Properties of resonant interatomic Coulombic decay in Ne dimers

Properties of the interatomic Coulombic decay (ICD) process in Ne dimers have been obtained by tracking the formation of energetic Ne+ ions. The double photoionization cross section, deduced from the Ne+/Ne+ coincidence signal, is dominated by the ICD process and presents a threshold 280 meV below the atomic Ne+2s(-1) threshold. Rydberg excitation of a 2s electron in the dimer creates molecular Rydberg states whose Sigma and Pi symmetries have been resolved. These excited states decay by a resonant ICD process releasing an energetic Ne+ ion and a neutral excited Ne* fragment. Subsequent autoionization of the Ne* fragment explains a double photoionization threshold below the dimer 2s ionization threshold.     

Femtosecond interatomic Coulombic decay in free neon clusters: large lifetime differences between surface and bulk

A quantitative determination of 2s vacancy lifetimes in surface and bulk atoms of free Ne clusters has been made. While for free atoms the 2s inner-valence hole has a ps lifetime, it reduces to 6+/-1 fs for cluster bulk atoms. For surface atoms, the lifetime is on average longer than 30 fs. The lifetime estimate was obtained from fits of high-resolution photoelectron spectra of Ne clusters. The shortening of the lifetime is attributed to the coordination dependent interatomic Coulombic decay, which is extremely sensitive to internuclear distances.     

Three-electron interatomic Coulombic decay from the inner-valence double-vacancy states in NeAr

We have unambiguously identified interatomic Coulombic decay in NeAr from the inner-valence double-vacancy state Ne-Ar(2+)(3s(-2)) to outer-valence triple-vacancy states Ne(+)(2p(-1))-Ar(2+)(3p(-2)) by momentum-resolved electron-ion multicoincidence. This is the first observation of interatomic Coulombic decay where three electrons (3e) participate. The results suggest that this 3e interatomic Coulombic decay is significantly faster than other competing processes like fluorescence decay and charge transfer via curve crossing.     

Electron-transfer-mediated decay and interatomic Coulombic decay from the triply ionized states in argon dimers

We report the first observation of electron-transfer-mediated decay (ETMD) and interatomic Coulombic decay (ICD) from the triply charged states with an inner-valence vacancy, using the Ar dimer as an example. These ETMD and ICD processes, which lead to fragmentation of Ar(3+)-Ar into Ar(2+)-Ar(2+) and Ar(3+)-Ar+, respectively, are unambiguously identified by electron-ion-ion coincidence spectroscopy in which the kinetic energy of the ETMD or ICD electron and the kinetic energy release between the two fragment ions are measured in coincidence.     

Partial widths and branching ratios for the emitted electron resulting from interatomic Coulombic decay in quantum wells heterostructure

Interatomic coulomb decay (ICD) is a decay process relying on the Coulombic interaction between neighbouring atoms, molecules or nanostructures. Due to this process, an electron is emitted into the continuum. We study the ICD process in a system of the double quantum well heterostructure and investigate how we can manipulate the structure's parameters such that a better detection of the ICD's emitted electron is achieved. For this purpose, we calculated the partial widths (PWs) and branching ratios (BRs) of the ICD's emitted electron to the left and right asymptotes of the heterostructure; these will give an estimation of the detection current. We manipulated the structure's parameters and took into account the repulsion from the electron in the ground state located in the left well. By introducing two small barriers in the vicinity of the right QW, we observed a BR three times larger than in the structure without the barriers. We also investigate the effect of repulsion due to the second electron. This work gives a better understanding of the dynamics of the scattered ICD's electron, and realisation of better design rules for future experimental observation of ICD in nanostructures.     

Compact free electron lasers: From Cerenkov to waveguide free electron lasers

The basic principles of free electron generators of coherent radiation and the practical considerations related to the development of far infrared and sub-millimeter wave sources are discussed in this paper. The attention is focused on compact devices, which reduce the size and cost issue. A review of the main results obtained at the Frascati FEL Facility is presented together with the planned experiments.     

X射线自由电子激光

X射线自由电子激光是一种基于电子直线加速器的大型科学研究装置,可以产生超高亮度、超短脉冲、波长可调的相干X射线辐射,在物理、化学、材料、生命科学等诸多领域都有非常广泛和极为重要的应用。文章介绍了X射线自由电子激光的基本原理、运行模式、发展历程、国内外X射线自由电子激光大科学装置的发展态势以及近年来我国X射线自由电子激光装置的现状。     

Assessment of free electron lasers

Free electron lasers represent a radical alternative to conventional lasers, being potentially the most flexible, high power and efficient generators of coherent radiation from the ultra-violet to the far infra-red. In this review the properties of free electron lasers are discussed from both a theoretical and an experimental viewpoint, and some likely areas of application for these devices are outlined.     

Orbit stability in free electron lasers

Helical magnetic wigglers for free electron lasers can produce non-helical electron trajectories if a uniform axial guide magnetic field is imposed. Friedland's necessary criterion for the existence of helical orbits is reviewed and shown to apply for non-relativistic electron energies. An experiment designed to test this criterion is described and results are compared with theory.This research was ponsored by the U.S. Office of Naval Research, and by the U.S.-Israel Binational Science Foundation.     

Nonstationary generation in free electron lasers

Evolution of the alternating field structure in the high-Q multimode cavity excited by the beam of relativistic electron oscillators is described by a self-consistent set of equations. Stationary and pulse injection regimes are investigated. The injection current increasing over the threshold, three consecutive stages take place: a) stationary single-mode generation (or, correspondingly, generation of identical pulses), b) periodic and c) stochastic self-modulation.     

Common properties of free electron lasers

A general theory of microwave devices (in particular, ubitron, gyrotron) based on the ultrarelativistic Doppler up-conversion of electron oscillation frequency is developed. The wave growth rate for the amplifier and the start current for the generator are obtained. A universal relation between the frequency conversion factor and the efficiency of the laser is derived.     

Two-stream instability in free electron lasers

The analysis of the interaction between electromagnetic waves and electron beams in semi-infinite wigglers is presented. The beam is assumed to include two cold streams of electrons; hence, its space-charge waves may be unstable (two-stream instability). In the wiggler, this instability is shown to yield much larger growth rates (up to seven times greater) than in the conventional one-stream free electron laser (FEL). Accordingly, the gain per pass is enhanced by orders of magnitude. The enhancement of the two-stream instability is shown to be most effective for short-period wigglers     

A planar electromagnet microwiggler for free electron lasers

The authors have designed and tested an electromagnet planar microwiggler for use in free electron lasers (FELs), constructed of current conductors wound on ferromagnetic cores. A prototype with a period of 1 cm and a gap of 0.5 cm produced a peak field on axis in excess of 4.6 kG, with a linear B/H characteristic to about 3.2 kG. The field of each half-period of the wiggler is independently tunable by adjusting the current delivered to each, thus allowing for precision tuning and/or wiggler tapering. General scaling laws are employed to predict the performance of a geometrically similar design with a period of 5 mm     

X射线自由电子激光单颗粒成像研究

具有超高亮度、超短脉冲、全相干特性的X射线自由电子激光(X-ray Free Electron Lasers,XFELs)的出现为超快时间研究与超微结构探索带来新的机遇,使得获取单分子、单颗粒原子分辨率图像及电影成为可能。随着德国FLASH、意大利FERMI、美国LCLS以及日本SACLA等装置的建成与投入使用,X射线自由电子激光已经进入了快速发展的阶段,一系列物理、化学、生物、材料科学领域的前沿研究成果不断涌现。为突破实验技术、工程设备及软件算法上的技术壁垒,相关科研机构通过国际合作,拟实现纳米颗粒、细菌、细胞、病毒、团簇及生物学大分子等单颗粒的原子分辨率成像。文章将聚焦单颗粒成像的发展历史、科学意义、研究背景、研究目标、研究规划、研究现状及世界各国的布局,并展望单颗粒成像未来的发展。     

Transverse mode frequency pulling in free electron lasers

We consider the spectral evolution of transverse modes in free electron lasers. While the gain maxima of the various transverse modes occur at different frequencies, coupling between the modes via the electron beam causes the modes to shift frequency such that the final laser frequency differs from what one would expect if only the TEM₀₀ mode were present. This can strongly effect the turn-on behavior in lasers with short electron pulses.     

Echo-enabled谐波放大自由电子激光参数

通过数值模拟,结合解析计算分析,研究了Echo-enabled谐波放大自由电子激光对装置参数和主要束流参数变化的独立敏感性,给出了谐波聚束因子和饱和功率随参数变化的数值模拟结果;对色散段由相干同步辐射效应引起的能散和发射度增长进行了估算和数值模拟。结果表明：相干同步辐射效应引起的切片能散和切片发射度增长在5%以内,对最后辐射影响不大。