Partial Focusing of Coherent Optical Transition Radiation and Measurement of Transverse Size of Femtosecond Electron Bunches

Russian Physics Journal - Coherent optical transition radiation (COTR) from femtosecond electron bunches is simulated using an earlier proposed model. It is shown that partial screening of the COTR...     

A proposed method to characterize high harmonic microbunching in EEHG operation of SDUV-FEL

The echo-enabled harmonic generation （EEHG） scheme offers remarkable efficiency for generating high harmonic microbunching with a relatively small energy modulation. A proof of principle experiment of the EEHG scheme has been proposed at the Shanghai deep ultraviolet （SDUV） free electron laser （FEL） facility, where the 4th harmonic of the seed laser is amplified in the 9 m long radiator. To explore the advantages of the EEHG scheme, in this paper, a method of measuring the coherent high harmonic radiation of the radiator is proposed to investigate the electron beam microbunching corresponding to the 10th-20th harmonics of the seed laser. The principle of the proposed method, comparisons with existing methods and the simulation results are presented and discussed.     

Study of hot electrons by measurement of optical emission from the rear surface of a metallic foil irradiated with ultraintense laser pulse

Hot electrons and optical emission are measured from the rear surface of a metallic foil. The spectra of the optical emission in the near infrared region have a sharp spike around the wavelength of the incident laser pulse. The optical emission is ascribed to coherent transition radiation due to microbunching in the hot electron beam. It is found that the optical emission closely correlates with the hot electrons accelerated in resonance absorption.     

Generating optical orbital angular momentum in a high-gain free-electron laser at the first harmonic

A scheme to generate intense coherent light that carries orbital angular momentum (OAM) at the fundamental wavelength of an x-ray free-electron laser (FEL) is described. The OAM light is emitted as the dominant mode of the system until saturation provided that the helical microbunching imposed on the electron beam is larger than the shot-noise bunching that leads to self-amplified emission. Operating at the fundamental, this scheme is more efficient than alternate schemes that rely on harmonic emission, and can be applied to x-ray FELs without using external optical mode conversion elements.     

Observation of fine structures in laser-driven electron beams using coherent transition radiation

We have measured the coherent optical transition radiation emitted by an electron beam from laser-plasma interaction. The measurement of the spectrum of the radiation reveals fine structures of the electron beam in the range 400-1000 nm. These structures are reproduced using an electron distribution from a 3D particle-in-cell simulation and are attributed to microbunching of the electron bunch due to its interaction with the laser field. When the radiator is placed closer to the interaction point, spectral oscillations have also been recorded, signature of the interference of the radiation produced by two electron bunches delayed by 74 fs. The second electron bunch duration is shown to be ultrashort to match the intensity level of the radiation. Whereas transition radiation was used at longer wavelengths in order to estimate the electron bunch length, this study focuses on the ultrashort structures of the electron beam.     

Cumulative "roof effect" in high-resolution in vivo 31P NMR spectra of human calf muscle and the Clebsch-Gordan coefficients of ATP at 1.5 T

NMR spectra of non-weakly coupled spin systems exhibit asymmetries in line intensities known as "roof effect" in 1D spectroscopy. Due to limited spectral resolution, this effect has not been paid much attention so far in in vivo spectroscopy. But when high-quality spectra are obtained, this effect should be taken into account to explain the quantum-mechanical fine structure of the system. Adenosine 5'-triphosphate (ATP) represents a 31P spin system with multiple line splittings which are caused by J-couplings of medium strength at 1.5 T. We analyzed the ATP roof effect in vivo, especially for the beta-ATP multiplet. The intensities of its outer resonances deviate by ca. 12.5% from a symmetrical triplet. As this asymmetry reflects the transition from Paschen-Back to Zeeman effect with total spin that is largely broken up, the Clebsch-Gordan coefficients of the system can be indicated in analogy to the hyperfine structure of hydrogen. Taking the roof effect into account, the chi2 of fitting in vivo ATP resonances is reduced by ca. 9% (p<0.005).     

Observation of broadband self-amplified spontaneous coherent terahertz synchrotron radiation in a storage ring

Bursts of coherent synchrotron radiation at far-infrared and millimeter wavelengths have been observed at several storage rings. A microbunching instability has been proposed as the source for the bursts. However, the microbunching mechanism has yet to be elucidated. We provide the first evidence that the bursts are due to a microbunching instability driven by the emission of synchrotron radiation in the bunch. Observations made at the Advanced Light Source are consistent with the values predicted by the proposed microbunching model. These results demonstrate a new instability regime for high energy synchrotron radiation sources and could impact the design of future sources.     

Ground-State-Moment-Analysis: A quantitative tool for X-ray magnetic circular dichroism analysis for <Emphasis Type="Italic">3d</Emphasis> transition metals

By the use of sum rules and X-ray magnetic circular dichroism (XMCD) integral spectral values, experimentally determined element specific spin- and orbital-magnetic moments could be extracted. Therefore, sum rules neglect all additional spectral shape information. On the basis of so called ground state moments and their spectral representations, XMCD spectra could be fitted. The gain of information due to this method is directly related to the analysis of the spectral shape. Simple and complex spectra, with many different observable spectral features, could be quantitatively analyzed. Different unoccupied parts of the bandstructure have been resolved and intuitively interpreted by the moment analysis procedure. Focusing on this fitting method, we will review recent applications to Iron-Garnets and CrO₂ and show new results for Fe-, Mn-, and V-L_2,3 XMCD spectra. In addition, spectral overlap between the L₂ and L₃ edges at the light transition metal site could be handled by this method, and quantitative magnetic moments have been extracted from the spectra. This is the basis for a possible future element specific renormalization technique, used for the whole series of the transition metals, which is very important at the light transition metal site. PACS 87.64.Ni; 75.70.-i; 75.50.Cc; 75.70.Ak     

THE PHASE TRANSITION FROM THE "ROTATION" TO VIBRATION SPECTRA IN THE LIPKIN SU(2) MODEL

The collective monopole excitation spectra of the Lipkin SU(2) model aresolveaexactly in the truncated monopole phonon representation and also approximately in the continuous variable representation. The transition from the "rotation" to vibration spectra for lowly-lying exclted states with the increase of the particle number in the upper level and the disappearance of the monopole "deformation" with the increase of the excitation energy are exhibited. The origin of these phase transitions is explained and the shape transition of real nuclei with the number of valence nucleons i s briefly discussed.     

First Observation of z-Dependent Electron-Beam Microbunching Using Coherent Transition Radiation

We report the first measurements of the electron-beam microbunching z dependence in a self-amplified spontaneous-emission (SASE) free-electron laser (FEL) experiment by the observation of visible wavelength coherent transition radiation (CTR). In this case the fundamental SASE wavelength was at 537 nm, and the CTR exhibited an exponential intensity growth similar to the SASE radiation. In addition, we observed for the first time structure in the CTR angular distribution patterns that may be useful for optimizing SASE FEL performance.     

锡和氙离子在13.5 nm波长附近的光谱特性

 应用多组态Dirac-Fock方法,系统计算了锡（Sn）和氙（Xe）离子在13.5 nm波长附近的辐射跃迁波长和跃迁几率。深入分析了^(7~13)+Sn和Xe^(7~13)+离子跃迁的原子光谱特性,研究了相对论效应和电子关联效应对能级位置和跃迁几率的影响发现,相对论效应和电子关联效应对光谱能量的影响分别为2%和5%。并从光谱特性的角度对比讨论了Sn和Xe作为极端远紫外光源的优劣,发现Sn更有优势。     

“磁氢原子”的光谱

对于以荷正电的磁单极子为核的原子(简称“磁氢原子”),本文计算了它的能级、选择规则、跃迁几率、光谱和谱线强度。其中三条特征吸收谱线(2774?,2734?,2191?)的相对强度是37:50:6。 关键词：     

Raman scattering in a ferroelectric sodium nitrite crystal

Raman spectra of the ferroelectric sodium nitrite are recorded over a wide spectral range at different temperatures, including the ferroelectric phase transition interval. The room-temperature Raman spectrum reveals the overdamped А₁(z) soft mode which is attributed to the ferroelectric phase transition.     

Electronic structure of hexagonal rare-earth manganites RMnO<Subscript>3</Subscript>

The optical spectra of single crystals of hexagonal rare-earth manganites RMnO₃ (R=Sc, Y, Er) are studied in the range from 0.7 to 5.4 eV. It is found that the spectra substantially differ from the spectra of orthorhombic manganites in both the positions of spectral features and their polarization anisotropy. It is shown that the optical absorption edge is determined by an abnormally strong (k?1) and narrow electric dipole transition with the center at approximately 1.6 eV with light polarization in the basal plane of the crystal. This transition can be treated with confidence as charge transfer from oxygen to manganese. The experimental results are in many instances substantially different from the first-principles calculations of the electronic structure of YMnO₃ published recently and, hence, may serve as a reliable basis for the further improvement of computational methods.     

超短超强激光与固体靶相互作用中背表面光发射的实验研究

从金属箔背表面测量了超热电子穿越固体靶产生的光发射.光发射积分成像图案呈圆环状，在圆环边缘附近出现局部化明亮光信号确定为光学渡越辐射；光发射光谱在300—500nm之间出现一系列非周期锐利尖峰，在400nm（2ω）附近的尖峰较明显，这个光发射取决于v×B加 热机制产生的超热电子束的微束团引起的相干渡越辐射，(v为电子电度，B为磁场强度)，光 强随靶厚度的增加而减小. 关键词： 超热电子 光发射 光学渡越辐射 v×B加热机制 相干渡越辐射     

Optical transmission and reflection spectroscopy of single quantum dots

An analytical formulation of the interband optical transmission and reflectivity spectra of a single quantum dot embedded in a semiconductor is presented. We consider the effect of the sample surface as well as other reflecting surfaces on the shape of the spectra near the ground state exciton resonance. The saturation of the transmission and reflectivity spectra due to the quantum optical saturation of the transition at higher light power is presented.     

Studies on coordination and hydrogen bond intermolecular interaction using 1D & 2D FTIR spectroscopy

Two-dimensional (2D) correlation spectroscopy is a powerful method to study the intermolecular interactions between different molecules/functional groups. In the present paper, variable concentrations were selected to construct 2D synchronous spectrum for studying the weak intermolecular interactions in solutions. Mathematical analysis performed on 2D synchronous spectra using variable concentration as an external perturbation shows that the "Orthogonal Sample Design Scheme" is necessary for eliminating the interfering cross peaks in 2D synchronous spectra. The authors prepared four mixed-solutes-solutions whose concentration series satisfy the "Orthogonal Sample Design Scheme" for each chemical system and the consequent 2D synchronous spectrum was calculated from the corresponding four 1D spectra. Thus, by 1D & 2D FTIR spectra together with solid grinding reaction, the intermolecular interactions in two chemical systems (Sodium 2-Aminobenzoate/NdCl3 in aqueous solution, and 2-ethylhexyl phosphonic acid mono 2-ethylhexyl ester (PC88A)/Naphthenic Acid (NA) in heptane solution) were studied, where the intermolecular interactions only induce subtle spectral variations in conventional 1D spectra. First, the cross peaks between f-f transition bands of Nd3+ ion at 521, 574, 741, 795 and 865 nm and pi-pi transition band of Sodium 2-Aminobenzoate at 308 nm in 2D synchronous spectrum confirm the coordination interaction between Sodium 2-Aminobenzoate and Nd3+. Solid grinding reaction between Sodium 2-Aminobenzoate and NdCl3 and FTIR spectra of the product indicate that the vibration bands of amino, carboxyl groups from sodium 2-aminobenzoate show considerable changes. Based on the spectral result above, a conclusion is drawn that Nd3+ can coordinate with Sodium 2-Aminobenzoate by amino and carboxyl groups. Second, the cross peaks between POH stretching band of PC88A at 983 cm(-1) and COOH stretching band of NA at 1 710 cm(-1) in 2D spectra confirm the interaction between PC88A and NA. Subtraction spectrum demonstrates that when PC88A is mixed with NA in heptane solution, and P=O stretching band of PC88A shifts from 1 199 to 1161 cm(-1), and POH stretching band shifts from 983 to 965 cm(-1). Based on the spectral result above, a conclusion was made that PC88A and NA can interact with each other by forming new assemblies with POH and COOH groups.     

Spectral characteristics of subcritical carbon dioxide in nanopores

Coherent anti-Stokes Raman scattering spectra measured within the Q-branch of the vibrational transition ν₁ are used to gain insights into the state of carbon dioxide molecules in nanopores of Vycor™ glass at room temperature (20.5°C) and a subcritical temperature of 30.5°C and gas pressures up to the saturation point P _sat for each temperature. Along with the main spectral component, belonging to gaseous CO₂ molecules, the spectra recorded at pressures close to P _sat feature a second (low-frequency) component. The second component is associated with the contribution from the CO₂ molecules trapped inside pores. A spectral deconvolution with account for the interference of these two bands makes it possible to estimate the spectral characteristics of the second (low-frequency) component at each temperature. At 20.5°C, the bandwidth of the low-frequency component decreases with CO₂ pressure, a behavior that can be explained by the transition of CO₂ from the adsorbed to the condensed state in the pore. At the subcritical temperature of 30.5°C, the spectral width of the second component is pressure-independent and close to the value measured in the bulk of the supercritical fluid, a result likely associated with a low-temperature shift of the critical point of the substance trapped in nanopores.     

2-D electron spin transient nutation spectroscopy of Lanthanoid ion Eu2+(8S7/2) in a CaF2 single crystal on the basis of FT-pulsed electron spin resonance spectroscopy: Transition moment spectroscopy

This work demonstrates the usefulness of pulsed electron spin resonance (ESR)-based two-dimensional electron spin transient nutation (2-D ESN) spectroscopy for complete assignments of complicated fine-structure hyperfine ESR spectra including hyperfine forbidden transitions from electronic and nuclear high-spin systems. The 2-D ESN spectroscopy is termed transition moment spectroscopy as spectra are acquired as a function of transition moment instead of transition energy used in conventional spectroscopy. We have applied the novel spectroscopic technique to Eu²⁺ ion (S=7/2,I=5/2), which has two isotopes (¹⁵¹Eu [47.9%] and¹⁵³Eu [52.1%]), in a CaF₂ single crystal as a model system. We have completely identified the complicated fine-structure hyperfine ESR spectra by invoking the spectral simulation of the 2-D ESN spectra on the basis of transition moment analyses. The analyses are based on exact numerical calculations of the transition moments as well as a perturbation-based analytical approach combined with reduced rotation matrices for the nuclear part of the transition moment. This is the first example of the spectral simulation for 2-D ESN spectra including the hyperfine allowed and forbidden transitions in high-spin systems. In addition, we have made simulation of the fine-structure forbidden transitions, which reproduces the angular variations of the observed spectra at liquid helium temperatures.     

Cl_2~+离子P支高激发振转态的跃迁谱线

基于描述双核分子体系包含转动光谱常数Lv所在高阶项在内的跃迁能谱的经典表达式,采用差分收敛法(DCM)的物理思想,推导获得了求解双核分子体系P支跃迁谱线的另一解析表达形式。对于某一双核分子体系的P支跃迁带,新的谱线表达式仅需要11条精确的实验跃迁谱线就可以正确预言高激发振转态的谱线结构,同时,利用谱线展开系数C还可以进一步给出计算谱线的不确定度以更可靠地预言高激发振转态的跃迁谱线。推导获得了相应的计算公式以正确获得对应跃迁带的转动光谱常数(B_(v′),B_(v″),D_(v′),D_(v″),H_(v′),H_(v″),L_(v′),L_(v″))。应用新的谱线表达式,对~(35) Cl_2~+和~(35) Cl ~(37) Cl~+离子电子态A~2Π_u-X~2Π_g(Ω=1/2)跃迁体系(3,7)跃迁带的P支跃迁谱线进行了详细研究,获得的计算结果不但能完全重复已知的实验数据,还获得了实验上未能获得的包含高激发振转态在内的跃迁谱线。另外,通过计算同时也得到了各跃迁带的高阶转动光谱常数。该研究方法和计算结果为人们进一步正确认识Cl_2~+离子电子态的内部能级结构及物理化学的相关性质提供了更完整的参考数据。