Ab initio absorption spectra and optical gaps in nanocrystalline silicon

The optical absorption spectra of Si(n)H(m) nanoclusters up to approximately 250 atoms are computed using a linear response theory within the time-dependent local density approximation (TDLDA). The TDLDA formalism allows the electronic screening and correlation effects, which determine exciton binding energies, to be naturally incorporated within an ab initio framework. We find the calculated excitation energies and optical absorption gaps to be in good agreement with experiment in the limit of both small and large clusters. The TDLDA absorption spectra exhibit substantial blueshifts with respect to the spectra obtained within the time-independent local density approximation.     

Infrared absorption spectra and compositions of evaporated silicon oxides (SiOx)

Clear and simple relationships among oxygen content, peak position in the 9–10 μm region and intensity of infrared absorption were obtained for a wide range of x value in evaporated silicon oxides (SiO_x). Using these relationships, the origin of 875 cm⁻¹ peak was proposed to be due to a vibrational mode from a structural combination of Si---(O_ySi_4-y) (y = 2, 3 and 4).     

Photoacoustic investigations of the optical absorption spectra of porous silicon layers on a silicon backing

This paper presents a series of experimental photoacoustic spectra of porous silicon layers on crystalline silicon and their numerical analysis performed in the proposed two-layer model. The goal of the analysis was to calculate the optical absorption spectra of porous silicon from the photoacoustic spectra of porous silicon layers on a silicon background. The experimental character of the observed absorption band associated with the porous silicon was revealed. This is the first attempt at a theoretical interpretation of the photoacoustic spectra of porous silicon on a silicon backing.     

Comparison of thermally detected optical absorption and electromodulation spectra of GaAs/AlGaAs quantum wells

We report for the first time a comparative study of GaAs/AlGaAs quantum well (QW) spectra obtained by thermally detected optical absorption (TD-OA) and photo- (PR) and electroreflectance (ER) experiments, respectively. The excitonic transition energies, obtained at low temperature with these methods, agree within 1 meV. It is demonstrated, using samples with different AlGaAs overlayer thicknesses, that the shape of the TD-OA response is strongly influenced by interference effects for QW's grown on absorbing substrates. A complete PR lineshape analysis yields phase angles for all transitions, whose differences can be attributed to the change of the AlGaAs front barrier thicknesses. ER investigations following successive top layer removal confirm this interpretation.     

LII,III emission, LIII absorption, Kβ5 emission, K absorption spectra of chromium from pure metal and nitrides with the electron microprobe

The characteristic L_{II, III} and Kβ₅ X-ray emission bands of chromium in the pure metal and the nitrides Cr₂N and CrN are investigated with an electron microprobe. Also presented are the chromium L_III and K absorption replicas obtained using the self-absorption method. The results are discussed with reference to the theories proposed in the literature.     

ESR and optical absorption spectra of Cu2+ in LiKSO4

ESR spectra of Cu²⁺ in LiKSO₄ have been studied at different temperatures. The measured g-values suggest a rhombic field for Cu²⁺ ion in the lattice. Optical absorption spectra of the crystal have been studied at room and liquid nitrogen temperatures. From the nature and position of the observed bands, they have been attributed to Cu²⁺ ions in D_2h symmetry. The orbital reduction parameters obtained indicate a substantial degree of covalency in the bonding of the copper ion.     

Low-temperature optical absorption spectra of hematoporphyrins

Low-temperature optical absorption spectra of both free base and metal complexes of hematoporphyrin have been obtained. Vibrational structure, inferred by others from MORD data, is observed here directly in bands III and IV of the free base. Structure is also observed in the β band of the Ni and Pd metallohematoporphyrins. Implications of these results on the presently accepted theory of porphyrin optical spectra are discussed.     

The electron energy-loss and Auger spectra of Ir4(CO)12: Comparison with XPS shake-up and UV-absorption spectra

Electron energy-loss and Auger spectra of condensed Ir₄ (CO)₁₂ are presented in relation to corresponding phenomena for molecularly adsorbed CO and to XPS shake-up in transition-metal carbonyls. Distinctions between the properties of mono-metal-atom clusters and multi-metal-atom clusters are established, and possible differences in the electronic excitation of weakly and strongly adsorbed CO are discussed.     

Infrared absorption spectra of 4H silicon carbide

We performed infrared absorption measurements on 4H-SiC samples with polarization E∥c and E⊥c at 8, 85 and 300 K. From the strong temperature dependence of the absorption lines, electronic transitions are separated from vibronic transitions. The electronic transition lines between 300 and 500 cm^-1 are assigned to the shallow nitrogen donor. It is found that the electronic transitions of the shallow nitrogen donor are polarization dependent. Zeeman spectroscopy was performed to study the influence of the magnetic field on the electronic transitions up to 15 T. The results show no linear Zeeman splitting and only a diamagnetic shift. This is consistent with the effective mass tensor of three different diagonal components in 4H-SiC. Received: 25 November 1999 / Accepted: 20 April 2000 / Published online: 2 August 2000     

Infrared absorption spectra of amorphous silicon with gold

We determined the optical constants below the absorption edge of amorphous films of Si_1?xAu_x (x between 0.07 and 0.30) prepared by getter sputtering in argon. In the range 0.15 to 0.5 eV we found that the absorption increased with increasing Au content and could be described by Mott's formula for a.c. conductivity σ(ω) ≈ ω²[In (I₀/ω)]⁴ with I₀ ? 4.5 eV at x = 0.11 and 3.8 eV at x = 0.29; in the range 10–100 K it changed only slightly with temperature. The absorption is interpreted as due to direct transitions involving Au atoms.     

InSb纳米晶体的生长和吸收光谱的研究

采用在惰性气体中蒸发的方法获得了沉积在ZnS基片上的InSb纳米晶体,其平均尺寸随惰性气体的压强增加而增大.从实验测量的室温吸收谱上看到,当纳米晶体的平均直径从27.9 nm减小到24.2nm再到21.4 nm时,其吸收边分别向高能方向移动了0.0151 eV和0.0145 eV.用有效质量近似模型计算了半导体纳米晶体的吸收边相对其体材料的移动,将理论计算与实验结果进行了比较.     

Electron-energy loss spectra and optical constants of uranium

The energy loss of 40 keV electrons in thin uranium films has been determined in transmission for the spectral region 2–220 eV. The energy-loss function, the complex dielectric constant and the optical absorption coefficient have been computed.     

Deconvolution of loss features from electron spectra

The problem of deconvoluting loss features from energy spectra of electrons emitted from solids is investigated. A new formula is found for the case of an exponentially decreasing intensity from the solid surface of electron emitters. This as well as a formula for the case of a homogeneous distribution of electron emitters is studied in detail. Central in the formulas is the cross section for inelastic scattering. Under the assumption that angular deflection of electrons can be ignored the formulas are exact, i.e. the effect of all multiple scattering events is included. Through numerical calculations on Al spectra the effect of deconvolution is tested, and it is demonstrated that a small deviation from the exact procedure can result in spurious structure in the deconvoluted spectrum.     

Photogeneration,optical absorption and transport in hydrogenated sputtered amorphous silicon

By combining direct optical transmission with steady state photocurrent and photoconductivity gain measurements we evaluated the optical absorption constant of sputtered hydrogenated a-Si between 10⁵ to 10^?1cm^?1. The photogeneration process involves excitation from the valence band or from defect states in the middle of the gap to the conduction band, with the electrons making the major contribution to the photocurrent. The electron drift mobility, determined from the photoconductivity studies, is considerably smaller than that determined from the time of flight technique, due to trapping at deep centers.     

Optical absorption and electron energy loss spectra of carbon and boron nitride nanotubes: a first-principles approach

We present results for the optical absorption spectra of small-diameter single-walled carbon and boron nitride nanotubes obtained by ab initio calculations in the framework of time-dependent density-functional theory. We compare the results with those obtained for the corresponding layered structures, i.e. the graphene and hexagonal boron nitride sheets. In particular, we focus on the role of depolarization effects, anisotropies, and interactions in the excited states. We show that the random phase approximation reproduces well the main features of the spectra when crystal local field effects are correctly included, and discuss to what extent the calculations can be further simplified by extrapolating results obtained for the layered systems to results expected for the tubes. The present results are relevant for the interpretation of data obtained by recent experimental tools for nanotube characterization, such as optical and fluorescence spectroscopies, as well as polarized resonant Raman scattering spectroscopy. We also address electron energy loss spectra in the small-q momentum-transfer limit. In this case, the interlayer and intertube interactions play an enhanced role with respect to optical spectroscopy. PACS 71.45.Gm; 77.22.Ej; 78.20.Bh; 78.67.Ch     

Electronic excitation in phosphorus-containing molecules. II. Inner shell electron energy loss spectra of PF5, OPF3 AND OPCl3

Electron energy loss Spectroscopy has been used to obtain the inner shell excitation spectra of PF₅, OPF₃ and OPCl₃ in the P 2p,2s (L-shell) region as well as in the respective ligand K shell (F 1s, O 1s) and L shell (Cl 2p and 2s) regions. The spectra are compared and contrasted with earlier reported spectra obtained on the trivalent phosphorus compounds (PH₃, PCl₃, PF₃ and P(CH₃)₃). The spectra were obtained using an impact energy of 2.5keV and a scattering angle of about 1°. The spectra reported here are typical of molecules with electronegative ligands in that the discrete portions of the spectra show strong transitions to virtual molecular orbitais. In addition, intense features are observed at or just beyond the ionization edge attributable to transitions to trapped inner well states, while broad features further into the continuum can be ascribed to σ^*(P—L) shape-resonances (L = ligand). This resonance assignment was supported by a comparison with the corresponding spectra for PF₃ and PCl₃.     

The absorption and fluorescence spectra of matrix-isolated CF2

The CF₂ absorption spectrum between 2700 and 2250 Å and its fluorescence spectrum excited by radiation near 2500 Å have been observed in matrix isolation experiments. The positions of the absorption bands are somewhat dependent on the environment in which the CF₂ molecule is trapped. The absorption spectrum can be explained in terms of a progression in the upper-state bending fundamental, with a band spacing of 499 cm⁻¹, and the fluorescence spectrum involves a progression in the ground-state bending fundamental, with a band spacing of 657 cm⁻¹. A significant deviation between the calculated and observed separation of any given absorption and emission band can be explained by considering the effect of phonon interactions. Arguments are presented suggesting that the band origin lies near 36 900 cm⁻¹ for CF₂ produced by the photolysis of CF₂N₂ in an argon matrix.     

Energy loss spectra,auger spectra and high energy electron diffraction of A-15 superconductors

We have measured the electron energy loss spectra of V₃Si, Nb₃Al, Nb₃Sn, Nb₃Ge (annealed) and Nb₃Ge (sputtered) from 3 to 70 eV. Numerous new structures were discovered in this study which could be related to interband transitions, plasma excitations, and core transitions. We have also measured the high energy electron diffraction patterns and Auger spectra which characterized the lattice structures and compositions of these surfaces.     

Birefringence and anisotropic optical absorption in porous silicon

The refractive indices and the coefficients of optical absorption by free charge carriers and local vibrations in porous silicon (por-Si) films, comprising nanometer-sized silicon residues (nanocrystals) separated by nanometer-sized pores (nanopores) formed in the course of electrochemical etching of the initial single crystal silicon, have been studied by polarization-resolved IR absorption spectroscopy techniques. It is shown that the birefringence observed in por-Si is related to the anisotropic shapes of nanocrystals and nanopores, while the anisotropy (dichroism) of absorption by the local vibrational modes is determined predominantly by the microrelief of the surface of nanocrystals. It is demonstrated that silicon-hydrogen surface bonds in nanocrystals can be restored by means of selective hydrogen thermodesorption with the formation of a considerable number of H-terminated surface Si-Si dimers.