Capture of hydroxymethylene and its fast disappearance through tunnelling

Singlet carbenes exhibit a divalent carbon atom whose valence shell contains only six electrons, four involved in bonding to two other atoms and the remaining two forming a non-bonding electron pair. These features render singlet carbenes so reactive that they were long considered too short-lived for isolation and direct characterization. This view changed when it was found that attaching the divalent carbon atom to substituents that are bulky and/or able to donate electrons produces carbenes that can be isolated and stored. N-heterocyclic carbenes are such compounds now in wide use, for example as ligands in metathesis catalysis. In contrast, oxygen-donor-substituted carbenes are inherently less stable and have been less studied. The pre-eminent case is hydroxymethylene, H-C-OH; although it is the key intermediate in the high-energy chemistry of its tautomer formaldehyde, has been implicated since 1921 in the photocatalytic formation of carbohydrates, and is the parent of alkoxycarbenes that lie at the heart of transition-metal carbene chemistry, all attempts to observe this species or other alkoxycarbenes have failed. However, theoretical considerations indicate that hydroxymethylene should be isolatable. Here we report the synthesis of hydroxymethylene and its capture by matrix isolation. We unexpectedly find that H-C-OH rearranges to formaldehyde with a half-life of only 2 h at 11 K by pure hydrogen tunnelling through a large energy barrier in excess of 30 kcal mol(-1).     

一种分数间隔恒模算法的宽带盲均衡器

提出了一种基于子带分解技术的分数间隔恒模算法(FSE-CMA)的宽带均衡器结构.该结构将子带分解和全频带的子卷积方法有机地结合在一起.子带分解的预白化作用能明显加快FSE-CMA的收敛速度;同时,子带分解和子卷积方法都允许对数据进行降速率的并行处理,因此能减小运算复杂度,有利于算法的实时实现.仿真实验结果验证了该结构的有效性.     

分数Hamilton图的充分必要条件

分数Hamilton图是比Hamilton图更广泛的图类.作者借用线性规划的知识给出了判定分数Hamilton 图的新的充分必要条件,并利用新的充分必要条件对特殊图类的分数Hamilton图进行了研究.     

Direct observation of hole transfer through DNA by hopping between adenine bases and by tunnelling.

The function of DNA during oxidative stress and its suitability as a potential building block for molecular devices depend on long-distance transfer of electrons and holes through the molecule, yet many conflicting measurements of the efficiency of this process have been reported. It is accepted that charges are transported over long distances through a multistep hopping reaction; this 'G-hopping' involves positive charges moving between guanines (Gs), the DNA bases with the lowest ionization potential. But the mechanism fails to explain the persistence of efficient charge transfer when the guanine sites are distant, where transfer rates do not, as expected, decrease rapidly with transfer distance. Here we show experimentally that the rate of charge transfer between two guanine bases decreases with increasing separation only if the guanines are separated by no more than three base pairs; if more bridging base pairs are present, the transfer rates exhibit only a weak distance dependence. We attribute this distinct change in the distance dependence of the rate of charge transfer through DNA to a shift from coherent superexchange charge transfer (tunnelling) at short distances to a process mediated by thermally induced hopping of charges between adenine bases (A-hopping) at long distances. Our results confirm theoretical predictions of this behaviour, emphasizing that seemingly contradictory observations of a strong as well as a weak influence of distance on DNA charge transfer are readily explained by a change in the transfer mechanism.     

Scanning tunnelling microscopy of Z-DNA

Scanning tunnelling microscopy (STM) has been used to map the surface topography of inorganic materials at the atomic level, and is potentially one of the most powerful techniques for probing biomolecular structure. Recent STM studies of calf thymus DNA and poly(rA).poly(rU) have shown that the helical pitch and periodic alternation of major and minor grooves can be visualized and reliably measured. Here we present the first STM images of poly(dG-me5dC).poly(dG-me5dC) in the Z-form. Both the general appearance of the fibres and measurements of helical parameters are in good agreement with models derived from X-ray diffraction.     

Direct observation of second-order atom tunnelling

Tunnelling of material particles through a classically impenetrable barrier constitutes one of the hallmark effects of quantum physics. When interactions between the particles compete with their mobility through a tunnel junction, intriguing dynamical behaviour can arise because the particles do not tunnel independently. In single-electron or Bloch transistors, for example, the tunnelling of an electron or Cooper pair can be enabled or suppressed by the presence of a second charge carrier due to Coulomb blockade. Here we report direct, time-resolved observations of the correlated tunnelling of two interacting ultracold atoms through a barrier in a double-well potential. For the regime in which the interactions between the atoms are weak and tunnel coupling dominates, individual atoms can tunnel independently, similar to the case of a normal Josephson junction. However, when strong repulsive interactions are present, two atoms located on one side of the barrier cannot separate, but are observed to tunnel together as a pair in a second-order co-tunnelling process. By recording both the atom position and phase coherence over time, we fully characterize the tunnelling process for a single atom as well as the correlated dynamics of a pair of atoms for weak and strong interactions. In addition, we identify a conditional tunnelling regime in which a single atom can only tunnel in the presence of a second particle, acting as a single atom switch. Such second-order tunnelling events, which are the dominating dynamical effect in the strongly interacting regime, have not been previously observed with ultracold atoms. Similar second-order processes form the basis of superexchange interactions between atoms on neighbouring lattice sites of a periodic potential, a central component of proposals for realizing quantum magnetism.     

利用相干渡越辐射测量自由电子激光的群聚

探讨了利用相干渡越辐射(CTR)来测量自由电子激光(FEL)群聚电子束的测量方法。根据相干渡越辐射(CTR)的特性,结合FEL中电子束群聚的特点,建立了理论模型。利用CAEP光阴极RF腔注入器、30 M eV射频加速器产生的30 M eV电子束及相应的FEL装置进行了实验验证。结果表明:群聚后电子束渡越辐射的强度大大高于非群聚电子束的辐射强度,而且辐射角也很小,反映了该辐射的相干性,由此可以给出电子束的群聚效果。     

咸化湖盆优质烃源岩的形成与分布

应用沉积学、地球化学理论和油源对比等方法,对我国咸化湖盆优质烃源岩的形成与分布进行了研究。结果表明,断陷湖盆和前陆湖盆优质烃源岩（有机质含量特别高的烃源岩）是与碳酸盐、硫酸盐和氯化盐等蒸发盐类矿物相伴生的,有机质沉积的有利环境是半咸水、咸水和盐水湖泊。湖水在重力作用下易形成盐度分层,表层水盐度小,适于广盐和嗜盐性浮游生物的生存;深水部位的底层水盐度大、缺乏游离氧,适于有机质的保存;表层水生物高产率区与底层水缺氧区的叠合部位就是优质烃源岩的发育区。由气候变化引起的突发性洪水会破坏分层水体,使得湖水淡化,形成有机质含量较低的烃源岩,因此咸化优质烃源岩与淡化普通烃源岩常呈互层分布,咸化沉积物数量越多,生烃潜力越大。     

Tunable quantum tunnelling of magnetic domain walls

Perhaps the most anticipated, yet experimentally elusive, macroscopic quantum phenomenon is spin tunnelling in a ferromagnet, which may be formulated in terms of domain wall tunnelling. One approach to identifying such a process is to focus on mesoscopic systems where the number of domain walls is finite and the motion of a single wall has measurable consequences. Research of this type includes magnetotransport measurements on thin ferromagnetic wires, and magnetization experiments on single particles, nanomagnet ensembles and rare-earth multilayers. A second method is to investigate macroscopic disordered ferromagnets, whose dynamics are dominated by domain wall motion, and search the associated relaxation-time distribution functions for the signature of quantum effects. But whereas the classical, thermal processes that operate in these experiments are easily regulated via temperature, the quantum processes have so far not been tunable, making difficult a definitive interpretation of the results in terms of tunnelling. Here we describe a disordered magnetic system for which it is possible to adjust the quantum tunnelling probabilities. For this material, we can model both the classical, thermally activated response at high temperatures and the athermal, tunnelling behaviour at low temperatures within a unified framework, where the domain wall is described as a particle with a fixed mass. We show that it is possible to tune the quantum tunnelling processes by adjusting the 'mass' of this particle with an external magnetic field.     

Atomic-scale imaging of DNA using scanning tunnelling microscopy

The scanning tunnelling microscope (STM) has been used to visualize DNA under water, under oil and in air. Images of single-stranded DNA have shown that submolecular resolution is possible. Here we describe atomic-resolution imaging of duplex DNA. Topographic STM images of uncoated duplex DNA on a graphite substrate obtained in ultra-high vacuum are presented that show double-helical structure, base pairs, and atomic-scale substructure. Experimental STM profiles show excellent correlation with atomic contours of the van der Waals surface of A-form DNA derived from X-ray crystallography. A comparison of variations in the barrier to quantum mechanical tunnelling (barrier-height) with atomic-scale topography shows correlation over the phosphate-sugar backbone but anticorrelation over the base pairs. This relationship may be due to the different chemical characteristics of parts of the molecule. Further investigation of this phenomenon should lead to a better understanding of the physics of imaging adsorbates with the STM and may prove useful in sequencing DNA. The improved resolution compared with previously published STM images of DNA may be attributable to ultra-high vacuum, high data-pixel density, slow scan rate, a fortuitously clean and sharp tip and/or a relatively dilute and extremely clean sample solution. This work demonstrates the potential of the STM for characterization of large biomolecular structures, but additional development will be required to make such high resolution imaging of DNA and other large molecules routine.     

Attosecond real-time observation of electron tunnelling in atoms

Atoms exposed to intense light lose one or more electrons and become ions. In strong fields, the process is predicted to occur via tunnelling through the binding potential that is suppressed by the light field near the peaks of its oscillations. Here we report the real-time observation of this most elementary step in strong-field interactions: light-induced electron tunnelling. The process is found to deplete atomic bound states in sharp steps lasting several hundred attoseconds. This suggests a new technique, attosecond tunnelling, for probing short-lived, transient states of atoms or molecules with high temporal resolution. The utility of attosecond tunnelling is demonstrated by capturing multi-electron excitation (shake-up) and relaxation (cascaded Auger decay) processes with subfemtosecond resolution.     

盾构运动过程的数值分析

采用模拟盾构推进过程的方法,对土的工程性质和盾构尺寸对盾构姿态的影响进行数值分析.通过分析得出了盾构单位距离角度的变化与地层基床系数和盾构尺寸之间的关系,为盾构姿态的控制提供理论依据,并为建立盾构运动过程的数学模型打下基础.     

Images of single-stranded nucleic acids by scanning tunnelling microscopy

The scanning tunnelling microscope has the potential to resolve the structure of biological molecules with atomic detail. Progress has been made in the imaging of dried, unshadowed double helices of DNA4-7 and in recording images of DNA under water. Also, images of unshadowed complexes of DNA with the RecA protein from Escherichia coli indicate that this technique may not be restricted to thin biological samples. Here we present images of polydeoxyadenylate molecules aligned in parallel, with their bases lying flat on a surface of highly oriented pyrolytic graphite and with their charged phosphodiester backbones protruding upwards. Based on these images, a molecular model has been built which suggests the presence of a hydrogen bond that could stabilize the parallel alignment. Our micrographs demonstrate the potential application of scanning tunnelling microscopy in structural studies of nucleic acids and provide evidence that it could be used to sequence DNA.     

Natural engineering principles of electron tunnelling in biological oxidation-reduction

We have surveyed proteins with known atomic structure whose function involves electron transfer; in these, electrons can travel up to 14 A between redox centres through the protein medium. Transfer over longer distances always involves a chain of cofactors. This redox centre proximity alone is sufficient to allow tunnelling of electrons at rates far faster than the substrate redox reactions it supports. Consequently, there has been no necessity for proteins to evolve optimized routes between redox centres. Instead, simple geometry enables rapid tunnelling to high-energy intermediate states. This greatly simplifies any analysis of redox protein mechanisms and challenges the need to postulate mechanisms of superexchange through redox centres or the maintenance of charge neutrality when investigating electron-transfer reactions. Such tunnelling also allows sequential electron transfer in catalytic sites to surmount radical transition states without involving the movement of hydride ions, as is generally assumed. The 14 A or less spacing of redox centres provides highly robust engineering for electron transfer, and may reflect selection against designs that have proved more vulnerable to mutations during the course of evolution.     

Real-time detection of electron tunnelling in a quantum dot

Nanostructures in which strong (Coulomb) interactions exist between electrons are predicted to exhibit temporal electronic correlations. Although there is ample experimental evidence that such correlations exist, electron dynamics in engineered nanostructures have been observed directly only on long timescales. The faster dynamics associated with electrical currents or charge fluctuations are usually inferred from direct (or quasi-direct) current measurements. Recently, interest in electron dynamics has risen, in part owing to the realization that additional information about electronic interactions can be found in the shot noise or higher statistical moments of a direct current. Furthermore, interest in quantum computation has stimulated investigation of quantum bit (qubit) readout techniques, which for many condensed-matter systems ultimately reduces to single-shot measurements of individual electronic charges. Here we report real-time observation of individual electron tunnelling events in a quantum dot using an integrated radio-frequency single-electron transistor. We use electron counting to measure directly the quantum dot's tunnelling rate and the occupational probabilities of its charge state. Our results provide evidence in favour of long (10 micros or more) inelastic scattering times in nearly isolated dots.     

双线隧道施工对既有基桩的影响研究离心机模型试验研究

为了确定双线地铁隧道开挖对近接建筑桩的影响,采用一系列室内离心机模型来模拟不同覆土直径比(C/D)下双线隧道近接已有基桩的施工过程,通过定量控制隧道外套的水囊注放水的方式模拟和控制隧道的地层损失。通过离心机试验测定了不同覆土直径比下隧道近接施工横截面地表沉降和基桩的沉地表与基桩的沉降量降,以及单桩轴力与侧摩阻力在隧道推进过程中的变化情况随着隧道推进距离的变化规律和单桩轴力与侧摩阻力的变化情况。结果表明,隧道与桩和地表的相对位置对沉降的量值和分布规律以及基桩的轴力、侧摩阻力有显著影响。     

非旋波近似下两个原子与场模型中的聚束与反聚束效应

用完全量子化方法，研究在非施波近似下两个二能级原子与场相互作用过程中的聚束与反聚束效应，揭示了光场频率的变化对场的二阶相干度的影响，并讨论了光场频率与原子间耦合系数的变化民场的二阶相干度的关系。