The absorption edges of GeS and Urbach's rule

The low-energy absorption tail of the E∥a exciton in GeS obeys Urbach's rule over at least three orders of magnitude in absorption coefficient for 4.2 ? T ? 240 K. The E∥b polarization (for which no exciton is observed) does not obey Urbach's rule. The E∥a Urbach edge is caused by interactions between the excitons and a 8.7 meV rigid-layer vibrational mode. Our results are consistent with Sumi and Toyazawa's theory of the Urbach edge, and inconsistent with the models of Dow, Redfield and Skettrup. The absorption edge of GeS is confirmed as a direct edge.     

Optical absorption band edge in single-crystal GeS

The absorption band edge of single-crystal orthorhombic GeS has been measured in all three polarizations at 4.2, 77 and 300°K. Strong excitonic absorption is observed for E ∥ a, and the transitions in this direction are identified as direct-allowed. Transitions in the E ∥ b and E ∥ c polarizations are tentatively identified as direct forbidden and direct allowed, respectively, with values for E_g similar to those found for E ∥ a. Evidence for the partially two-dimen?ional character of GeS is discussed.     

Thermomodulated exafs of soft x-ray absorption edges

Thermotransmission measurements exhibit structure for over 100 eV above the L_2,3 edge in Al and the M_2,3 edge in Ni, structure attributed to the temperature modulation of EXAFS. It appears to arise from both thermal expansion and a Debye-Waller factor.     

Exponential absorption edge in hydrogenated α-Si films

Optical and photoelectric measurements demonstrate that hydrogenated amorphous silicon prepared by glow discharge decomposition of silane has an exponential optical absorption edge, over the photon energy range 1.4–1.8 eV with a slope of 0.05–0.08 eV. Evidence is presented that the photogeneration efficiency is unity at room temperature and independent of electric field (10²?10⁴ V/cm^?1) and photon energy (1.2–2.2 eV).     

“Forbidden” soft-X-ray absorption edges in the alkali halides

Transitions from s core levels to the conduction-band edge and to excitons derivable therefrom are forbidden by symmetry in the alkali halides. Excepting K spectra, all indications from the literature seem to be that L₁, M₁, N₁, O₁ spectra are in fact unobservable. By untangling some data available for NaBr in the region where the Na⁺L₁ and Br^?M_{4 5} spectra overlap, we suggest that “forbidden” spectra may indeed by observable, and more detailed scans in certain regions of photon energy might be useful. The importance of excitons in X-ray spectra is further elucidated and earlier conclusions are confirmed.     

Exponential frequency dependence of multiphonon-summation infrared absorption

The recently observed exponential frequency dependence of the infrared absorption coefficient β in NaCl- and CaF₂-structure crystals is explained in terms of multiphonon absorption processes. A calculation gives the correct frequency dependence and order of magnitude of β, and predicts the temperature dependence, which should be easy to measure.     

Photoelectric effect in some alloys in the region of absorption edges

Summary Photoelectric effect in alloys using the transmission method in the energy range (17.8÷138) keV, at energies in the region of absorption edges of the various elements in the alloys, have been measured and compared with the respective theoretical estimates.     

银吸收边附近X射线衰减系数测量

在北京同步辐射装置的4B7A中能X光束线上,光源能区为2.1~6.0keV,能量分辨大于5000,高次谐波小于0.1%,光源强度大于109光子/s。通过全能区多能点的透过率精确测量Ag样品质量厚度,然后采用Ag薄膜对单能X光子的透过率进行测量,给出了Ag薄膜在吸收边(3.4~3.9keV)的衰减系数。建立了Ag样品吸收边附近衰减系数同步辐射测量方法。通过不确定度分析给出衰减系数测量不确定度小于1%,填补了在该区间衰减系数的空白。     

银吸收边附近X射线衰减系数测量

在北京同步辐射装置的4B7A中能X光束线上,光源能区为2.1~6.0 keV,能量分辨大于5000,高次谐波小于0.1%,光源强度大于109光子/s。通过全能区多能点的透过率精确测量Ag样品质量厚度,然后采用Ag薄膜对单能X光子的透过率进行测量,给出了Ag薄膜在吸收边（3.4~3.9 keV）的衰减系数 。建立了Ag样品吸收边附近衰减系数同步辐射测量方法。通过不确定度分析给出衰减系数测量不确定度小于1％,填补了在该区间衰减系数的空白。     

Thermoreflectance of GeS

The thermoreflectance of GeS has been studied in the energy range 1.2–5.2 eV at 100 and 320 K in polarized light for orientations E∥a and E∥c. The energy values of critical points have been determined from experimental curves.     

Edge absorption and circular photogalvanic effect in 2D topological insulator edges

The electron absorption on the edge states and the edge photocurrent of a 2D topological insulator (TI) are studied. We consider the optical transitions within linear edge branches of the energy spectrum. The interaction with impurities is taken into account. The circular polarization is found to produce the edge photocurrent, the direction of which is determined by light polarization and edge orientation.     

Luminescence of exciton in GeS

The luminescence of one free and four bound excitons has been investigated in the layer compound GeS. Up to six phonon replicas of the bound excitons are observable. The energies of these phonons are in reasonable agreement with Raman- and IR reflectivity measurements. The luminescence is highly anisotropic and is only seen in the polarization E ∥ a which confirms that the transitions E ∥ b are forbidden. Measurements with E ∥ c were inconclusive because of the difficulties in the sample preparation. Time resolved measurements revealed decay times between 20 nsec and 1.5 μsec for the different bound excitons. One exciton has been identified as bound to a Si impurity. This is the first observation of intentional doping of GeS. Thermal dissociation energies for the bound excitons lie between 6.5 and 20 meV.     

On the role of scattering and absorption edges in waveguide collimation of X-rays

The role of incoherent scattering in damping of transmitted radiation is studied on the basis of the statistical theory of X-ray scattering in a narrow rough collimator. Approximate expressions for the damping coefficients of waveguide modes are obtained, and damping associated with scattering is found to be proportional to the product of a root-mean-square amplitude of roughnesses by the correlation length. The effect of a considerable suppression of scattering near the absorption edges due to dispersion is predicted; this can also be observed from a decrease in the back-scattered beam intensity. The roughnesses not only reduce the radiation transmission by diffractive filters, but also adversely affect the selectivity of radiation filtering. The analytic expressions for the damping coefficients of waveguide modes make it possible to generalize the results to collimators with various properties of the surfaces.     

Dipolar excitations at the LIII x-ray absorption edges of the heavy rare-earth metals

We report measured dipolar asymmetry ratios at the LIII edges of the heavy rare-earth metals. The results are compared with a first-principles calculation and excellent agreement is found. A simple model of the scattering is developed, enabling us to reinterpret the resonant x-ray scattering in these materials and to identify the peaks in the asymmetry ratios with features in the spin and orbital moment densities.     

The edges and terrace effect of Ag particles on optical resonance absorption property

Alternative Ag and SiO₂ multilayers are prepared by using radio frequency magnetron sputtering. The Ag particles are found to diffuse toward and mostly accumulate near the surface of the Ag—SiO₂ composite film via a rapid thermal treatment. Different shapes of the Ag particles are obtained by changing the thickness of each Ag and SiO₂ layer. The response absorption property of the Ag composite film is also investigated. We relate the resonance absorption to the surface level and the Fermi level. To induce the obvious resonance absorption in an Ag composite film, it is necessary to maintain special shapes with sharp edges and wide terraces and to maintain the particle sizes ranging from 0 nm to 100 nm.     

An alternate empirical relationship for the optical absorption edges of certain ferroelectric materials

No theoretical justification has been found for the empirical relationship which has been used previously to describe the optical absorption edges of BaTiO₃ and KTaO₃. In this communication, another, equally valid, empirical relationship is presented that describes both the absorption edges of these materials and new data on gadolinium molybdate.     

Investigation of GeS (001)

Various possibilities of the preparation of the clean GeS surface were studied. The LEED pattern of the (001) face of the GeS cleaved in situ exhibits clear ordinary reflections. The dimensions of the surface unit mesh agree well to within experimental error (6%) with the corresponding dimensions of the bulk unit cell.The author wishes to thank Dr M.imeková for performing scanning electron microscopy pictures and Dr K.Jurek CSc. for EDAX measurements. Special thanks go to Dr M.Závtová who called my attention to this interesting material.