X-ray laser research at the LULI (Palaiseau)

The X-ray laser program at Palaiseau is based on the recombination scheme in lithiumlike ions, which requires a moderate pump power and seems to be promising for the purpose of scaling to shorter wavelengths. In aluminum plasmas, peak gain values of 2-2.5 cm^-1 have been obtained at a wavelength of 105.7 Å corresponding to the 3d -5f transition, 6 ns after the top of a 2 ns laser pulse. The same transition in sulfur is emitted at 65.2 Å and has shown a gain of 1 cm^-1 in a preliminary time-integrated experiment. Simulations using a collisional-radiative model as the postprocessor of a hydrodynamic numerical code predict amplifications for the 3d-4f, 3d-5f, and 4d-5f transitions. A new experiment, in progress at the present time, has been designed to enhance the gain-length product up to 10-15 at 105.7 Å. The recently extended facilities of the LULI make it possible to produce a 6-cm-long plasma column, keeping the flux density at the same level as in the previous experiments     

A slow wave free-electron laser in a longitudinal wiggler field

Linearized Vlasov-Maxwell equations are solved to obtain the growth rate of free electron laser instability from a tenuous relativistic electron beam propagating in a partially dielectric loaded waveguide immersed in combined axial and longitudinal wiggler magnetic fields. The instability appears via cyclotron resonance interactions for wave perturbations very close to w-kV_z-w_c=nk ₀V_Z where n is the general harmonic number. For n=0, the gain is similar to a slow wave cyclotron amplifier. For n⩾1, the growth rate is substantially larger than the standard slow wave free electron laser scheme utilizing a transverse wiggler field     

Adsorption of hydrogen and carbon monoxide on Rh(1 1 1)/V surface alloys

A detailed study of the interaction of hydrogen and carbon monoxide with two different Rh(1 1 1)/V surface alloys (1/3 monolayer of V in the second atomic layer or 1/3 monolayer of V in form of islands on the surface) is presented in comparison to the clean Rh(1 1 1) surface. For hydrogen a decrease in the sticking coefficient is found for both alloy surfaces. The sticking coefficient of H₂ as a function of the translational energy is similar to the Rh(1 1 1) surface, showing a direct activated adsorption mechanism. For low translational energies hydrogen adsorption is dominated by dynamical steering on Rh(1 1 1) and by a precursor mechanism on the Rh(1 1 1)/V subsurface alloy. The H₂ TPD desorption peaks are shifted to lower temperatures on the alloy surfaces, caused by the downshift of the metal d-band due to V alloying. On all three surfaces the saturation coverage of hydrogen was measured, giving 1.2, 1.0 and 0.8 monolayer for Rh(1 1 1), the Rh(1 1 1)/V subsurface alloy and for the Rh(1 1 1)/V islands, respectively. For CO the sticking coefficients and the saturation coverages are basically the same on the Rh(1 1 1) and the alloy surfaces. There is an extrinsic precursor on the ordered CO (√3×√3) phase on the Rh(1 1 1) surface, but there is no evidence for such a precursor on the Rh(1 1 1)/V subsurface alloy. On the Rh(1 1 1)/V islands surface, the extrinsic precursor exists on the Rh(1 1 1) surface between the V islands. Apparently this precursor is only stable on the ordered CO layer on Rh(1 1 1).     

Highly stable passivation of a Si(1 1 1) surface using bilayer-GaSe

As a stable and ‘epitaxial’ passivation of a Si surface, we propose the bilayer-GaSe termination of a Si(1 1 1) surface. This surface is fabricated by depositing one monolayer of Ga on a clean Si(1 1 1) surface and subsequent annealing in a Se flux at around 520 °C, which results in unreconstructed 1×1 termination of the Si(1 1 1) surface by bilayer-GaSe. We found by scanning tunneling microscopy observation that slow cooling of the clean Si(1 1 1) surface from 850 to 520 °C with simultaneous deposition of a Ga flux results in better termination of the Si(1 1 1) surface. It was also found that this surface is stable against heating around 400 °C in O₂ atmosphere of 3×10⁻³ Pa. By utilizing these properties of the bilayer-GaSe terminated surface, we have succeeded in fabricating ZnO quantum dots on this substrate.     

灵芝红外光谱的特征峰研究

利用红外光谱法及双指标序列法对不同产地和不同品种灵芝进行红外光谱的特征峰研究,结果表明：不同产地和不同品种的绝大多数灵芝红外光谱具有相似的特征峰, 其共有峰率可达到91.67%以上,但菌丝体品种其共有峰率达到52.94%～58.82%之间,变异峰率可达29.41%～66.67%之间,主要变异峰在指纹图谱区域1 460.9～1 423.7 cm-1之间出现阶梯峰。灵芝红外光谱明显的特征峰主要显示为多糖的特征峰型并伴有蛋白质谱带。在3 377.8～3 396.5 cm-1处有明显的宽而强的吸收峰, 在2 924.2～2 925.1 cm-1有小肩峰, 在1 635.8～1 650.3 cm-1处有中等强度的吸收峰, 在1 372.5～1 375.2 cm-1处有中等强度的吸收峰, 在1 074.8～1 075.3和1 043.2～1 045.2 cm-1处有很强的吸收分叉尖峰, 在指纹图谱区域891.0～894.8 cm-1处有明显的弱峰。在指纹图谱区域563.10～574.7 cm-1有中等强度的吸收峰。     

某型高性能常规涡轮对转改型研究

本文对某高性能常规涡轮开展了对转改型研究。首先对该常规涡轮进行基本分析,详细探讨了将其改进为1+1及1+1/2对转涡轮(无低压涡轮导叶)时遇到的关键问题,并给出了一些解决问题的方案.研究表明,1+1对转方案现实可行,1+1/2对转方案将很难实现.在方案研究的基础之上,本文进行了保留过渡段的1+1对转涡轮详细设计并采用三维黏性程序对其性能进行了初步的分析。     

Electrical characteristics of pulsed glow discharges

The i-v characteristics, energy partitioning, and time evolution of the discharge current and reduced field (E/N) for a nitrogen discharge are simulated using a self-consistent calculation of the electron energy distribution function and the vibrational level populations. The model includes diffusion losses and takes account of the external circuit parameters. The results discussed are for pressures of 1-100 torr, discharge currents in the range of 10^-3-5.0 A, and a reduced field (E/N) in the range of 150-250 Td. For a typical discharge in a tube of 2-cm diam. and a current of a few amperes, the results show that the energy stored in the vibrational manifold saturates a few milliseconds after the initiation of the discharge     

Triggered alternans in an ionic model of ischemic cardiac ventricular muscle

It has been known for several decades that electrical alternans occurs during myocardial ischemia in both clinical and experimental work. There are a few reports showing that this alternans can be triggered into existence by a premature ventricular contraction. Detriggering of alternans by a premature ventricular contraction, as well as pause-induced triggering and detriggering, have also been reported. We conduct a search for triggered alternans in an ionic model of ischemic ventricular muscle in which alternans has been described recently: a one-dimensional cable of length 3 cm, containing a central ischemic zone 1 cm long, with 1 cm segments of normal (i.e., nonischemic) tissue at each end. We use a modified form of the Luo-Rudy [Circ. Res. 68, 1501-1526 (1991)] ionic model to represent the ventricular tissue, modeling the effect of ischemia by raising the external potassium ion concentration ([K(+)](o)) in the central ischemic zone. As [K(+)](o) is increased at a fixed pacing cycle length of 400 ms, there is first a transition from 1:1 rhythm to alternans or 2:2 rhythm, and then a transition from 2:2 rhythm to 2:1 block. There is a range of [K(+)](o) over which there is coexistence of 1:1 and 2:2 rhythms, so that dropping a stimulus from the periodic drive train during 1:1 rhythm can result in the conversion of 1:1 to 2:2 rhythm. Within the bistable range, the reverse transition from 2:2 to 1:1 rhythm can be produced by injection of a well-timed extrastimulus. Using a stimulation protocol involving delivery of pre- and post-mature stimuli, we derive a one-dimensional map that captures the salient features of the results of the cable simulations, i.e., the {1:1-->2:2-->2:1} transitions with {1:1<-->2:2} bistability. This map uses a new index of the global activity in the cable, the normalized voltage integral. Finally, we put forth a simple piecewise linear map that replicates the {1:1<-->2:2} bistability observed in the cable simulations and in the normalized voltage integral map. (c) 2002 American Institute of Physics.     

A study of a parallel mode launch into a magnetic beach

A parallel mode launch in the electron cyclotron range of frequencies is performed on axis in the east end cell of the Phaedrus-B tandem mirror. Propagation of the energy launched toward a magnetic beach in a radially hollow density profile was studied experimentally. A strong refractive dispersal of the launched energy was observed and shown to be in good agreement with a WKB-type ray tracing model using a cold plasma Appleton-Hartree dispersion relation. Measurements of plasma density and electron temperature indicate that the use of this dispersion relation is well justified. These results are compared with additional ray tracing studies for radially hollow and nonhollow, high ( f_launch<f_p) and low (f _launch>f_p) density models. For the experimental conditions examined, a reversal of refractive behavior is shown to exist as the energy propagates from a low density (f_launch>f_p) region to a high density (f_launch<f_p) region     

Formation of two-dimensional solid and liquid Pb on Ge(1 1 1) surfaces

When more than 0.7 monolayer of Pb was deposited onto Ge(1 1 1) surfaces at room temperature, a (√3×√3)R30° structure appeared at the surfaces, which was interpreted as resulting from the formation of a pseudo-Pb(1 1 1) overlayer. On heating, the Pb overlayer melted into a two-dimensional liquid Pb and a few diffuse rods newly emerged in (1×1) RHEED patterns from the surfaces. The diffuse rods looked alike in various incidence azimuths. This shows that the structure in the liquid is nearly random in respect to azimuthal orientation, implying that the liquid Pb is not under the strong influence of the Ge(1 1 1) substrate.     

In situ STM study of nanosized Ru and Os islands spontaneously deposited on Pt(1 1 1) and Au(1 1 1) electrodes

We provide an overview of structure and reactivity of selected bimetallic single crystal electrodes obtained by the method of spontaneous deposition. The surfaces that are described and compared are the following: Au(1 1 1)/Ru, Pt(1 1 1)/Ru and Pt(1 1 1)/Os. Detailed morphological information is presented and the significance of this work in current and further study of nanoisland covered surfaces in the catalytic and spectroscopic perspective is highlighted. All surfaces were investigated by in situ STM and by electroanalytical techniques. The results confirm our previous data that nanosized Ru islands are formed with specific and distinctive structural features, and that the Ru growth pattern is different for Au(1 1 1) and Pt(1 1 1). For Au(1 1 1), Ru is preferentially deposited on steps, while a random and relatively sparse distribution of Ru islands is observed on terraces. In contrast, for Ru deposited on Pt(1 1 1), a homogeneous deposition over all the Pt(1 1 1) surface was found. Os is also deposited homogeneously, and at a much higher rate than Ru, and even within a single deposition it forms a large proportion of multilayer islands. On Au(1 1 1), the Ru islands on both steps and terraces reach the saturation coverage within a short deposition time, and the Ru islands grow to multilayer heights and assume hexagonal shapes. On Pt(1 1 1), the Ru saturation coverage is reached relatively fast, but when a single deposition is applied, Ru nanoislands of mainly monoatomic height are formed, with the Ru coverage not exceeding 0.2 ML. For Ru deposits on Pt(1 1 1), we demonstrate that larger and multilayer islands obtained in two consecutive depositions can be reduced in size--both in height and width--by oxidizing the Ru islands and then by reducing them back to a metallic state. A clear increase in the Ru island dispersion is then obtained. However, methanol oxidation chronoamperometry shows that the surface with such a higher dispersion is less active to methanol oxidation than the initial surface. A preliminary interpretation of this effect is provided. Finally, we studied CO stripping reaction on Pt(1 1 1)/Ru, Au(1 1 1)/Ru and on Pt(1 1 1)/Os. We relate CO oxidation differences observed between Pt(1 1 1)/Ru and Pt(1 1 1)/Os to the difference in the oxophilicity of the two admetals. In turn, the difference in the CO stripping reaction on Pt(1 1 1)/Ru and Au(1 1 1)/Ru with respect to the Ru islands is linked to the effect of the substrate on the bond strength and/or adlayer structure of CO and OH_ads species.     

Some interruption criteria for short high-frequency vacuum arcs

If the contacts of a vacuum interrupter open shortly before a current zero, the transient recovery voltage (TRV) can cause a reignition and reestablish the arc. When the current in a diffuse vacuum arc passes through zero, there is a distinct pause before the TRV builds up (approximately 40 ns for copper). During this pause the gap carries conduction current only with an ion component which depends on dI /dt, varying between 3 A for dI/dt=60 A/μs and 60 A for dI/dt=1235 A/μs. The ion current subsequently decays in tens or hundreds of nanoseconds. It can be distinguished from the displacement current at this time by varying dV/dt, keeping the other parameters constant. Among the interruption criteria for short high-frequency vacuum arcs, dI /dt prior to current zero and initial dV/dt are the most important. High values of dI/dt are more likely to precipitate reignitions, but breakdowns can occur after lower dI/dt's if the gap has been subjected to a high current for a relatively long time (>100 μs)     

Hysteresis and bistability in the direct transition from 1:1 to 2:1 rhythm in periodically driven single ventricular cells

The transmembrane potential of a single quiescent cell isolated from rabbit ventricular muscle was recorded using a suction electrode in whole-cell recording mode. The cell was then driven with a periodic train of current pulses injected into the cell through the same recording electrode. When the interpulse interval or basic cycle length (BCL) was sufficiently long, 1:1 rhythm resulted, with each stimulus pulse producing an action potential. Gradual decrease in BCL invariably resulted in loss of 1:1 synchronization at some point. When the pulse amplitude was set to a fixed low level and BCL gradually decreased, N+1:N rhythms (N>/=2) reminiscent of clinically observed Wenckebach rhythms were seen. Further decrease in BCL then yielded a 2:1 rhythm. In contrast, when the pulse amplitude was set to a fixed high level, a period-doubled 2:2 rhythm resembling alternans rhythm was seen before a 2:1 rhythm occurred. With the pulse amplitude set to an intermediate level (i.e., to a level between those at which Wenckebach and alternans rhythms were seen), there was a direct transition from 1:1 to 2:1 rhythm as the BCL was decreased: Wenckebach and alternans rhythms were not seen. When at that point the BCL was increased, the transition back to 1:1 rhythm occurred at a longer BCL than that at which the {1:1-->2:1} transition had initially occurred, demonstrating hysteresis. With the BCL set to a value within the hysteresis range, injection of a single well-timed extrastimulus converted 1:1 rhythm into 2:1 rhythm or vice versa, providing incontrovertible evidence of bistability (the coexistence of two different periodic rhythms at a fixed set of stimulation parameters). Hysteresis between 1:1 and 2:1 rhythms was also seen when the stimulus amplitude, rather than the BCL, was changed. Simulations using numerical integration of an ionic model of a single ventricular cell formulated as a nonlinear system of differential equations provided results that were very similar to those found in the experiments. The steady-state action potential duration restitution curve, which is a plot of the duration of the action potential during 1:1 rhythm as a function of the recovery time or diastolic interval immediately preceding that action potential, was determined. Iteration of a finite-difference equation derived using the restitution curve predicted the direct {1:1<-->2:1} transition, as well as bistability, in both the experimental and modeling work. However, prediction of the action potential duration during 2:1 rhythm was not as accurate in the experiments as in the model. Finally, we point out a few implications of our findings for cardiac arrhythmias (e.g., Mobitz type II block, ischemic alternans). (c) 1999 American Institute of Physics.     

Temperature dependence of surface electronic structure of Si(111) surface

Temperature dependence of angle-resolved ultraviolet photoelectron spectra has been obtained for Si(111) surfaces starting with a thermally quenched “1 × 1” surface and ending with a high temperature “1 × 1” surface. it has been found that the surface state at 0.8 eV below the Fermi level exhibits degradation with the increase in temperature, which explains the difference of surface electronic structures between a quenched “1 × 1” surface and a high temperature “1 × 1” surface. Electron correlation effect in a dangling-bond derived surface state is postulated as a cause for the phenomenon.     

Influence of surface band gaps on the lifetimes of charge transfer states

Using a first principles electronic structure method, we investigate how a band gap in the surface projected density of states of a noble metal influences the resonant tunneling lifetime of an atomic level near the surface. Comparing Li atoms outside Cu(1 1 1), where the Li 2s level is within the gap, with Cu(0 0 1), where the Li 2s level is below the gap, reveals that the lifetime of the ionization level of Li is longer on Cu(1 1 1). This difference is caused both by the decreased final state phase space for the tunneling electron on Cu(1 1 1) and by a reduction in intra-atomic hybridization in Li outside a surface whose band gap encompasses the ionization level. Our results are consistent with previous experiments and theory that predict a similar trend for the ionization level of Cs above Cu(1 1 1) and Cu(0 0 1).     

三大产地（波罗的海、多米尼加和缅甸）琥珀红外光谱鉴别特征

采用镜面反射法和溴化钾压片透射法对波罗的海、多米尼加和缅甸琥珀进行红外光谱测试,获得了不同产地的红外光谱鉴别特征。经仔细分析认为,尽管不同产地琥珀的红外光谱具有共同特征,但某些吸收峰位置或相对强度仍存在差异。3 000～2 800 cm-1范围内,波罗的海琥珀有两个明显的吸收峰和一个弱肩峰,多米尼加琥珀有两个吸收峰,缅甸琥珀具有三个特征吸收峰;1 740～1 690 cm-1范围内,波罗的海琥珀出现1 732 cm-1附近强吸收峰,多尼米加和缅甸琥珀有两个吸收峰,分别位于1 730和1 695 cm-1附近,但是多米尼加琥珀1 730 cm-1峰强度小于1 695 cm-1,与缅甸琥珀相反;1 300～1 000 cm-1范围内,波罗的海琥珀具有典型的“波罗的海肩峰”及1 163 cm-1附近的强吸收,多米尼加琥珀出现1 240 cm-1附近吸收峰,缅甸琥珀在此范围内的强吸收峰组成“山”字型;波罗的海琥珀R（A_{1 381 cm-1}/A_{1 456 cm-1}）值约为0.9,多米尼加琥珀约0.8,而缅甸为0.7。上述差异可以作为判断琥珀产地的鉴别依据。不同产地琥珀的红外光谱出现差异,可能与其形成年代、植物种源和地质环境不同有关。根据红外光谱中与环外C=C双键有关的吸收峰的存在和强度,推断三个产地的琥珀形成时间由早到晚分别为缅甸、波罗的海、多米尼加。红外光谱在识别琥珀产地方面具有重要的宝石学意义,同时在考古学研究与应用中有潜在意义。     

Density functional theory investigation of CN on Cu(1 1 1), Ni(1 1 1) and Ni(1 0 0)

The adsorption of CN on Cu(1 1 1), Ni(1 1 1) and Ni(1 0 0) has been investigated using density functional theory (DFT). While experimental studies of CN on Cu(1 1 1) show the molecular axis to be essentially parallel to the surface, the normally-preferred DFT approach using the generalised gradient approximation (GGA) yields a lowest energy configuration with the C-N axis perpendicular to the surface, although calculations using the local density approximation (LDA) do indicate that the experimental geometry is energetically favoured. The same conclusions are found for CN on Ni(1 1 1); on both surfaces bonding through the N atom is always unfavourable, in contrast to some earlier published results of ab initio calculations for Ni(1 1 1)/CN and Ni(1 0 0)/CN. The different predictions of the GGA and LDA approaches may lie in subtly different relative energies of the CN 5σ and 1π orbitals, a situation somewhat similar to that for CO adsorbed on Pt(1 1 1) which has proved challenging for DFT calculations. On Ni(1 0 0) GGA calculations favour a lying-down species in a hollow site in a geometry rather similar to that found experimentally and in GGA calculations for CN on Ni(1 1 0).     

Single-photon generation from stored excitation in an atomic ensemble

Single photons are generated from an ensemble of cold Cs atoms via the protocol of Duan et al. [Nature (London), ()]]. Conditioned upon an initial detection from field 1 at 852 nm, a photon in field 2 at 894 nm is produced in a controlled fashion from excitation stored within the atomic ensemble. The single-quantum character of field 2 is demonstrated by the violation of a Cauchy-Schwarz inequality, namely w(1(2),1(2)|1(1))=0.24+/-0.05 not > or = 1, where w(1(2),1(2)|1(1)) describes the detection of two events (1(2),1(2)) conditioned upon an initial detection 1(1), with w-->0 for single photons.     

Spin dynamics in iron-based layered superconductor (La0.87Ca0.13)FePO revealed by 31P and 139La NMR studies

We report 31P and 139La NMR studies of (La0.87Ca0.13)FePO, which is a family member of the recently discovered superconductor LaFeAs(O1-xFx). In the normal state, Knight shift and nuclear spin-lattice relaxation rate divided by T (1/T1T) show that a Fermi-liquid state with moderate ferromagnetic fluctuations emerges below 30 K. From 1/T1T of 31P and 139La, a quasi-two- dimensional electronic structure is suggested, in which the FeP layer is more conductive than the LaO layer. In the superconducting (SC) state, although a clear Meissner signal was observed, 1/T1T increases below Tc, in contrast to a decrease of 1/T1T due to the opening of a SC gap, suggesting that novel low-energy spin dynamics develop in the SC state.     

荧光法测定镧原子高激发态的超精细结构

在Ｌａ空心阴极灯中，用荧光法测定了二级跃迁（１６８５６ｃｍ＾－１→３４２７２ｃｍ＾－１），（２００８２ｃｍ＾－１→２６８５３ｃｍ＾－１）和（２００８２ｃｍ＾－１→３７５４４ｃｍ＾－１）的超精细结构光谱，得到了３４２７２ｃｍ＾－１，３６８５３ｃｍ＾－１和３７５４４ｃｍ＾－１能级的Ａ常数，这些镧原子高激发态超精细结构数据是首次报道。