Attosecond synchronization of high-harmonic soft x-rays

Subfemtosecond light pulses can be obtained by superposing several high harmonics of an intense laser pulse. Provided that the harmonics are emitted simultaneously, increasing their number should result in shorter pulses. However, we found that the high harmonics were not synchronized on an attosecond time scale, thus setting a lower limit to the achievable x-ray pulse duration. We showed that the synchronization could be improved considerably by controlling the underlying ultrafast electron dynamics, to provide pulses of 130 attoseconds in duration. We discuss the possibility of achieving even shorter pulses, which would allow us to track fast electron processes in matter.     

Taste flashes: reaction times, intensity, and quality

Human simple reaction times and magnitude estimates of taste intensity increased as the duration of 500-millimolar sodium chloride or 2-millimolar saccharin sodium pulses lengthened from 100 to 1000 milliseconds. Responses to "What was the taste?" ranged from 94 to 100 percent "sweet" for saccharin and 68 to 83 percent "salty" for salt across all pulse durations when both substances were randomized with water pulses.     

Time-resolved holography with photoelectrons

Ionization is the dominant response of atoms and molecules to intense laser fields and is at the basis of several important techniques, such as the generation of attosecond pulses that allow the measurement of electron motion in real time. We present experiments in which metastable xenon atoms were ionized with intense 7-micrometer laser pulses from a free-electron laser. Holographic structures were observed that record underlying electron dynamics on a sublaser-cycle time scale, enabling photoelectron spectroscopy with a time resolution of almost two orders of magnitude higher than the duration of the ionizing pulse.     

Ultrafast X-ray Pulses from Laser-Produced Plasmas

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.     

Photosynthate and nitrogen requirements for seed production by various crops

Seed biochemical composition was the basis for segregating 24 crops into four distinct groups. Nitrogen requirements of pulses and soybeans were so great that sustained seed growth demanded continued nitrogen translocation from vegetative tissues. This translocation must eventually induce senescence in these tissues, restrict the duration of the seed-fill period, and limit seed yield.     

Fusion contour for intermittent photic stimuli of alternating duration

It is generally thought that fusion of intermittent photic stimuli occurs when the duration between successive pulses of light is reduced to a certain value, this value being a function of the illumination and viewing conditions. The findings described in this report show that fusion is determined not only by the duration between successive stimuli but also by the temporal pattern of successive stimuli.     

Ultrashort light pulses

Experimental techniques are now available for the generation of repetitive and single coherent optical pulses of extremely short time duration and high peak power. These pulses should find extensive application in basic and applied research. Additional shortening of optical pulse durations can be obtained by means of the stimulated Raman effect, second-harmonic generation, or amplification with nonlinear laser amplifiers.     

1Localization effects with steady thermal noise in one ear and pulsed thermal noise in the other

When the duration or repetition rate of pulses in the left ear is increased, while steady, in-phase, thermal noise sounds in the right ear, the pulses are heard to move toward the median plane. At still longer durations (for a given repetition rate) the loudness of noise on the right diminishes, until finally all sound is localized at the median plane.     

脉冲场强和波形对小鼠卵母细胞电活化效果的影响

实验以一定持续时间与不同场强搭配的一次电脉冲对注射ｈＣＧ后１７和１９ｈ的小鼠卵母细胞进行电刺激，研究脉冲场强和波形对印母细胞活化效果的影响．结果说明：（１）在一定持续时间下，只有当场强增至一定值时才获得较高活化率（ｈＣＧ后１７ｈ卵，４０％～５５％左右；ｈＣＧ后１ｇｈ卵，４２％～７０％左右）；（２）在某一持续时间，获得较高活化率的场强不止一个，而是有一定范围；低于这一范围的场强因刺激强度不够至使许多卵母细胞不能活化，高于这一范围的场强则因刺激强度过大造成卵大量死亡而使活化率下降；（３）随脉冲持续时间倍减，获较高活化率的场强约以０．２ｋＶ／ｃｍ的强度递增；（４）与注ｈＣＧ后１７ｈ卵相比，ｈＣＧ后１９ｈ卵获较高活化率的场强范围更宽；（５）在同等条件下，不规则波形脉冲的卵活化率、碎裂率和死亡率都明显高于方波脉冲．     

小鼠卵母细胞的电激活及胚胎电融合

为提高核移植的效率，采用SHIMADZU生产的SSH－2电融合仪进行小鼠卵母细胞电激活及2－细胞胚胎电融合，结果表明：（1）不同脉冲的卵激活率有极显著差异（P＜0．01），在2次脉冲和30μs的脉冲宽度的条件下，脉冲强度以0．1kV/cm最好，激活率达76．9％；（2）不同脉冲强度、脉冲次数和脉冲宽度的胚胎融合率亦有极显著差异（P＜0．01），最佳融合参数为0.1kv/cm,30μs的脉冲宽度及2脉冲，胚胎融合率达84．6％。     

Generation of an artificial aurora

On 26 January 1969 an Aerobee 350 rocket was fired from Wallops Island, Virginia, carrying an electron accelerator. Above 230 kilometers the electron guns put out a beam of 0.5 ampere of 10 kev electrons in pulses of 1-second duration aimed downward along the earth's magnetic field lines. The interaction of electron beam with the atmosphere at an altitude of about 100 kilometers generated enough light so that the auroral rays produced could be photographed on the ground by television camera systems.     

X-ray pulses approaching the attosecond frontier

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.     

Auditory system of noctuid moths

Insect-eating bats find their aerial food by sonar, through emitting ultrasonic chirps and locating sources of echoes. Certain moths have ears sensitive to these chirps and can detect bats well beyond the range of the bats' sonar. On hearing a distant bat, many moths turn and fly directly away from the source of ultrasound. Only one sense cell in each ear of a moth provides the primary nervous information for this response. This article describes my initial attempts to find out how a moth's central nervous system processes the train of chirps reaching its two ears. The ear of a restrained moth is exposed to a sequence of artifically generated ultrasonic pulses that approximates the cries made by a bat. This stimulus can be varied with respect to ultrasonic frequency (pitch), pulse intensity, pulse duration, the interval between pulses, and pulse-train duration. The more sensitive acoustic sense cell responds to all frequencies between about 15,000 and 80,000 cycles per second, but the signal that it transmits to the moth's central nervous system contains no measure of frequency within this range. However, this nerve signal reports variations in the other parameters of the stimulus. The acoustic fiber connects, in the central nervous system, with various nerve cells that transform the signal farther. The signal from a pulse-marker neuron contains no measures of pulse intensity or pulse duration, reporting only changes in interpulse interval and pulse-train duration. A train-marker neuron reports only the duration of the pulse train. The stimulus parameters may be likened to keys, each of which is necessary to gain admittance through a given door but becomes superfluous once this door has been passed. This analogy suggests one of the ways in which a signal is transformed in its passage through the nervous system, and how its specificity is assured in eliciting a given response. In addition to undergoing this kind of transformation, neural signals generated in the two directionally sensitive ears must be combined if a flying moth is to steer a course away from a distant bat. Neurons have been discovered in the central ganglia which summate signals from the right and left ears. Other neurons are inhibited in their activity by stimulation of one ear. The moth may combine signals from these neurons with motor-nerve information on the attitude of its own wings, which act as oscillating baffles modifying its directional acoustic sensitivity 20 to 40 times a second as it flaps an erratic path through the darkness.     

Effect of adenosine 3',5'-monophosphate on neuronal pacemaker activity: a voltage clamp analysis

Bursting pacemaker activity in nerve cells can be modified for long periods by synaptic input of short duration. There is evidence that cyclic nucleotides may play a role in these modifications. The predominant effect of elevated levels of adenosine 3',5'-monophosphate in Aplysia neurons was an increased slope conductance to hyperpolarizing pulses, evident in voltage clamp records. A similar increase in slope conductance was seen as one component of maximum strength synaptic stimulation, which is consistent with the idea that cyclic nucleotides are important in the expression of synaptic alteration of bursting pacemaker activity.     

Enhanced optical emission during Crab giant radio pulses

We detected a correlation between optical and giant radio pulse emission from the Crab pulsar. Optical pulses coincident with the giant radio pulses were on average 3% brighter than those coincident with normal radio pulses. Combined with the lack of any other pulse profile changes, this result indicates that both the giant radio pulses and the increased optical emission are linked to an increase in the electron-positron plasma density.     

大黄鱼的发声及信号特性研究

对大黄鱼Pseudosciaena crocea觅食及产卵时的发声及信号特性进行了研究。结果表明:不同行为下大黄鱼发声信号的频谱特性一致,即约在800 Hz频率处有一个明显的谱峰;但不同行为下发声的时域信号差距较大,即觅食时大黄鱼的发声信号都是简单的单脉冲,脉冲间隔大多集中在1~30 ms,脉冲间隔比较小,而产卵时大黄鱼的发声信号则大部分是连续的双脉冲或三脉冲,只有极个别的单脉冲或多脉冲,脉冲间隔则集中在100~130 ms,脉冲间隔要大得多。     

褐菖鲉发声系统形态结构及其发声特征研究

分别采用形态学和模拟领地入侵的方法研究了褐菖鲉发声系统和发声特征。结果表明,褐菖鲉发声系统由鱼鳔肌和鱼鳔组成。鱼鳔肌前方附着于颅骨的翼耳骨,后经第1～7肋骨,与鱼鳔背后侧部相连。在领地入侵模拟实验中,褐菖鲉发出了连续脉冲的“咕噜噜”声音,其平均脉冲周期为(32.6±2.6) ms,平均脉冲间隔为(51.8±81.4) ms, 主频为68～175 Hz。对单个声音脉冲中包涵的正弦波型的能量周期分析发现：其声音主频与最大能量周期的倒数一致,表明褐菖鲉是采用鱼鳔肌收缩的发声机制。     

影响兔胚胎细胞核移植效率的因素

 对兔胚胎细胞核移植（NT）的有关影响因素进行了系统研究。结果发现电场强度100 V·mm-1，脉冲时间15μs，电脉冲3～4次的融合率显著高于其它组（P<0.05）；融合前激活卵母细胞，分裂率和囊胚率显著高于融合后激活（P <0.05）；当用8～16-细胞胚胎的卵裂球作供核时，卵裂率和囊胚率显著高于致密桑椹胚卵裂球为供核组；当重组胚在含3%发情牛血清（OCS）的TCM199中培养48 h后，转入含10% FCS的TCM199中继续培养，卵裂率和囊胚率显著高于一直在3% OCS或10% 胎犊血清（FCS）中培养的重组胚（P < 0.05）。将22枚2～4-细胞期重组胚移入同期发情的受体母兔输卵管内，34 d后产下NT仔兔1只。结果表明，电融合参数和供体胚胎的发育阶段以及受体卵母细胞的状态对NT效果有显著影响，在NT胚胎的不同发育阶段采用不同的血清种类和浓度可提高其胚胎发育能力。     

Factors Affecting the Efficiency of Embryonic Cell Nuclear Transfer in Rabbits

Factors affecting the efficiency of nuclear transfer （NT） in rabbits were examined in the present study. When 100 V mm of pulse strength and 15 us of pulse duration were employed, 3 and 4 electronic pulses resulted in significantly more cytoplasts fused with donor cells compared with 2 electronic pulses （P〈 0.05）, but no significant difference was found in the cleavage rate of reconstructed embryos among the three groups （P〉0.05）. When the duration and number of electronic pulse were fixed at 15 ps and 3 times, increase of pulse intensity from 100 V mm 1 to 150 V mm^-1 and 200 V mm^-1 resulted in a significantly decrease in the cleavage rate of reconstructed embryos （P〈 0.05）, although the fusion rate did not significantly differ among the three groups （P〉 0.05）. Significantly more reconstructed embryos cleaved and developed to blastocysts when they were derived from donor embryos at the 8-16-cell stage, in comparison with the reconstructed embryos derived from donor embryos at the compact morula stage （P 〈 0.05）, although the fusion rate was similar （P 〉 0.05）. Activation of cytoplasts prior to fusion increased the cleavage rate （P〈 0.05） and blastocyst development （P〈 0.05） of reconstructed embryos, but decreased the fusion rate （P 〈 0.05） compared with cytoplasts activated post fusion. More reconstructed embryos developed to blastocysts when they were cultured in TCM ＋ 3% OCS at the first 48 h and then cultured in TCM199＋ 10% FCS, in comparison with the reconstructed embryos cultured in either TCM199＋ 10% FCS or TCM199＋3% OCS （P 〈 0.05）. When 22 NT embryos were transferred into the oviducts of one recipient rabbit, one recipient rabbit delivered a female rabbit at 34 days of gestation. In conclusion, either electrofusion parameter or developmental stage of donor embryos have a significant effect on the efficiency of NT, NT embryos require different concentration of serum at their different development stages.     

Periodical cicadas as resource pulses in North American forests

Resource pulses are occasional events of ephemeral resource superabundance that occur in many ecosystems. Aboveground consumers in diverse communities often respond strongly to resource pulses, but few studies have investigated the belowground consequences of resource pulses in natural ecosystems. This study shows that resource pulses of 17-year periodical cicadas (Magicicada spp.) directly increase microbial biomass and nitrogen availability in forest soils, with indirect effects on growth and reproduction in forest plants. These findings suggest that pulses of periodical cicadas create "bottom-up cascades," resulting in strong and reciprocal links between the aboveground and belowground components of a North American forest ecosystem.