共查询到20条相似文献，搜索用时 15 毫秒
1.
The compelling need for standoff detection of hazardous gases and vapor indicators of explosives has motivated the development of a remotely pumped, high-gain air laser that produces lasing in the backward direction and can sample the air as the beam returns. We demonstrate that high gain can be achieved in the near-infrared region by pumping with a focused ultraviolet laser. The pumping mechanism is simultaneous resonant two-photon dissociation of molecular oxygen and resonant two-photon pumping of the atomic oxygen fragments. The high gain from the millimeter-length focal zone leads to equally strong lasing in the forward and backward directions. Further backward amplification is achieved with the use of earlier laser spark dissociation. Low-divergence backward air lasing provides possibilities for remote detection. 相似文献
2.
A high-temperature plasma is created when an intense laser pulse is focused onto the surface of a solid. An ultrafast pulse of x-ray radiation is emitted from such a plasma when the laser pulse length is less than a picosecond. A high-speed streak camera detector was used to determine the duration of these x-ray pulses, and computer simulations of the plasmas agree with the experimental results. Scaling laws predict that brighter and more efficient x-ray sources will be obtained by the use of more intense laser pulses. These sources can be used for time-resolved x-ray scattering studies and for the development of x-ray lasers. 相似文献
3.
High-temperature (about 2800 degrees C) vapor fractionation of a silicate glass that originally contained 82 percent by weight of SiO(2) resulted in a decrease of the silica content to 45 percent. Oxygen isotope analyses show that the O(18)/O(16) ratio increased from 13.80 per mil in the starting material to 14.47 and 15.03 per mil in the residuum. This suggests that bediasites, which also exhibit an increase in this ratio with decreasing content of silica, have been subjected to a process of vapor fractionation. 相似文献
4.
Jin C Haufler RE Hettich RL Barshick CM Compton RN Puretzky AA Dem'yanenko AV Tuinman AA 《Science (New York, N.Y.)》1994,263(5143):68-71
Laser radiation (XeCl laser, 308-nanometer wavelength) focused into a cell containing Mo(CO)(6) vapor produced ultrafine particles in the extended waist of the laser beam. Negative ion mass spectrometry revealed molybdenum carbide cluster ions with a stoichiometry MonC4n (n = 1 to 4). The MonC4n(-) (n = 2 to 4) ions are completely unreactive with NH(3), H(2)O, and O(2), suggesting structures in which the molybdenum atoms are unavailable for coordination to additional ligands. Collision-induced dissociation studies of these anions show the loss of MoC(4) units as the main fragmentation pathway. This observation, together with the lack of addition reactions, provides a basis for structures in which a planar cluster of two, three, or four molybdenum atoms is surrounded by, and bonded to, carbon dimers. 相似文献
5.
沉积物中磷形态的化学连续提取法应用研究 总被引：35，自引：1，他引：35
对水体沉积物中磷的环境地球化学研究中较普遍采用的7步法连续提取磷分级方案在实际应用中出现的一些问题进行了探讨。从该方法对水系沉积物成分分析标准物质GSD—12及太湖沉积物的分析结果看，该方法的应用中各形态的重现性问题必须得到重视，尤其是闭蓄态磷的提取中，实际操作中要尽量避免因沉积物固结容器底部而降低提取剂的提取效率，对于提取离心液过滤难的问题也一定要解决好。可交换态磷、铝磷和铁磷的提取液由于磷浓度低，可以不将其后的漂洗液混入后测定，以提高待测液中磷的浓度。另外，从GSD—12与太湖沉积物中磷形态特征的相似性看，GSD—12可以考虑作为淡水水体沉积物磷形态分级的标准物质，从而解决不同文献、不同研究项目在沉积物形态分析方面缺乏可比性的问题。 相似文献
6.
Dupuis RD 《Science (New York, N.Y.)》1984,226(4675):623-629
Metalorganic chemical vapor deposition (MOCVD) is a process in which two or more metalorganic chemicals (for instance, trimethylgallium) or one or more metalorganic sources and one or more hydride sources (for instance, arsine, AsH(3)) are used to form the corresponding intermetallic crystalline solid solution. MOCVD materials technology is a vapor-phase growth process that is becoming widely used to study the basic physics of novel materials and to grow complex semiconductor device structures for new optoelectronic and photonic systems. The MOCVD process is described and some of the device applications and results that have been realized with it are reviewed, with particular emphasis on the III-V compound semiconductors. 相似文献
7.
Wiechert U Halliday AN Lee DC Snyder GA Taylor LA Rumble D 《Science (New York, N.Y.)》2001,294(5541):345-348
We have determined the abundances of 16O, 17O, and 18O in 31 lunar samples from Apollo missions 11, 12, 15, 16, and 17 using a high-precision laser fluorination technique. All oxygen isotope compositions plot within +/-0.016 per mil (2 standard deviations) on a single mass-dependent fractionation line that is identical to the terrestrial fractionation line within uncertainties. This observation is consistent with the Giant Impact model, provided that the proto-Earth and the smaller impactor planet (named Theia) formed from an identical mix of components. The similarity between the proto-Earth and Theia is consistent with formation at about the same heliocentric distance. The three oxygen isotopes (delta17O) provide no evidence that isotopic heterogeneity on the Moon was created by lunar impacts. 相似文献
8.
Sedimentation field flow fractionation is a powerful, new, high-resolution separation method for a wide variety of colloids, micelles, particulates, and soluble macromolecules of biological interest. Advances in instrumentation allow sedimentation field flow fractionation operation with rotor speeds up to 32,000 revolutions per minute ( approximately 85,000 gravities), which permits separation of materials as small as 5 x 10(5) molecular weight, depending on sample density. Compared to conventional centrifugation techniques, the gentle, mass-separating sedimentation field flow fractionation method is capable of higher resolution in shorter times. 相似文献
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10.
We report on an all-optical switch that operates at low light levels. It consists of laser beams counterpropagating through a warm rubidium vapor that induce an off-axis optical pattern. A switching laser beam causes this pattern to rotate even when the power in the switching beam is much lower than the power in the pattern. The observed switching energy density is very low, suggesting that the switch might operate at the single-photon level with system optimization. This approach opens the possibility of realizing a single-photon switch for quantum information networks and for improving transparent optical telecommunication networks. 相似文献
11.
Absorption lines in the v, band of water vapor at 6.3 micrometers have been fully resolved by using a tunable semiconductor laser. Three attnospheric water vapor lines near 5.32 micrometers were studied in detail and found to have linle widths two to four times narrower than the width calculated by Benedict and Kaplan. 相似文献
12.
Waterlogged molecules 总被引：4，自引：0，他引：4
R Wolfenden 《Science (New York, N.Y.)》1983,222(4628):1087-1093
Measurements of vapor pressures over their aqueous solutions indicate that organic compounds show profound differences in hydrophilic character. These differences are of such magnitude as to suggest an important role for changing solvation in determining free energy changes associated with metabolic transformations in water, and in governing structural equilibria of proteins and other large molecules in water. When two or more functional groups are present within the same solute molecule, their combined effects on its free energy of solvation are commonly additive. Striking departures from additivity, observed in certain cases, indicate the existence of special interactions between different parts of a solute molecule and the water that surrounds it. Similar considerations presumably apply to activated intermediates in the interconversion of biological materials. 相似文献
13.
利用微生物从低廉的纤维素、半纤维素及其水解产物生产生物燃料乙醇受到高度重视。代谢工程技术构建和改造高产乙醇重组菌成为研究的重点。着重概述了产乙醇重组大肠杆菌的研究进展。以大肠杆菌作为模式菌株进行乙醇代谢工程改造的研究和探索,将为新型的产乙醇重组微生物提供技术依据和借鉴。 相似文献
14.
Ultraviolet laser microprobe analyses of a calcium-aluminum-rich inclusion (CAI) from the Allende meteorite suggest that a line with a slope of exactly 1.00 on a plot of delta17O against delta18O represents the primitive oxygen isotope reservoir of the early solar nebula. Most meteorites are enriched in 17O and 18O relative to this line, and their oxygen isotope ratios can be explained by mass fractionation or isotope exchange initiating from the primitive reservoir. These data establish a link between the oxygen isotopic composition of the abundant ordinary chondrites and the primitive 16O-rich component of CAIs. 相似文献
15.
Water isotope ratios have been measured by laser absorption spectroscopy in and out of cirrus clouds formed in situ and convectively generated in anvils over subtropical regions. Water vapor in the tropical and subtropical upper troposphere shows a wide range of isotopic depletion not observed previously. The range suggests that dehydration of upper tropospheric air occurs both by convective dehydration and by gradual dehydration mechanisms. Twenty-five percent of upper tropospheric water sampled is in ice particles whose isotopic signatures are used to identify those grown in situ from those lofted from below. 相似文献
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厘清饱和水汽压差(VPD)的年际动态特征有助于深入理解大气干湿程度对气候变化的响应。本研究基于我国西南地区105个台站1982—2015年的气象观测资料,利用Mann-Kendall突变检验分析VPD的突变特征,基于统计分析探讨突变点前后的VPD变化趋势及影响因素。研究结果表明,西南地区1982—2015年VPD变化以2000年为突变年,增速由0.001 kPa/a变为0.005 kPa/a,并且VPD呈增加趋势的站点在2000年后增多。VPD在时间动态上出现变化的主要原因是在2000—2015年间相对湿度下降幅度增加,导致西南地区出现更干燥的气候特征。作为大气主要水汽来源的外来水汽通量和局地实际蒸散发在2000年后表现为增长趋势,但由于2000年后更高的空气温度引起饱和水汽压比实际水汽压增幅更大,即大气中实际水汽增加远低于饱和水汽容量增加,进而导致VPD增幅更大。本研究表明在突变点后西南地区VPD显著变化的原因主要是温度上升对相对湿度的影响,研究结果有助于深入理解中国西南地区的气候变化,明晰VPD变化背后的相关机制,为该地区的生态系统构建和未来气候变化应对提供参考。 相似文献
18.
Li YY Cunin F Link JR Gao T Betts RE Reiver SH Chin V Bhatia SN Sailor MJ 《Science (New York, N.Y.)》2003,299(5615):2045-2047
Elaborate one-dimensional photonic crystals are constructed from a variety of organic and biopolymers, which can be dissolved or melted, by templating the solution-cast or injection-molded materials in porous silicon or porous silicon dioxide multilayer (rugate dielectric mirror) structures. After the removal of the template by chemical dissolution, the polymer castings replicate the photonic features and the nanostructure of the master. We demonstrate that these castings can be used as vapor sensors, as deformable and tunable optical filters, and as self-reporting, bioresorbable materials. 相似文献
19.
Matsubara H Yoshimoto S Saito H Jianglin Y Tanaka Y Noda S 《Science (New York, N.Y.)》2008,319(5862):445-447
Shorter-wavelength surface-emitting laser sources are important for a variety of fields, including photonics, information processing, and biology. We report on the creation of a current-driven blue-violet photonic-crystal surface-emitting laser. We have developed a fabrication method, named "air holes retained over growth," in order to construct a two-dimensional gallium nitride (GaN)/air photonic-crystal structure. The resulting periodic structure has a photonic-crystal band-edge effect sufficient for the successful operation of a current-injection surface-emitting laser. This represents an important step in the development of laser sources that could be focused to a size much less than the wavelength and be integrated two-dimensionally at such short wavelengths. 相似文献
20.
Modern laser technology has revolutionized the sensitivity and precision of spectroscopy by providing coherent light in a spectrum spanning the infrared, visible, and ultraviolet wavelength regimes. However, the generation of shorter-wavelength coherent pulses in the x-ray region has proven much more challenging. The recent emergence of high harmonic generation techniques opens the door to this possibility. Here we review the new science that is enabled by an ability to manipulate and control electrons on attosecond time scales, ranging from new tabletop sources of coherent x-rays to an ability to follow complex electron dynamics in molecules and materials. We also explore the implications of these advances for the future of molecular structural characterization schemes that currently rely so heavily on scattering from incoherent x-ray sources. 相似文献