Direct Energy Absorption Research Articles

Optical and Structural Properties of Manganese‐Doped Zinc Oxide Grown by Metal‐Organic Chemical Vapor Deposition

AbstractThis systematic study investigates the optical properties and process−structure−property relationships of Mn‐doped zinc oxide (ZnMnO) grown by metal‐organic chemical vapor deposition with varying Mn‐doping concentration and growth conditions. ZnMnO exhibits a good crystal quality oriented in the (002) direction and contains intermixtures of zinc oxide (ZnO)‐like and manganese oxide (MnxOy)‐like phases. The material exhibits a direct energy absorption band‐edge and a reduction in bandgap with Mn‐doping. Photoluminescence studies show that Mn‐doping can simultaneously tailor broad green band luminescence and ultraviolet edge emissions. Post‐growth air‐annealing results in broad MnxOy‐related photoluminescence emissions at 3.3–4.5 eV. A further reduction in the absorption band‐edge is also observed with annealing. Results indicate that luminescence wavelengths and intensities, and absorption band‐edge can be tuned with the Mn‐doping process. This paper promotes a thorough understanding of defect centers in ZnO with transition metal doping and their interrelation with optical characteristics. The work provides a solid foundation for the development of optoelectronic devices, such as light emitting diodes, solar cells, lasers, and photodetectors using ZnO‐based materials.

Persistent luminescence excitation spectroscopy of BaAl2O4:Eu2+,Dy3+

BaAl2O4:Eu2+,Dy3+ is related, both by structure and luminescence, to one of the best persistent luminescent phosphors, SrAl2O4:Eu2+,Dy3+. At room temperature (RT), the green persistent emission of BaAl2O4:Eu2+,Dy3+ remains visible for hours after ceasing irradiation. Similar to SrAl2O4, BaAl2O4 with hexagonal P63 structure, has two M2+ sites, but, limited optical activity from the 2nd site is observed in the emission of BaAl2O4:Eu2+,Dy3+ - even at 77 K. Using combined approach of photoluminescence, thermoluminescence (TL), and persistent (excitation) luminescence measurements, the origin and properties of persistent luminescence of BaAl2O4:Eu2+,Dy3+ were studied in detail. Ultraviolet (UV) excited and persistent emission are identical and no contribution from the Eu2+ in the high-symmetry Ba site was observed. TL excitation spectra clarified the unstructured conventional excitation spectrum; now it is evident that defects or the Dy3+ co-dopant do not contribute to persistent luminescence via direct energy absorption. Mechanisms for persistent luminescence should thus be revised.

Microwave plasma enhancement of multiphase flames: On-demand control of solid propellant burning rate

This effort explores the microwave-supported plasma enhancement of an aluminized, ammonium perchlorate composite solid propellant flame. The technique is enabled through novel alkali metal doping, which provides increased levels of ionization and allows efficient microwave energy deposition to the flame structure and subsequent perturbation of the steady-state propellant burning rate. Three potential modes of energy deposition are identified in composite propellants: (1) plasma-enhancement promoted by the presence of sodium, (2) strong absorption by high temperature metallic oxide products, and (3) direct absorption of energy by the propellant burning surface condensed phase. Equilibrium calculations are used to identify propellant compositions with high free-electron populations, and solution of the Boltzmann equation using the BOLSIG + code is used to show the significant effects of sodium doping on microwave absorption of the flame. Experimental emission spectroscopy and imaging of the microwave-enhanced propellant flame structure show evidence of these mechanisms. Propellant burning rates can be increased by up to 60% through the application of 1 kW, continuous 2.46 GHz microwave radiation. Propellant doping with 3.5% by weight of sodium nitrate shows significant improvement in the effectiveness of microwave application in modifying the propellant burning rate. This technique provides high levels of dynamic propellant combustion control from a solid-state system. The on-demand control of energetic material burning rates through low level doping and microwave irradiation is a promising technique which could lead to the development of a new class of ‘smart’ dynamically controllable energetic materials.

Picosecond Pulse Radiolysis of Highly Concentrated Phosphoric Acid Solutions: Mechanism of Phosphate Radical Formation.

Eight solutions containing phosphoric acid with concentrations ranging from 2 mol L(-1) to neat acid have been studied by picosecond pulse radiolysis. The absorbance of the secondary radical H2PO4(•) formed within 7 ps of the electron pulse is observed using pulse-probe method in the visible. Kinetic analysis shows that the radicals of phosphoric acid are formed via two mechanisms: direct electron detachment and oxidation by the radical cation of water, H2O(•+). On the basis of molar extinction coefficient value of 1850 L mol(-1) cm(-1), at 15 ps the radiolytic yield of H2PO4(•) formation by direct energy absorption is 3.7 ± 0.1 × 10(-7) mol J(-1) in neat phosphoric acid. In highly concentrated phosphoric acid solutions, the total yield of phosphate radical at 15 ps exhibits an additional contribution that can be explained by electron transfer from phosphoric acid to H2O(•+). The efficiency of the electron transfer to this strongly oxidizing species in phosphoric acid solutions is lower compared with the one in sulfuric acid solutions. Two explanations are given to account for a relatively low efficiency of H2O(•+) scavenging in concentrated phosphoric acid solutions.

A New FAPAR Analytical Model Based on the Law of Energy Conservation: A Case Study in China

The fraction of absorbed photosynthetically active radiation (FAPAR) characterizes the energy-absorption ability of the vegetation canopy. It is a critical input to many land-surface models such as crop growth models, net primary productivity models, and climate models. There is a great need for FAPAR products derived from remote-sensing data. The objective of this research is to develop a new instantaneous quantitative FAPAR model based on the law of energy conservation and the concept of recollision probability (p). Using the ray-tracing method, the FAPAR-P model separates direct energy absorption by the canopy from energy absorption caused by multiple scattering between the soil and the canopy. Direct sunlight and diffuse skylight are also considered. This model has a clear physical meaning and can be applied to continuous and discrete vegetation. The model was validated by Monte Carlo (MC) simulation and field measurements in the Heihe River basin, China, which proved its reliability for FAPAR calculations.

Spur Reactions Observed by Picosecond Pulse Radiolysis in Highly Concentrated Bromide Aqueous Solutions

The formation of the well-known product Br3(-), observed in the steady-state radiolysis of highly concentrated Br(-) aqueous solutions, has now been directly observed at ultrashort times corresponding to the relaxation of the spur. The transient absorption induced by picosecond pulse radiolysis of 6 M Br(-) aqueous solution was probed simultaneously at 260 nm with the third harmonic laser wave and from 350 to 750 nm with a supercontinuum generated by the fundamental laser wave. This approach allows several transient radiolytic species to be followed in parallel, particularly the solvated electron, BrOH(-•), Br2(-•), and Br3(-). The kinetics measured within 4 ns at 260 and 370 nm clearly exhibit that the decay of Br2(•-) is correlated with the formation of Br3(-). In highly concentrated Br(-) solutions, the OH(•) radical is fully replaced by Br2(•-), and the spur kinetics of OH(•) radical in pure water is comparable with that of Br2(-•). Model calculations indicate that the main OH(•) radical combination product H2O2 in pure water has formation kinetics similar to that of Br3(-) in 6 M Br(-) solutions. Moreover, they point out that oxidation of Br(-) occurs within the electron pulse both by direct energy absorption and by scavenging of the water radical cation, H2O(•+).

Endovenous Laser Ablation: A Review of Mechanisms of Action

The aim of this article is to summarize and review the proposed theories on the laser action during endovenous ablation. Laser mechanics and laser-tissue interaction are summarized from articles found in literature. Several theories, like the "steam bubble theory," the "direct contact theory," the "heat pipe," and "direct light energy absorption" are discussed. The laser light emitted intraluminally can be absorbed, scattered, or reflected. Reflection is negligible in the near-infrared spectrum. By combining absorption and scattering, the optical extinction of different wavelengths related to different biological tissues can be determined. The direct contact of the fiber tip and the vein wall may be a way of destroying the vein wall, but results in ulcerations and perforations of the vein wall. Avoiding this contact, and allowing direct light absorption into the vein wall, results in a more homogenous vein wall destruction. If the energy is mainly absorbed by the intraluminal blood, the laser fiber will act as a heat pipe. Histological studies show that a more circumferential vein wall destruction can be obtained when the vein is emptied of its intraluminal blood. The use of tumescent liquid reinforces spasm of the vein and protects the perivenous tissue. Several factors play an important role in the mechanism of endovenous laser ablation. Direct energy absorption by the vein wall is the most efficient mechanism. It is important to empty the vein of its intraluminal blood and to inject tumescent liquid around the vein.

Energy absorption and dissipation associated with mass activation and deactivation for open systems

Energy dissipation at the macroscopic scale, applied to large bodies, relies on using the average bulk or global properties. The normal procedure is to load/unload a uniaxial tensile specimen, and account for the difference of the area under the stress and strain curve, even though unloading does not occur in reality. The same procedure, however, is not feasible for treating the energy dissipation at the microscopic scale, applied to small bodies, where the space/time dependency of the local material properties plays a role. That is the transient character of the energy transfer between the specimen surface and surrounding can no longer be neglected. Moreover, there is no way to simulate microscopic unloading. Besides, the coupon test scheme of load/unload, an artifact, that has been used because of no other choice. Energy lost is an intrinsic process that defies empirical determination in the true sense. Incomplete homogeneous loading and/or unloading rate at all locations of the material gives rise to dissipation. This effect has been described by using the transitional function that entails multiscaling and segmentation for the simulation of material damage at the different scales. Segmented damage initiation/termination thresholds are invoked that may consist of, say pico- to nano-cracking and nano- to micro-cracking followed by micro- to macro-cracking and so on. The idealized “crack tip” is used to model the sink and source that can absorb and dissipate energy, respectively. The mass surrounding the crack tip is said to be activated by direct-absorption of energy and deactivated by self-dissipation of energy. The threshold for each scale range is assumed to depend on the square of the crack tip velocity a ˙ 2 and mass densities M ↓ ↑ and M ↓ ↑ such that W = M ↓ ↑ a ↑ ↓ 2 and D = M ↓ ↑ a ↑ ↓ 2 . The quantities W and D are referred to, respectively, as the direct-absorption and self-dissipation energy density. They fluctuate in time and determine whether the surrounding crack tip field of inhomogeneity is expanding or contracting. Singularity representation is used to capture the character of the crack tip field strength while scaling is reflected by the characteristic length. Demonstrated will be a pico/nano/micro/macro fatigue cracking model of a 2024-T3 aluminum panel. Only the undamaged material properties are employed. Time degradation of the pico/nano/micro/macro material structure behavior is derived by using nine scale transitional physical parameters: three for the pico/nano range ( μ pi / na ∗ , σ pi / na ∗ , d pi / na ∗ ), three for the nano/micro range ( μ na / mi ∗ , σ na / mi ∗ , d na / mi ∗ ) and three for the micro/macro range ( μ mi / ma ∗ , σ mi / ma ∗ , d mi / ma ∗ ). The subscripts pi, na, mi, and ma designate, respectively, pico, nano, micro, and macro. Only the ratios of two successive scale sensitive parameters need to be known. The time dependent physical parameters at the lower scale are implicit and needed only for analytical continuation. More specifically, the transitional character of picocracks, nanocracks, microcracks, and macrocracks are determined from the specified life expectancy of time arrow according to pico → nano → micro → macro with the respective singularity strength of λ given by 1.25/1.00/0.75/0.50. Recall that λ = 0.5 corresponds to the inverse square root r −0.5 in fracture mechanics with r being the distance from the macrocrack tip. The microcrack, nanocrack, and picocrack tips are assigned with the singularities r −0.75 and r −1.00and r −1.25, respectively. The time of arrow in years will depend on the problem definition. Progressive damage is assumed to occur in the direction of pico → nano → micro → macro. The same scheme is applied to the fatigue damage of a 2024-T3 panel with a total life time of 18.5 + to 20 years that may be distributed over the pico, nano, micro, macro, and structure scale according to 1.5 +/2.5 +/3.5 +/5.5 +/7.0 +. Such a specification can only be satisfied by matching the energy dissipated in damaging the internal material structure at each scale range. Hence, the precise time dependent material property degradation process over the total life span can be enforced. Material inhomogeneities at the different scales are thus compensated by the inhomogeneous reinforcements at the same different scales such that energy release rate at each scale would be relatively homogeneous and controlled.

Microwave activated combustion synthesis of bulk cobalt silicides

CoSi and CoSi 2 bulk intermetallic compounds have generated some interest as potential electronic and structural materials. Combustion synthesis has been shown to be an effective method for the preparation of many intermetallic compounds, but requires a high adiabatic temperature to propagate. Thermodynamic calculations show that cobalt silicides require activation. Microwave activation is attractive for combustion synthesis reactions, allowing rapid heating and cooling rates, volumetric heating, and direct absorption of energy by the reaction zone. In this study, a microwave activated combustion synthesis process was used to produce CoSi and CoSi 2 bulk cobalt silicides. Thermal activation and conventional combustion synthesis were also attempted, and the results of the three processes are compared.

The Studies of Microwave Effects on the Chemical Reactions

AbstractMicrowave heating involves direct absorption of energy by functional groups that bear ionic conductivity or a dipole rotational‐effect, and this energy is then released into the surrounding solution. This absorption of energy causes the functional groups involved to have higher reactivity to other surrounding reactants than when they are simply incubated with the reactants at the same temperature. In other word the enhanced rate of the reaction can be due to the reactant stirred by the molecular dipole rotation and molecules themselves acting as a stirring bar. In contrast to conventional heating, the salient feature of “dipole rotation” constitutes one efficient form of “molecular agitation” or “molecular stirring” many aspects of which can be explore in chemical reactions. We will discuss some of the useful applications of this “molecular agitation” by means of microwave irradiation. Using this unique technology, we have developed: 1) a method to control the cleavage sites of peptide bonds, especially those bonds connected to aspartic acid residues inside the native peptides and proteins, 2) a method to increase coupling efficiency in solid‐phase peptide synthesis using a common microwave oven, 3) a novel procedure that increases the rate of alcalase‐catalyzed reactions using microwave irradiation in peptide‐bond formation with proline as a nucleophile and selective benzoylation of a pyranoside derivative, 4) a procedure to solubilize and hydrolyze retrograded starch, 5) a novel procedure to enhance the rate of saponification in a serum sample for very long chain fatty acid analysis.

Controlled generation of internal cracks in polymethyl methacrylate

A simple technique is presented for generating internal cracks in polymethyl methacrylate (PMMA) using laser irradiation. With precise control of the laser beam, a well-defined single crack can be produced in the matrix. The shape of the crack is flat and regular, close to an ellipse with nearly equal major and minor axes. The size and orientation of the crack can be controlled through the laser energy and the adjustment of the optical system. The laser-induced crack in PMMA consists of three regions: the central damage region caused by direct energy absorption, the intermediate cracking region, and the outer circumferential region associated with crazing. These controlled internal cracks can be used to study crack propagation, interaction, and related problems.

Computational study on propagation and two-species-ion-plasma heating due to the ion-Bernstein wave

The propagation of the ion-Bernstein wave and resulting heating at 5 Omega D resonance are investigated in a two-component ion plasma (D-H) by means of particle simulation. The propagation of the ion-Bernstein wave observed in the simulation can be characterized by the linear theory. The wave energy is deposited locally around the 5 Omega D resonance region located in the center of the system due to the direct energy absorption into D and H through the 5 Omega D resonance and 5/2 Omega H subharmonic resonance. The bulk of the velocity distribution of H is heated. For the deuterium species, generation of high energy tail ions is observed as well as the heating of the bulk of the velocity distribution.

The mechanism of radiosensitization by electron-affinic compounds

“Electron-affinic” radiosensitizers generally mimic oxygen in modifying cellular radiosensitivity, although not necessarily by the same molecular mechanisms. Useful mechanistic probes include quantitative relationships between chemical properties and efficacy, although the latter can usually be expressed only in terms of extracellular concentrations. The response vs concentration curves can also provide a test of a mechanistic model. Some possible mechanisms of radiosensitization are outlined: sensitization of damage from direct energy absorption; oxidation of radicals on the sugar/phosphate backbone of DNA; oxidation of base radicals; and radiation-induced binding of radiosensitizers. Each model is approached from the viewpoint of the known redox relationships with radiosensitizers and competition with thiols which serve to reduce the effect.

Radical Processes Following γ-Irradiation of Frozen Concentrated Matrices of Heparin and Dextran Sulphate

Unlike D-glucose and sucrose ices, frozen concentrated aqueous solutions of heparin and sodium dextran sulphate after γ-irradiation do not efficiently trap electrons $(G{\rm e}_{t}^{-} . Using electron spin resonance, radical yields were measured for these polyanion ices, and G(radical) is shown to increase linearly with solute concentration. The radical yield, extrapolated from the concentrated polyanion ices to the solid state, is in good agreement with the experimental value determined at 77°K for γ-irradiated solid dextran sulphate. For heparin ices, however, the extrapolated value is approximately double the experimental value for the solid. The results indicate that whereas direct action effects adequately account for the radical production in dextran sulphate ices, there is evidence of electronic energy transfer from water molecules in association with the heparin which results in greater heparin degradation than from direct energy absorption. Similar effects could be responsibl...

Photomorphogenesis in Arabidopsis thaliana (L.) Heynh

Arabidopsis seeds were germinated on sterile mineral agar supplemented with 1% glucose and cultured under continuous light regimes. With 4-hour incandescent plus 20-hour monochromatic illumination in the region from 400 to 485 nanometers there was effective floral induction at an intensity of 100 microwatts per square centimeter. Exclusion of far red wave lengths from the 4-hour incandescent period sharply reduced the effectiveness of subsequent monochromatic blue light in promoting floral induction. Delayed floral induction occurred under continuous incandescent light lacking far red and was attributable to the blue wave lengths. Continuous 485 nanometer (100 microwatts per square centimeter) exposure without any white light treatment during the postgermination growth period was ineffective in floral induction and meristem development. Light at 730 nanometers under the same conditions was partially effective, whereas energy between 500 and 700 nanometers was completely ineffective. When continuous monochromatic light at a 3-fold higher energy level was administered, all photomorphogenic responses were accomplished with 485 nanometer light, including germination and 100% floral induction without any white light treatment at any time during the experiment. Almost equal quantum effectiveness was calculated when equivalent quantum flux densities in the region from 710 to 740 nanometers or at 485 nanometers were used. It is postulated that floral induction in Arabidopsis may be the result of a continuous excitation of a stable form of far red-absorbing phytochrome localized in or on a membrane, and that excitation can be either by direct absorption of energy by far red-absorbing phytochrome or by transfer from an accessory pigment.

Electron-affinic sensitization. I. A structural basis for chemical radiosensitizers in bacteria.

The bacterium Serratia marcescens has been used as a test system for demon- strating the radiosensitizing properties of a class of electron-affinic organic compounds. These include: unsaturated diesters and diketo compounds, mono-, di- and triketohydrindene (indanetrione), derivatives of pyruvic acid, quinones and several miscellaneous compounds. With few exceptions, the sensitization is restricted to anoxic systems. An electron-transfer mechanism, involving interaction of the sensitizer with a charged intermediate produced by direct energy absorption in a target molecule, is proposed as the basis of a model for radiosensitization. Bridges (1969) has discussed radiosensitization at different levels of biological complexity and has made a critical evaluation of the various mechanisms pro- posed in the literature. In this paper, an account is given of the development of a physico-chemical model for cellular radiosensitization based on the electron-affinic properties of the dose-modifying agents. In a previous publication (Adams and Dewey 1963), a mechanism was suggested in which the sensitizing effects of N-ethylmaleimide, diacetyl and benzophenone were attributed to their ability to act as electron-transport agents in enhancing the damaging effects of hydrated electrons produced by energy absorption in the aqueous component of the cell. Although this earlier model was empirically successful, in that it led to the characterization of most of the bacterial radiosensitizers discussed in this paper, it has serious limitations. An alternative model, also based directly on electron-affinity concepts, but more consistent with recent data, is therefore proposed in the discussion. However, since this mechanism was developed from the earlier hypothesis, a brief des- cription of the latter is included in this introduction. The main postulate of this model was that the sensitizing properties of N-ethylmaleimide were attributed to its ability to capture and stabilize a hydrated

Energy Migration and Transfer in Solid Argon and Krypton at Low Temperatures

The electron spin resonance spectra of CD/sub 3/ and D radicals produced in solid Ar and Kr by gamma irradiation at 4.2 deg K are studied. CD/sub 4 mole fractions of 10/sup -5/ to 10/sup -1/ are irradiated with doses up to 1.6 x 10/ sup 5/ r. It is concluded that a large fraction of the CD/sub 3/ and D radicals are produced by energy transfer from the rare gas to the CD/sub 4/, rather than by direct energy absorption by the CD/sub 4/ molecule from the photons. (T.F.H.)

Effects of Hydration on X-Ray Sensitivity in Hordeum

Shortly after Stadler (1) conducted his classical experiments with barley seeds showing that X-rays could induce mutations in plants, one of his students, Tascher (2), demonstrated that soaked barley seeds were more sensitive to X-rays than dry seeds. Since that time wet and dry seeds have been irradiated by numerous workers (Gelin, 3; Gustafsson, 4; Wertz, 5; Froier and Gustafsson, 6), and it has become widely accepted in radiobiology, primarily as a result of the studies with seeds, that, at the cellular level, as the water content of a biological system increases, so also does its sensitivity to X-rays (cf. 46a). The studies with soaked and dry seeds were conducted mostly during the 1930's and early 1940's when many workers considered that radiation-induced events were largely or solely a result of the direct absorption of energy by a sensitive site. Little attempt was made to clarify or interpret the nature of the modification of the radiosensitivity that occurred when water was added to seeds. However, in the 1940's when the was rediscovered (cf. Mottram, 7, 8) through the demonstration in a variety of organisms (9-12) that X-radiation in the presence of oxygen increased radiosensitivity, biologists began seriously considering the likelihood of the existence of other mechanisms by which radiation affected living tissues-other, that is, than by the direct absorption of energy at a sensitive site. The fact that radiation chemists early postulated the production of active radiation products from the radiodecomposition of water served to emphasize this possibility (13-20). Thus it developed that the theory of the indirect effect of X-rays came into prominence. This theory has as its basic tenet the concept that much radiation damage is mediated by active radicals, or other chemical entities, that are produced by radiation and that those radicals may be transported more or less distance in the cell to produce a detectable biological event-presumably, for the most part, by chemical modification of macromolecules. Additional support for the indirect action theory came from the findings that