Thermomodulation Spectra of Al, Au, and Cu

Thermotransmission and thermoreflection measurements were made on semitransparent films of Al, Au, and Cu at about 370 and 120 K in the range 0.5-5 eV. The data yield the thermomodulation spectrum Δε2 of the imaginary part of the dielectric constant directly, without Kramers-Kronig analysis. A comparison of the interband region of Δε2 for Cu with the piezo-modulation spectrum of a single crystal shows that broadening of the Fermi distribution and volume strain caused by thermal expansion are the principal causes of the Δε2 spectrum. The Δε2 spectrum for Al is particularly simple and can be discussed using closed-form expressions for the optical conductivity. It appears that the temperature dependence of the interband relaxation time for transitions across gaps produced by |V111| is smaller than that for transitions across gaps caused by |V200|, which in turn is smaller than that for the infrared intraband transitions

Effect of hybridization on 4d→4f spectra in light lanthanides

The effect of the hybridization of 4f electrons on La, Ce, and Pr tripositive ions with ligand states or conduction electrons was simulated by reducing the Slater integrals involving the 4f electrons by 10% or 20%, depending on whether the integrand involves one or two 4f electrons, respectively. The dipole-allowed 4d→4f spectra were calculated. Observable effects, changes in line energies, and oscillator strengths, were compared with limited data available for ionic and metallic solids containing these lanthanides, and for Ce vapor. Many of the observed changes in the 4d→4f spectra attributed to hybridization effects are found qualitatively in the calculation

Optical Properties of Dilute Ag-In Alloys

The optical constants of silver and five dilute silver-indium alloys (\u3c5% In) were determined from reflectance and transmittance measurements in the spectral region between 3.35 and 4.28 eV on vacuum-evaporated films. The data are consistent with the assumption that the onset of interband absorption in silver arises primarily from direct interband transitions from the d band near L3 to the Fermi surface, but with a small, though not insignificant, contribution due to direct transitions from the conduction band near L2′ to the conduction band near L1. The changes in the complex dielectric constant induced by alloying were interpreted to indicate a shift of the L3→EF transition to higher energy and a shift of the L2′→L1 transition to lower energy. Because contributions to the dielectric constant of the two transitions overlap and because of the limited energy range investigated, no firm estimates of the magnitudes of the shifts could be made, but it seems clear that the L2′−L1 energy gap decreases as indium is added to silver. The strength of the interband absorption in the alloys at energies below that of the edge in silver appeared to be too strong to be accounted for by indirect transitions. Alloying-induced changes in the energy-loss function showed that the relatively undamped plasma resonance in silver near 3.8 eV becomes strongly damped as indium is added. The increase in damping was attributed partly to a decrease in the conduction-electron relaxation time, and partly to the shift of interband transitions so that they occur at the plasma energy. Data from measurements on films cooled to liquid-helium temperature showed some sharpening of structure in the complex dielectric constant and of the peak in the loss function

Optical absorption in Al and dilute alloys of Mg and Li in Al at 4.2 K

The absorptances of Al and alloys of up to 5.5 at.% of Li and Mg in Al were measured at 4.2 K in the 0.2-3.0-eV range. The theory of Ashcroft and Sturm was fitted to the data. The fit obtained is reasonably good, provided three relaxation times are used, one for the Drude term and one for each type of transition between parallel bands. The fit is slightly better for the more concentrated alloys. In all cases the discrepancy between the fit and the data is greatest in the region between the two interband peaks in the optical conductivity, and known extensions to the theory do not qualitatively improve the fit. Pseudopotential Fourier coefficients U200 and U111 obtained from the fits vary with solute type and concentration. The variation can be explained semiquantitatively with a virtual-crystal pseudopotential

Optical Properties of Single-Crystal Cadmium

Measurements were made of the reflectivity and/or absorptivity of cadmium from 0.15 to 20 eV, using polarized light with the electric vector both perpendicular and parallel to the c axis of the crystal. In the visible and infrared spectra both polarizations yielded strong absorption peaks. For perpendicular polarization, the main peak was at 0.98 eV and was attributed mainly to transitions between bands along the L−H symmetry line. Also in this polarization there was an absorption edge at 0.29 eV. This may be due to transitions near the point K in the Brillouin zone. For parallel polarization, the main absorption peak was at 1.10 eV and was attributed to transitions between bands along Γ−K, Γ−M, and L−H. No low-energy absorption edge was found for parallel polarization. Agreement between the experimental data and the calculations of Kasowski based on a nonlocal-pseudopotential model was fairly good. There was no agreement between the data and calculations based on a local-pseudopotential model. At low temperatures, the long-wavelength absorptivities were approximately constant, in agreement with theory for the anomalous-skin-effect region. Using the theory of Kliewer and Fuchs and the experimental data, parallel and perpendicular effective masses were calculated to be 1.09m0 and 1.61m0, respectively. The weighted average of these is in good agreement with the thermal effective mass for cadmium. The low-energy data support the theory of Kliewer and Fuchs and tend to confirm the volume absorption process suggested by Holstein

Thermomodulation spectra of high-energy interband transitions in Cu, Pd, Ag, Pt, and Au

Thermotransmission and thermoreflectance spectra were obtained for Cu, Pd, Ag, Pt, and Au in the 10-30 eV spectral region. Structures due to transitions from the Fermi level to high-density bands 15 eV above the Fermi level were identified in Pt. All metals showed structures arising from interband transitions between the d bands and the same flat bands, 15-20 eV above the Fermi energy. Attempts to fit to interband critical points in Au revealed over 40 possible critical points in the region of these structures, most of them near the Brillouin-zone centers. Systematic trends in the series of metals make the qualitative identification of the structures more secure, and no energy shifts of calculated energy bands are required. The observed widths of structures are sometimes much narrower than the widths of free-electron-like bands at comparable energies

Multiplet Structure below Threshold in Appearance-Potential Spectra—Lanthanum N4,5

We present the N4,5 appearance-potential spectrum of La. Structure exists at energies lower than the threshold energy for transitions terminating at the Fermi level. Several similarities between optical absorption spectra and our results are discussed. These results contradict the simple model usually used to interpret appearance-potential spectra and emphasize the need for a new theory

Optical Properties of the Rubidium and Cesium Halides in the Extreme Ultraviolet

The absorption spectra of evaporated thin films of all rubidium and cesium halides in the 50- to 250-eV region are reported. In this range, transitions from the 3d shell of Rb+ and from the 4d and 4p shells of Cs+ can be seen, as well as some transitions from inner shells of the halogen ions. Besides the absorption fine structure near the threshold for inner-shell transitions, broad absorption structure is observed and explained as due to d→f continuum transitions. The number of effective electrons whose oscillator strength has been exhausted in our spectral region has been computed from the absorption data; it is particularly strong for materials containing either Cs or I. Differences in the spectra of materials with NaCl and with CsCl structure are discussed. The measurements were performed using the DESY electron synchrotron as a light source

Optical properties of β′-CoAl

The optical absorptance of β′-CoAl in the (0.1-2.5)-eV region was measured and analyzed by the Kramers-Kronig method. The optical conductivity shows interband features beginning below 0.1 eV, with several structures below 2 eV. These structures agree with those calculated from the energy bands of Moruzzi, Williams, and Gelatt. The experimentally observed systematics in CoAl and NiAl confirm our previous interpretation of the spectra of β′-NiAl, and disagree with other experimental and theoretical attempts to understand the optical properties of these compounds

Low-energy interband absorption in bcc Fe and hcp Co

We have examined the electronic structure of bcc Fe and single-crystal hcp Co by using optical absorptivity and thermoreflectance techniques for 0.2≤hν≤5 eV. The optical conductivities σ were calculated by Kramers-Kronig analyses. A prominent structure was observed in σ for Fe at 2.37 eV and a shoulder was observed near 0.8 eV; the latter structure was the dominant feature in the thermoreflectance spectrum. These were discussed in terms of minority-spin band interband absorption and spin-flip interband transitions. The anisotropic optical conductivities of hcp Co were discussed in terms of recent energy-band calculations