Luminescence centers in porous silicon

Photoluminescence studies on porous silicon show that there are luminescence centers present in the surface states. By taking photoluminescence spectra of porous silicon with respect to temperature, a distinct peak can be observed in the temperature range 100–150 K. Both linear and nonlinear relationships were observed between excitation laser power and the photoluminescence intensity within this temperature range. In addition, there was a tendency for the photoluminescence peak to red shift at low temperature as well as at low excitation power. This is interpreted as indicating that the lower energy transition becomes dominant at low temperature and excitation power. The presence of these luminescence centers can be explained in terms of porous silicon as a mixture of silicon clusters and wires in which quantum confinement along with surface passivation would cause a mixing of andX band structure between the surface states and the bulk. This mixing would allow the formation of luminescence centers.     

CW diode pumping and FM mode locking of a Nd: KGW laser

We have demonstrated cw diode end pumping of Nd: KGW, a novel solid-state gain medium, with up to 30% conversion efficiency into near-TEM₀₀ (M² < 1.05) output at = 1.067 µm for a pump level of 2.7 W. The slope efficiency was limited by intracavity reflections to 36%; however, direct comparison to a similar Nd:YAG laser indicates the same intrinsic slope efficiency of 60%. FM mode locking of this laser at 200 MHz has produced 12 ps pulses (compared to 16 ps for Nd: YAG), although an intracavity etalon was required. Considerable reduction in pulse width is possible (the line width limit is 0.5 ps) but different techniques may be necessary. Spatial hole burning was evident in both the 120 GHz free-running spectrum and the etalon-limited mode-locked spectrum.     

Fixed-point densities for a quasiperiodic kicked-oscillator map

Suppose that a one-dimensional harmonic oscillator is subjected to instantaneous kicks q times per natural period, with the kick amplitude varying sinusoidally with position. Viewed stroboscopically in phase space, the motion has an infinitely extended periodic or quasiperiodic array of fixed points, as well as an infinite web of chaotic orbits. In the present work (restricted to the quasiperiodic case q=5) the fixed points are classified according to their local linear behavior, which depends essentially on a single variable, the residue R. With the aid of a five-dimensional embedding, a function rho(R) is calculated which for infinitesimal DeltaR gives the average density of fixed points in the plane with residue in the range (R,R+DeltaR). The location and strength of the singularities and discontinuities of rho(R) are extracted from relatively simple transcendental equations, and this makes possible efficient numerical determination of rho(R). An exact equality for the densities of positive-R and negative-R fixed points is proved using decagonal symmetry and the integral representation of rho(R). For parameter values below the period-doubling threshold, there are no unstable fixed points with R greater, similar 0, and so we have equality of the densities of stable centers and unstable saddles. (c) 1995 American Institute of Physics.     

Reaction studies of carbon clusters

Reaction studies of carbon clustersC _n in the rangen=8–37, produced by laser vaporisation in a supersonic nozzle, have been investigated using time-of-flight mass spectrometry. Clear differences in reaction products formed on hydrogenation are detected which show that even and odd clusters behave quite differently and furthermore thatat least two different types of even cluster appear to exist. The reactivity patterns for clusters C _n withn=16, 18 and 22 are in a different class from those withn=20, 24, 26 ..., a behaviour consistent with the existence of closed cage fullerene structures for even clusters with 20 or more carbon atoms (other thann=22).     

The development of a sampler-sensor using a vanadium-oxinate-polymer film for the selective and direct determination of atmospheric ethanol

The development of a sensor for the direct and selective determination of atmospheric ethanol is in the initial stages. The sensor takes advantage of the selective chemical reaction between ethanol and vanadium oxinate. This reaction occurs in an organic medium where a red colored complex is the product. This reaction is determined spectrophotometrically where the absorbance maxima is 475 nm. The focus of this paper is to discuss the parameters necessary to develop a solid sorbent sampling-sensor which can be used to determine atmospheric ethanol.     

Shed plane leaves as a source ofα-tocopherol

A natural source of -tocopherol — shed plane leaves — has been found. The dynamics of the accumulation of a-tocopherol over the vegetation period of the plant has been studied, and a method has been developed for its isolation and quantitative determination.Deceased.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent, fax (3712) 89 14 75. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 372–374, May–June, 1995. Original article     

The development of a versatile field program for measuring tritium in “real-time”

Robust sample handling and liquid scintillation counting (LSC) procedures have been developed to routinely monitor tritium in the field relative to the 20,000 pCi/L drinking water standard. This procedure allows tritium to be monitored hourly during 24 hour drilling operations at depths in the saturated zone potentially contaminated by sub-surface nuclear weapons testing at the Nevada Test Site. Using retrofitted, shock hardened, vibration damped counters and strict analytical protocols, tritium may be measured rapidly in the field under hostile conditions. Concentration standards and dead tritium backgrounds are prepared weekly in a central laboratory and delivered to remote drilling locations where they are recounted daily as a check on counter efficiency and calibration. Portable LSC counters are located in trailers and powered off a battery pack and line filter fed by mobile generator. The samples are typically groundwaters mixed with drilling fluids returned after circulation through a drill string. Fluids are aerated and de-foamed, filtered, mixed with scintillation cocktail and dark adapted before counting. Real-time monitoring affords drilling and field personnel early warning against intercepting down-gradient plumes of radioactivity. For routine operations, the tritium activity may not exceed a 10,000 pCi/L threshold.     

Hot atom chemistry of mixed cristals 35 years of research

A historical survey of the author's contribution to the progress of solid state hot atom chemistry including more personal commentaries is presented.Presented at the International Seminar on the Chemistry of High Energy Atoms, Tokyo Institute of Technology, Ookayama, Meguro-ko, Tokyo 152, Japan, October 28–30, 1992, organised by T. MATSUURA, Institute for Atomic Energy, Rikkyo University, Nagasaka, Yokosuka 240-01, co-editor of the Handbook of Hot Atom Chemistry. I thank Professor MATSUURA for the permission to publish my contribution to the seminar slightly modified compared with the version submitted in the seminar proceedings.     

Agent for chemical dissolution of oxide layers grown on heat transfer surfaces of carbon steel components of nuclear and classifical power plants

Inhibited buffer-complexing ferrous citrate+citric acid solutions, 2.5<pH<3.5, at 363±5K, is used to dissolve a compact magnetite layer grown on low alloyed steel in a nuclear plant generator. The average rate of oxide dissolution is 40 m/h. Similarly, a compact duplex layer of magnetite covered by hematite formed on a carbon steel pipe of a classical heat exchanger dissolves in less than 10 h. The mean rate of metal uniform corrosion ranges between 1 and 9 mg/cm² h depending on pH, inhibitors and citrate ion concentration.     

Investigations on the determination of NO2 with galvanic nitrate solid electrolyte cells

Solid electrolyte cells for detecting NO₂ with Ba(NO₃)₂ or Sr(NO₃)₂ partially replaced by -Al₂O₃ as solid electrolytes have been studied. The cell tension depends on the NO₂- as well as on the NO-concentration. Investigations of the establishment of the NO₂-NO-O₂-equilibrium by the catalytic effects of the used electrode materials Pt and Au have shown that a decomposition of NO₂ below 400°C cannot be expected.17th Communication on oxoanionic solid electrolytes and their possible applications; 16th Communication see Solid State Ionics 51:183–185 (1992)Extension of a part of the lecture of V. Brüser, S. Jakobs, H.-H. Möbius and U. Schönauer Galvanische NO₂-Sensoren mit Nitrat-Festelektrolyten given in Feldberg, 3. 6. 1993