Study of the reactione + e −→K + K − in the energy range1350 \leqq \sqrt s \leqq 2400 MeV

Thee ⁺ e ^?→K ⁺ K ^? cross section has been measured from about 750 events in the energy interval \(1350 \leqq \sqrt s \leqq 2400 MeV\) with the DM2 detector at DCI. TheK ^± form factor |F _F ^±| cannot be explained by the ρ, ω, ? and ρ′(1600). An additional resonant amplitude at 1650 MeV has to be added as suggested by a previous experiment.     

Pulse laser acoustics for the characterization of inhomogeneities at interfaces of microstructures

Interfaces between neighbouring materials are often subjected to diffusion processes which cause layers having gradually varying mechanical properties--like densities, Young's moduli or shear moduli--perpendicular to the surface or interface. In this investigation particular interest is drawn on the question how the propagation characteristics of bulk acoustic waves are affected by diffusion layers. The reflection and transmission behavior of bulk acoustic waves encountering a continuum having a spatially dependent sound velocity is discussed based on numerical simulations as well as on experimental verifications. The simulated results are part of an on-going project in which material properties of MEMS devices are investigated by short pulse laser acoustic methods. Mechanical waves are excited and detected thermoelastically using laser pulses of 70 fs duration. For metals this leads to wavelengths of 10-20 nm and the corresponding frequencies amount to 0.3-0.6 THz. In contrast to previous work done in this field in which diffusion effects are generally considered as undesirable phenomena, the deliberate realization of microstructures having well defined gradually varying material properties in one or more dimensions represents a goal of this investigation. For metallic thin film multilayers thermally induced diffusion processes have shown to be an easy and reliable technique for the realization of layered structures having continuously varying mechanical properties within several 10 nm. Among the experimental methods suitable for the in-depth profiling of submicron metallic thin films providing resolutions of several nanometers, are short pulse laser acoustic methods, Rutherford backscattering spectroscopy (RBS), and glow discharge optical emission spectroscopy (GDOES). Short pulse laser acoustic methods and RBS have the advantage to be nondestructive. The short pulse laser acoustic method is described in detail and RBS measurements are presented for verification purposes. Finally potential engineering applications like micro-machined spectrum analyzers, acoustic isolation layers, and band pass filters, operating at very high frequencies are presented.     

Radiative decay ofJ/ψ into γ π+ π−

The radiative decayJ/ψ → γ π⁺ π^? has been studied using the 8.6 millionJ/ψ produced in the DM2 experiment at the DCIe ⁺e^? storage rings at Orsay. The π⁺ π^? mass spectrum shows a cleanf ₂ (1270) signal, and the possible presence of two other states at thef ₂ (1720) andf ₄ (2030) masses. For thef ₂ (1270), the branching ratio BR(J/ψ →γf)xBR(f→π⁺ π^?) is measured to be (7.50±0.30±1.12)×10^?4, and the spin analysis prefers theJ=2 assignment, with helicity parametersx=0.83±0.06 andy=0.01±0.06. The existence of higher mass states is discussed.     

Non-planar smoulder propagation governed by diffusion

The steady propagation of a thin smouldering front in a half-spacehas been considered. A suitable coordinate transformation hasallowed the region near the leading edge of the front to beexamined for both a maintained planar surface and with surfacecollapse due to material shrinkage. The change in the oxidizerconcentration for a small increment in the propagation speedfor large time and surface collapse has been determined. Theinfluence of two types of nonlinear diffusion on the shape ofthe smouldering front has been found; other cases can be dealtwith in a similar manner.     

Determination of the material properties of microstructures by laser based ultrasound

In most applications of MEMS the mechanical properties of the used materials are key parameters for the perfect working of the microsystems. Measuring bulk acoustic waves excited in MEMS structures with ultra-short laser pulses is a powerful method for the accurate and non-destructive evaluation as well as for the characterization of material properties. The pump-probe laser-based acoustic method generates bulk acoustic waves in a thermo-elastic way by absorbing the pump laser pulses. The acoustic waves are partly reflected at any discontinuity of the acoustic impedance. At the surface of the specimen the reflected acoustic pulses cause changes of the optical reflection coefficient, which are measured with the probe laser pulses. Thin membranes are part of numerous microelectromechanical systems (MEMS) like sensors, activators and bulk acoustic wave (BAW) filters for example. The described non-destructive and non-contact method is the right approach for testing such thin and brittle structures like membranes. Results of measurements on freestanding aluminium-silicon nitride multi-layer membranes with total thicknesses in the order of several hundred nanometers are presented and compared with thermo-elastic models and with measurements of the supported case. The measured results are used for the determination of the moduli of the membranes.     

KMgF3:Ni2+晶体的占位、局部结构、吸收光谱和顺磁g因子的统一解释

利用完全对角化方法和强场耦合方案,采用半自洽场(semi-SCF)自由Ni2+的d轨道模型和Ni2+-6X-(x=F,Cl,Br,I)络合物的μ-κ-α模型研究,建立了含有过渡族金属离子的晶体的局域结构与吸收光谱和顺磁g因子之间的定量关系,对KMgF3:Ni2+晶体的占位、局域结构、吸收光谱和顺磁g因子作出了统一解释,预测了KMgF3:Ni2+晶体的光谱精细结构.所得理论计算结果与实验值符合得很好.     

Imaging ripples on phononic crystals reveals acoustic band structure and Bloch harmonics

Broadband surface phonon wave packets on a phononic crystal made up of a microstructured line pattern are tracked in two dimensions and in real time with an ultrafast optical technique. The eigenmode distribution and the 2D acoustic band structure are obtained from spatiotemporal Fourier transforms of the data up to 1 GHz. We find stop bands at the zone boundaries for both leaky-longitudinal and Rayleigh waves, and show how the structure of individual acoustic eigenmodes in k space depends on Bloch harmonics and on mode coupling.     

Sensitivity improvement of a pump-probe set-up for thin film and microstructure metrology

This investigation deals with various new aspects of the sensitivity improvement of a pump-probe laser based acoustic method. A short laser pulse is used to excite a mechanical pulse thermo-elastically. Echoes of these mechanical pulses reaching the surface are causing a slight change of the optical reflectivity. The surface reflectivity is scanned versus time with a probe pulse. Thus the time of flight of the acoustic pulse is measured. The quantity to be measured i.e. the optical reflectivity change deltaR caused by acoustic pulses, is rather small. A set-up having an estimated sensitivity deltaR/R of about 10(-5) has shown to be sufficient to detect up to the fifth echo in a 50 nm aluminum film on sapphire substrate. A key challenge is the reduction of optical and electrical cross-talk between the excitation and the detection. Therefore the concepts of double-frequency modulation, cross-polarization, and balanced photodetection are implemented. Practical aspects like beam guiding, modulation techniques, beam focus minimization, and beam focus matching are discussed. Measurements for single- and multi-layer metallic films demanding higher sensitivity are presented.     

Characterization of Ta and TaN diffusion barriers beneath Cu layers using picosecond ultrasonics

In computer chips, aluminum is being replaced with copper in order to produce smaller, faster and more efficient electronic devices. The usage of copper allows higher current densities and thus higher packaging densities than aluminum. However, copper leads to new challenges and problems. It has different mechanical properties and a tendency to migrate into the surrounding dielectric and/or semiconducting layers. These diffusion processes can be prevented by so called diffusion barriers. A diffusion barrier is a very thin layer consisting of tantalum and tantalum nitride or titanium and titanium nitride, deposited between the copper and the substrate. A pump-probe setup is used to determine the mechanical properties of the barrier layers and of the copper layer. This short-pulse-laser-acoustic method is contact-free and non-destructive. Mechanical waves are excited and detected thermoelastically using laser pulses of 70 fs duration. Thin film measurements of buried diffusion layers are provided and compared with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Rutherford Backscattering Spectroscopy measurements (RBS). Results of a thermo-elasto-mechanical simulation are presented and a short overview of the simulation procedure is given. Current limits of the presented method are discussed and future directions of the on-going research project are presented.