Edge Onset Research Articles

VUV and UV Florescence and Absorption Studies of Tb3+and Tm3+Trivalent Ions in LiYF4Single Crystal Hosts

Abstract The laser induced fluorescence spectra of LiYF4:Tb3+ (YLF:Tb) and LiYF4:Tm3+ (YLF:Tm) single crystals, pumped by an F2 pulsed discharge molecular laser at 157 nm, were obtained in the vacuum ultraviolet (VUV) and ultraviolet (UV) regions of the spectrum, at room temperature. A number of new fluorescence peaks were observed for the first time. They were assigned to the dipole allowed transitions 4f75d → 4f8 and 4f115d → 4f12 of Tb3+ and Tm3+ ions respectively. The absorption spectra of the same crystal samples in the VUV and UV regions were taken as well. The edge (onset) and the energy of the states with 4fN − 15d configuration were determined.

L3 Near-edge Structure in Germanium

Ge-L3 spectra in the edge region are devided into two groups according to measurements of absorption for thin films or total electron yield for bulk crystals. The absorption spectrum with a distinctive peak at the just onset of L3 edge is concluded to be a near one of a true L3 spectrum in Germanium. This result is discussed in relation with other experiments and the electronic structure of the germanium crystal.

Radiation damage characteristics of silicon oxynitride ceramic

Silicon Oxynitride is a ceramic that often forms during sintering of Si3N4, as a result of reactions with the sintering aid or from oxygen adsorbed on the surface of the powder particles. It is an orthorhombic crystal in which Si is tetrahedrally coordinated, and it is quite susceptible to electron irradiation effects. We present here the first quantitative observations of radiation effects.Time-resolved Electron Energy Loss Spectroscopy (EELS) was used to follow the changes in elemental concentration with irradiation time in crystals of Si2N2O during electron-irradiation at 100kV. The experiments were performed in a Philips EM400ST microscope fitted with a field emission source using a focussed probe and a Gatan 666 Parallel EELS spectrometer. An LN2 cooled specimen holder was used to minimize contamination. Specimens were made from a ceramic containing plate-type Si2N2O microcrystals with long edges parallel to [010], described earlier. The special software for time resolved spectrum acquisition was developed in this laboratory. The Si-L, O-K and N-K edges were acquired as a function of irradiation time at known current density and then individually processed and numerically integrated over a 100eV interval, beginning at edge onset.

ZnGeP2: A Wide Bandgap Chalcopyrite Structure Semiconductor for Nonlinear Optical Applications

ABSTRACTIn this paper we report the growth and characterization of wide bandgap chalcopyrite structure semiconductor ZnGeP2 epitaxial layers by open tube MOCVD and ZnGeP2 single crystals by high-pressure vapor transport (HPVT). The electrical and optical properties of these crystals have been investigated by Hall measurements, absorption spectroscopy, photoconductivity and photoluminescence spectroscopy. The onset of the absorption edge for HPVT ZnGeP2 at 1.99 eV is in accord with the pseudodirect bandgap of this material. Both the meltgrown crystals and epitaxial layers show extended band tailing in the transparency range of ZnGeP2 depending on the growth conditions. Recently we have shown that epitaxial ZnGeP2 films can be grown with excellent surface morphology by MOCVD on (111) GaP substrates. These films should be suitable for phase matched second harmonic generation utilizing CO laser radiation in thick film heterostructures.

Investigation by X-Ray Absorption Spectroscopy of Platinum Clusters Supported on Zeolites

Abstract The onset of the LIII edge of the X-ray absorption spectrum of platinum in F-zeolites involves an electronic transition from core to band levels producing a so-called white line. The spectral area of this increases and the edge position shifts to higher energies, first when the platinum particle size becomes small enough so that the clusters are in the supercages, second when the clusters are covered with oxygen, and third when multivalent cations are introduced in the zeolite. Both features of the absorption edge indicate that under the three above circumstances, platinum becomes more electron deficient. This conclusion had been inferred earlier from the enhanced catalytic activity of Pt clusters in Y zeolites.

Dissociative photoionization of t-butyllithium

Tertiary-butyllithium, Li4(t-bu)4, was examined by photoionization spectroscopy and optical absorption in the wavelength range 2000–950 Å. An absorption edge and ionization onset were found together at 6.2±0.25 eV. The ion Li4(t-bu)3+ and neutral isobutylene were found as photolysis products between the onset and 8.1±0.25 eV. Additional ionic fragments, Li4(t-bu)2+, Li4(t-bu)+, and Li2(t-bu)+ appeared at 8.1±0.25 eV. Hydrocarbon ions appeared above 11 eV.

Fundamental Optical Phenomena in Infrared Window Materials

A wide variety of high-energy lasers and associated applications (1) and optical systems with very long path lengths [e.g. fiber optics (2)J have emerged in recent years. These developments have led to corresponding requirements for highly transparent window materials (3). Ideally, a window material would transmit light without any effect on either the light beam or the window. Obviously, no such ideal material exists or is really required; moreover, specific types of beam modi­ fications are often not only tolerable but desirable. Thus, for example, we may require a material which refracts light, modulates light intensity, modifies polariza­ tion, or the like, yet acts as a window in all other respects. Our principal emphasis here is on properties of materials for totally passive windows, although those appropriate for active components are considered briefly. We are concerned with the fundamental optical phenomena of absorption, refraction, and photo­ elasticity in window materials. The latter phenomena underly or contribute to more complex optical effects such as thermally induced lensing (4), laser-induced interference (5), laser-induced breakdown (6), ilnd self-focusing (7), for example. Typical observations of laser-induced lensing and interference in infrared materials are indicated in Figure 1. The primary emphasis here is on bulk infrared materials, especially those for 10 11m (C02) and 3-5 11m (chemical) laser applications. For this case, one is then concerned with the highly transparent frequency regime in insulating or semiconducting solids, which lies well above the principal lattice absorption bands, but well below the onset of the electronic absorption edge. Clearly, in order to achieve high transparency over an extended frequency region, materials with low lattice resonances Wo and high-energy gap frequencies Wg are desirable. It is useful to correlate the latter properties with molecular weight and with families of compounds of the same structure, as pointed out by Hilton (8). The graphs in Figure 2 vividly indicate the decrease in wyand increase in Wo as

Absorption and Electroabsorption of Trigonal Selenium near the Fundamental Absorption Edge

The absorption spectrum and the spectrum of electroabsorption of vapor-grown trigonal Se platelets have been measured at various temperatures down to 2\ifmmode^\circ\else\textdegree\fi{}K. The absorption curves for both $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ensuremath{\parallel}c$ and $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ensuremath{\perp}c$ show several steps at the onset of the absorption edge. No Urbach tail has been found. The steps are interpreted as arising from the indirect excitation of an exciton at 1. 853 eV (low-temperature value) with several phonons. The electroabsorption spectra confirm the step structure within the absorption edge. Finally, it is shown that the luminescence of Se can also be explained with the same model.