Vechten's theory the homopolar gap (E_h), ionic gap (E_c) and Penn gap (E_p) have been derived for the layered compound semiconductors SnS₂ and SnSe₂. The values of E_p so derived have been found to be in agreement with the values obtained from the Grimes-Cowley modification of the Penn model, and also with the reflectance spectra of the respective materials. Using the above data the Phillips ionicity has been calculated and the octahedral coordination of SnS₂ and SnSe₂ has been discussed on the basis of the calculated value of ionicity. The bond electronic polarizability has been evaluated using Chemla's approach and the values so obtained have been found in agreement with the values obtained from the Clausius-Mossottirelation. The Varshni formula has been shown to explain fairly well the temperature dependence of energy gap and the constants involved have been derived. The calculated isobaric temperature gradient of E_g[($\text{?E}{}_{\mathrm{g}}\text{?T)}{}_{\mathrm{p}}$] has been found to be in accordance with the reported values, and the electron lattice interaction has been evaluated.     

A combined LEED,AES and XPS study of the ZnO {0001} polar surfaces: I. LEED,AES and XPS of contaminated surfaces

The {0001} polar surfaces of ZnO single crystals have first been examined after a chemical treatment involving HCl and H₃PO₄ and a 24 hr bakeout at 250 °C. The impurities detected on the (000$\text{1}$)-O surface with AES were carbon, chlorine, phosphorus and to a lesser extent sulphur. On the (0001)-Zn surface, carbon, chlorine and sulphur were the dominant impurities, while the phosphorus signal was less important. These results were confirmed by XPS measurements on frehsly etched surfaces. The AES spectra were recorded as distribution curves N(E). Averaging, curve-fitting and related numerical techniques were used to obtain high resolution spectra, enabling the identification of the phosphorus L₁-transitions. The etched surfaces were cleaned progressively using argon ion bombardment and ohmic heating. It has been consistently observed that the clean surfaces exhibit primitive (1 × 1) structures. Superstructures such as $\text{(}\text{3}\text{×}\text{3}\text{)}$ on the (000$\text{1}$)-O surface, and $\text{(4}\text{3}\text{× 4}\text{3}\text{)}$ and (3 × 3) on the (0001)-Zn surface, were repeatedly observed at discrete spots of contaminated surfaces. A clear correlation with impurities as observed by AES however could not be found. Facetting was observed after prolonged heating.     

Proton spectra from the 6,7Li(γ,p) 5,6He reactions at Eγ = 60 MeV

The ${}^{\mathrm{6,7}}\text{Li}\text{(γ,}\text{p}\text{)}$ reactions have been investigated for E_γ = 60 MeV. Excitation of residual (1p)^?1 and (1s)^?1 hole states is evident from the proton spectra measured at ?_p = 45°. The data are compared with a theoretical calculation which includes short-range correlations.     

Fission fragment yield distributions as a function of their excitation energies for 235U(nth, f)

The recent ²³⁵U(n_th, f) data taken at the Grenoble high flux reactor have been reanalysed to determine the fragment mass distributions as a function of their excitation energies $\text{E}{}^1$. The mass distributions for low $\text{E}{}^1$ values show the known fine structures. However, the distributions for $\text{E}{}^1\text{> 30}\text{MeV}$ do not show the fine structure reported by Newson. Beyond $\text{E}{}^1\text{≈ 35}\text{MeV}$, the most probable masses of distributions decrease monotonously as $\text{E}{}^1$ goes up.     

Photoluminescence of glassy and single-crystalline GeSe2 measured at 77K

Photoluminescence (PL) and excitation (E) spectra of glassy and crystalline GeSe₂ at 77 K have a broad-band peaked at energy $\text{E}{}_{\mathrm{R}}\text{<}\text{1}\text{2}\text{E}{}_{\mathrm{g}}$. PL spectra have a Gaussian form: the Stokes shift (W_S=?ω?E_R) for crystalline GeSe₂ is greater than that for the glass. The absorption coefficient α at the maxima of E spectra is ～ 10²cm^-1 for the glassy form, and ～ 10⁴cm^-1 for the crystalline form. The results are interpreted in terms of the Mott-Davis-Street model supposing a strong electron-phonon coupling in these types of amorphous semiconductors which explains the large Stokes shift and the broad emission band.     

A folded diagram microscopic calculation of nuclear Coulomb displacement energies

We have performed a rather extensive microscopic calculation of the ¹⁷F-¹⁷O Coulomb energy differences ΔE_C. Our main purpose has been to study the effects of (i) folded diagrams, (ii) core polarization and (iii) the effects due to different nucleon-nucleon potentials, different single-particle spectra and different radial wave functions. Using a proton-neutron representation we have included higher-order Coulomb corrections like e.g. the coupling of valence particles to collective vibrations of the core.The inclusion of folded diagrams is very important; it is equivalent to a self-consistent treatment of the Q-box starting energies. It causes a significant suppression of the effect due to core polarization, its contribution to ΔE_C becoming small, about 0.03 MeV. As a consequence of this “self-correcting” behavior our results for ΔE_C are quite stable with respect to the choice of different single-particle spectra. Two rather different nucleon-nucleon potentials, the Reid soft-core potential and a meson exchange potential of the Bonn-Jülich group also lead to quite similar results for ΔE_C. In the case of $\text{J =}\text{5}\text{2}{}^{\mathrm{+}}$, for example, Δ_C calculated with the Reid potential ranges, depending on other assumptions, from 3.36 to 3.43 MeV and from 3.36 to 3.40 MeV with the Bonn-Jülich potential: Both are significantly smaller than the experimental value of 3.54 MeV. Optimizing the radial wave functions in the spirit of a Brueckner-Hartree-Fock theory improves the calculated values of ΔE_C for the $\text{J =}\text{1}\text{2}{}^{\mathrm{+}}$ state but not for $\text{J =}\text{5}\text{2}{}^{\mathrm{+}}\text{or}\text{3}\text{2}{}^{\mathrm{+}}$.We feel that other processes such as the explicit inclusion of core deformation or mesonic degrees of freedom or both are needed to explain the Coulomb displacement energies.     

Adsorbate-induced surface resonances observed in photoemission from a (2 × 2)R45° sulphur layer on Pd(100)

Angular-resolved photoemission spectra from a $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$ sulphur layer on Pd(100) reveal peaks independent of photon energy, which show strong dispersion in the electron energy range 3…9 eV relative to the vacuum level. The appearance of these levels can be correlated with absolute gaps in the projected bulk band structure of palladium. We interpret these features, which are also observed in secondary electron emission, as adsorbate-induced surface resonances above E_F. The sulphur 3p-derived levels below E_F show strong dispersion effects in the [110] but not in the [100] crystal azimuth. The projected band structure also shows that $\text{sd}$ hybridisation gives rise to an absolute gap at approximately this energy in the [110] $\text{(}\text{ΓX}\text{)}$ direction.