共查询到20条相似文献，搜索用时 24 毫秒
1.
Schmidt PO Rosenband T Langer C Itano WM Bergquist JC Wineland DJ 《Science (New York, N.Y.)》2005,309(5735):749-752
We present a general technique for precision spectroscopy of atoms that lack suitable transitions for efficient laser cooling, internal state preparation, and detection. In our implementation with trapped atomic ions, an auxiliary "logic" ion provides sympathetic laser cooling, state initialization, and detection for a simultaneously trapped "spectroscopy" ion. Detection is achieved by applying a mapping operation to each ion, which results in a coherent transfer of the spectroscopy ion's internal state onto the logic ion, where it is then measured with high efficiency. Experimental realization, by using 9Be+ as the logic ion and 27Al+ as the spectroscopy ion, indicates the feasibility of applying this technique to make accurate optical clocks based on single ions. 相似文献
2.
Significant advances have been made in the ability to control the motion of neutral atoms. Cooling and trapping atoms present new possibilities for studies of ultracold atoms and atomic interactions. The techniques of laser cooling and deceleration of atomic beams, magnetic and laser trapping of neutral atoms, and a number of recent advances in the use of radiative forces to manipulate atoms are reviewed. 相似文献
3.
Recent developments in laser spectroscopy of atomic ions stored in electromagnetic traps are reviewed with emphasis on techniques that appear to hold the greatest promise of attaining extremely high resolution. Among these techniques are laser cooling and the use of single, isolated ions as experimental samples. Doppler shifts and other perturbing influences can be largely eliminated. Atomic resonances with line widths of a few parts in 10(11) have been observed at frequencies ranging from the radio frequency to the ultraviolet. Experimental accuracies of one part in 10(18) appear to be attainable. 相似文献
4.
The motion of individual cesium atoms trapped inside an optical resonator is revealed with the atom-cavity microscope (ACM). A single atom moving within the resonator generates large variations in the transmission of a weak probe laser, which are recorded in real time. An inversion algorithm then allows individual atom trajectories to be reconstructed from the record of cavity transmission and reveals single atoms bound in orbit by the mechanical forces associated with single photons. In these initial experiments, the ACM yields 2-micrometer spatial resolution in a 10-microsecond time interval. Over the duration of the observation, the sensitivity is near the standard quantum limit for sensing the motion of a cesium atom. 相似文献
5.
Measurements of the equation of state of deuterium at the fluid insulator-metal transition 总被引：1，自引：0，他引：1
GW Collins Da Silva LB P Celliers DM Gold ME Foord RJ Wallace A Ng SV Weber KS Budil R Cauble 《Science (New York, N.Y.)》1998,281(5380):1178-1181
A high-intensity laser was used to shock-compress liquid deuterium to pressures from 22 to 340 gigapascals. In this regime deuterium is predicted to transform from an insulating molecular fluid to an atomic metallic fluid. Shock densities and pressures, determined by radiography, revealed an increase in compressibility near 100 gigapascals indicative of such a transition. Velocity interferometry measurements, obtained by reflecting a laser probe directly off the shock front in flight, demonstrated that deuterium shocked above 55 gigapascals has an electrical conductivity characteristic of a liquid metal and independently confirmed the radiography. 相似文献
6.
Waibel AE Peter T Carslaw KS Oelhaf H Wetzel G Crutzen PJ Poschl U Tsias A Reimer E Fischer H 《Science (New York, N.Y.)》1999,283(5410):2064-2069
Measurements from the winter of 1994-95 indicating removal of total reactive nitrogen from the Arctic stratosphere by particle sedimentation were used to constrain a microphysical model. The model suggests that denitrification is caused predominantly by nitric acid trihydrate particles in small number densities. The denitrification is shown to increase Arctic ozone loss substantially. Sensitivity studies indicate that the Arctic stratosphere is currently at a threshold of denitrification. This implies that future stratospheric cooling, induced by an increase in the anthropogenic carbon dioxide burden, is likely to enhance denitrification and to delay until late in the next century the return of Arctic stratospheric ozone to preindustrial values. 相似文献
7.
An isochoric cooling method for obtaining unprecedented tensions on liquids was used to determine the homogeneous nucleation limit for stretching of water at a variety of water densities. At densities in the range 0.55 to 0.68 gram per milliliter (g/ml), the data agree with the homogeneous nucleation temperatures measured by Skripov for superheated water at positive pressures. At densities between 0.68 and 0.93 g/ml, cavitation occurred only at negative pressures (that is, under tension). The cavitation tensions measured were in excellent agreement with those predicted by Fisher's 1948 vapor nucleation theory. A maximum tension of 140 megapascals (=1400 bars) was reached at 42 degrees C, which lies on an extrapolation of the line of isobaric density maxima. At higher densities, cavitation of droplets that survived heterogeneous nucleation failed to occur at all unless provoked, at much lower temperatures, by freezing. This observation confirms the existence of a density maximum at 42 degrees C and -140 megapascals and hence greatly strengthens the basis for Speedy's conjecture of a reentrant spinodal for water. 相似文献
8.
The morphology,particle size distribution and thermal properties of microcapsules were evaluated by scanning electron microscopy（SEM）,laser diffraction particle size analyzer and differential scanning calorimetry（DSC）.Vitamin A was used as model core material,HI-CAP 100（starch octenylsucciniate,OSA-starch） was used as wall material and prepared by spray drying.When emulsions were prepared with 40%（w/v） solution of total solids concentration at the core/wall material ratios of 40%（w/w）,the microencapsulation efficiency（ME） was（96.38 ± 0.71）%.Microcapsules exhibited spherical shapes with characteristic dents as evidence by SEM.With the vibrating frequency of the centrifugal granulation from 40,35,30,25 to 20 Hz,the volume diameter（D4,3） was 66.58,71.44,85.61,94.08,and 153.45 μm,respectively.Differential scanning calorimetry（DSC） results revealed that the glass transition temperature（Tg） and melting temperature（Tm） were 56.355 and 208.300°C,respectively.Vitamin A microcapsules produced with HI-CAP 100 exhibited spherical shapes with characteristic dents,which was attributed to drying and cooling solidification involved during spray-drying.The vibrating frequency of the centrifugal granulation had effect on the particle size distribution of microcapsules（P 〈 0.05）.The storage and heating stability of microcapsules was well by thermal properties. 相似文献
9.
Recently, with the high-speed development of industry and agriculture in China, the contamination of heavy metal has become a severe environmental problem caused by immoderate mining operation, "three wastes" emissions, vehicle exhaust, and misuse of agricultural chemical inputs. So, it is very important to monitor the contamination of heavy metals in soil. However, the national and industrial standards of detecting heavy metals in soil mainly focus on the traditional analytical approaches employed in laboratory at present. So, it is still difficult to achieve the on-site and fast detection of heavy metals in soil, which gives rise to such difficulty of monitoring and preventing the source contamination effectively and timely. In view of the matrix of soil sample, solid sampling analysis should be feasible to the fast detection of heavy metals, including electrothermal vaporization (ETV), X-ray fluorescence spectrometry (XRF), laser ablation (LA), laser induced breakthrough spectrometry (LIBS), X-ray photoelectron spectroscopy (XPS), and instrumental neutron activation analysis (INAA). The solid sampling techniques do not require digestion treatment and is thereby fast and efficient. However, among them, the detection limit and stability of XRF, LA, and LIBS cannot satisfy the all demands in standards of soil quality; on the other hand, it is too difficult to reach the miniaturization and on-site testing for LA, XAS, and INAA. By comparison, ETV is a kind of solid sampling tool with excellent advantages such as high analytical sensitivity, favorable stability, and being easy to be miniaturized, using electric heating to introduce analyses via aerosol from the sample into the atomizer or exciter for measurement. ETV is able to introduce heavy metals fast and efficiently, which is versatile to atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), atomic emission spectrometry (AES), and induced coupled plasma mass spectrometry (ICP-MS). As usual, many materials such as carbon, metals, and quartz can be utilized for ETV, which are frequently processed into graphite furnace, porous carbon tube, tungsten coil, rhenium coil, quartz tube and so on. Among various ETV approaches, electromagnetic induction ETV is characterized with no cold zone, fast heating or cooling and miniaturization. Considering the complicated soil matrices, however, ETV has been always confronted with the bottleneck problem, namely matrix interference. Through integrating these advanced techniques including gas phase enrichment (GPE), dielectric barrier discharge, matrix modifier, background correction and so on, the matrix interference will be eliminated completely for the detection of heavy metals in soil when solid sampling by using ETV atomic spectrometers. Especially for GPE, it can realize both two aims at one time: eliminating matrix interference and improving analytical sensitivity. This review is about to bring some valuable suggestions for innovating the fast detection of heavy metals in soil, to play parts in the environmental monitoring and protection in the future. 相似文献
10.
Absorption of pulsed laser radiation by a single particle generates a photoacoustic wave whose time profile can be measured with a wideband pressure transducer. Solution of the wave equation for pressure in one, two, and three dimensions shows that the photoacoustic wave is determined by the geometry and dimensions of the particle, and by its sound speed and density relative to the fluid that surrounds it. Photoacoustic waves, referred to here as signatures, are reported in experiments in which fluid droplets, cylinders, and layers are irradiated with 10-nanosecond laser pulses. 相似文献
11.
[目的]探讨532 nm激光选育高维生素C白菜的可行性。[方法]以白菜为实验材料,采用不同功率密度（2、6、10、14和20 mW/mm~2）和不同辐照时间（1、2、3、4和5 min）的Nd：YAG倍频激光对白菜种子胚进行辐照,研究激光作用方式对白菜幼苗功能叶片面积、叶绿素和维生素C含量的影响。功能叶片面积的测定：培养10 d时,取每颗白菜上最大的叶片,利用复印称重法对其面积进行测定,即将白菜叶片用激光复印机复印,沿叶片边缘剪下,准确称量其质量;用直径1.46cm的打孔器打50个纸圆片,算出单位面积纸张的质量（g/cm~2）,计算后得出叶面积。叶绿素含量的测定：每组取白菜叶片10 g,以2 ml磷酸缓冲溶液（50 mmol/L,pH≈6.8）将其研磨成均质。吸取上清液40μl,注入1.5 ml的Eppendorf管中,再加入960μl的无水乙醇,置于4℃黑暗条件下30 min,然后于4℃下以1 000 r/min离心15 min。以质量分数为95%的乙醇做空白对照,利用U-2800分光光度计（日立）分别测定待测液的A_（665）和A_（649）,由叶绿素总量=6.10×A_（665）＋20.04×A_（649）来计算其含量。Vc含量的测定：利用库仑滴定法测定Vc含量,即在预电解后的电解池中准确加入2.0 ml样品溶液,开动搅拌器,按下库仑仪工作键进行10 mA恒电流电解,滴定终点时指示灯亮,记录电解所消耗电量,通过法拉第定律计算Vc含量。[结果]激光辐照功率为2～20 mW/mm~2时,功能叶片面积和叶绿素含量与激光辐照功率、作用时间有关。适当辐照功率、短时间激光处理白菜种子对功能叶片面积表现为刺激效应,当较高功率密度激光辐照时间超过一定阈值（为4 min）时,激光处理表现出抑制效应。短时间激光处理白菜种子对叶绿素含量表现为刺激效应,当激光辐照时间超过一定阈值（5 min）时,激光处理表现出抑制效应,其中以14 mW/mm~2激光预处理白菜种子1 min的效果最佳。激光辐照功率密度为2～10 mW/mm~2时,白菜Vc含量随辐照时间增加而增加;辐照功率密度为14和20 mW/mm~2时,白菜Vc含量与辐照时间呈正态分布,白菜Vc含量与激光辐照能量呈正态分布。其中以2.8 J（20 mW/mm~2,3 min）激光预处理白菜种子效果最佳。当辐照剂量超过一定阈值即3.8 J（20 mW/mm~2,4 min）时,激光处理表现出抑制效应。[结论]适当辐照剂量的532 nm激光辐照白菜种子胚,可显著提高其幼苗维生素C含量。 相似文献
12.
Volatilization of high molecular weight DNA by pulsed laser ablation of frozen aqueous solutions 总被引：1，自引：0，他引：1
DNA has been volatilized by pulsed laser ablation of a thin film of a frozen aqueous DNA solution. The target film was irradiated in vacuum by a pulsed laser at power densities sufficient to ablate the ice matrix. The expanding ablated water vapor entrained embedded DNA molecules and expelled them into the gas phase. Ejection of DNA molecules as large as 410 kilodaltons was verified by collection of the ablation products and subsequent mass analysis by polyacrylamide gel electrophoresis with autoradiographic detection. 相似文献
13.
Lipid tubule self-assembly: length dependence on cooling rate through a first-order phase transition
The formation kinetics and self-assembly of multilamellar tubules of the diacetylenic phospholipid 1,2-bis(tricosa-10,12-diynoyl)-sn-glycerol-3-phosphocholine formed under controlled cooling rates were studied by x-ray diffraction and optical, atomic force, and scanning electron microscopy. Tubule formation was driven by a reversible first-order phase transition from an intralamellar, chain-melted L(alpha) phase to a chain-frozen L(beta), phase. These observations are the basis of a highly efficient method of tubule production in which tubule lengths can be controlled, between 1 and 100 micrometers, by varying the cooling rate. These tubules can be made in suspensions with 10 percent lipid by mass, far exceeding the lipid solubility limit. 相似文献
14.
Brace LH Theis RF Krehbiel JP Nagy AF Donahue TM McElroy MB Pedersen A 《Science (New York, N.Y.)》1979,203(4382):763-765
Altitude profiles of electron temperature and density in the ionosphere of Venus have been obtained by the Pioneer Venus orbiter electron temperatutre probe. Elevated temperatutres observed at times of low solar wind flux exhibit height profiles that are consistent with a model in which less than 5 percent of the solar wind energy is deposited at the ionopause and is conducted downward through an unmagnetized ionosphere to the region below 200 kilomneters where electron cooling to the neutral atmosphere proceeds rapidly. When solar wind fluxes are higher, the electron temperatures and densities are highly structured and the ionopause moves to lower altitudes. The ionopause height in the late afternoon sector observed thus far varies so widely from day to (day that any height variation with solar zenith angle is not apparent in the observations. In the neighborhood of the ionopause, measuremnents of plasma temperatures and densities and magnetic field strength indicate that an induced magnetic barrier plays an important role in the pressure transfer between the solar wind and the ionosphere. The bow, shock is marked by a distinct increase in electron current collected by the instrument, a featutre that provides a convenient identification of the bow shock location. 相似文献
15.
探讨532 nm激光选育高维生素C白菜的可行性。［方法］以白菜为实验材料,采用不同功率密度和不同辐照时间的Nd：YAG倍频激光对白菜种子胚进行辐照,研究激光作用方式对白菜幼苗功能叶片面积、叶绿素和维生素C含量的影响。［结果］激光辐照功率为2-20 mW/mm2时,功能叶片面积和叶绿素含量与激光辐照功率、作用时间有关,其中以14 mW/mm2激光预处理白菜种子1 min的效果最佳;辐照时间为1-5 min时,白菜幼苗维生素C含量与激光辐照剂量有关,其中以12 J （20 mW/mm2,3 min）激光预处理白菜种子效果最佳。［结论］适当辐照剂量的532 nm激光辐照白菜种子胚,可显著提高其幼苗维生素C含量。 相似文献
16.
《Science (New York, N.Y.)》1976,192(4246):1334-1336
Properties of the aerosols above Hawaii, Alaska, and the South Pole are derived from sun photometry at several wavelengths. The mass loading of aerosol material is several milligrams per square meter. At the South Pole the mean particle radius is 0.04 micrometer; at Hawaii in March 1975 there was a thin volcanic layer with a mean particle radius of 0.1 micrometer. The aerosols cause heating of the earth-atmosphere system at the poles and cooling at low latitudes. 相似文献
17.
Small metal particles (<5 nanometers), which are widely used in catalysis, have physical and chemical properties that are markedly different from those of the bulk metal. The differences are related to crystal structure, and it is therefore significant that structral rearrangements in small particles have been observed in real time by using high-resolution electron microscopy. A detailed investigation at the atomic level has been made of the factors affecting the dynamic activity of small gold crystals that are supported on thin films of amorphous carbon, silicon, and germanium. The rate of activity depends mainly on the current density of the incident electron beam and the degree of contact of the particle with the substrate, but this rate decreases rapidly as the particle size is increased. The activity of the particles is very similar on either carbon or silicon, but it is generally less marked on germanium because of increased contact between the particle and the substrate. The electron beam effectively heats the particles, and it appears that their dynamic behavior depends on their thermal contact with the substrate. 相似文献
18.
The corrosion inhibition effect of benzothiazole and its derivatives on dezincification of brass has been studied in simulated cooling water by weight loss, atomic adsorption spectrum and electrochemical technique. The relationship between moleculal struc 相似文献
19.
Witte S Zinkstok RT Ubachs W Hogervorst W Eikema KS 《Science (New York, N.Y.)》2005,307(5708):400-403
Precision spectroscopy at ultraviolet and shorter wavelengths has been hindered by the poor access of narrow-band lasers to that spectral region. We demonstrate high-accuracy quantum interference metrology on atomic transitions with the use of an amplified train of phase-controlled pulses from a femtosecond frequency comb laser. The peak power of these pulses allows for efficient harmonic upconversion, paving the way for extension of frequency comb metrology in atoms and ions to the extreme ultraviolet and soft x-ray spectral regions. A proof-of-principle experiment was performed on a deep-ultraviolet (2 x 212.55 nanometers) two-photon transition in krypton; relative to measurement with single nanosecond laser pulses, the accuracy of the absolute transition frequency and isotope shifts was improved by more than an order of magnitude. 相似文献
20.
Drescher M Hentschel M Kienberger R Tempea G Spielmann C Reider GA Corkum PB Krausz F 《Science (New York, N.Y.)》2001,291(5510):1923-1927
Single soft-x-ray pulses of approximately 90-electron volt (eV) photon energy are produced by high-order harmonic generation with 7-femtosecond (fs), 770-nanometer (1.6 eV) laser pulses and are characterized by photoionizing krypton in the presence of the driver laser pulse. By detecting photoelectrons ejected perpendicularly to the laser polarization, broadening of the photoelectron spectrum due to absorption and emission of laser photons is suppressed, permitting the observation of a laser-induced downshift of the energy spectrum with sub-laser-cycle resolution in a cross correlation measurement. We measure isolated x-ray pulses of 1.8 (+0.7/-1.2) fs in duration, which are shorter than the oscillation cycle of the driving laser light (2.6 fs). Our techniques for generation and measurement offer sub-femtosecond resolution over a wide range of x-ray wavelengths, paving the way to experimental attosecond science. Tracing atomic processes evolving faster than the exciting light field is within reach. 相似文献