MEASUREMENT OF IONIC ENERGY FROM FEMTOSECOND LASER-HEATED ARGON CLUSTERS

The high backing pressure argon gases adiabatically expand into vacuum through a pulsed gas jet to nucleate into large clusters. The clusters were heated by a 45fs, 2.3×10¹⁶ Wcm^-2 Ti: sapphire laser. The high energy of the ions produced in the cluster explosion was measured using time-of-flight spectrometry. The maximum and average kinetic energy of the ions were 0.2MeV and ～12.5keV, respectively, indicating that the femtosecond laser interactions with argon clusters are more energetic than interactions with atoms and molecules.     

Simulation of Electron Temperature in Argon Clusters Subjected to Intense Laser Fields

Argon clusters subjected to intense femtosecond laser pulses are ionized by the optical fields and inelastic electron-ion collisions. The cluster plasmas absorb the laser energy through collisional inverse bremsstrahlung, leading the argon cluster plasmas to very hot states. The calculated electron temperature in the clusters indicates that the intense laser-cluster interactions are more energetic than interactions with molecules.     

Simulation of Electron Temperature in Argon Clusters Subjected to Intense Laser Fields

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Large Molecules as Models for Small Particles in Aqueous Geochemistry Research

Many questions that geochemists now pose about mineral surfaces concern the properties of individual molecular functional groups. These questions can be answered directly with large aqueous molecules where the positions of atoms can be determined with accuracy and related to the reactive properties. It is time to abandon this approach with colloidal solid suspensions and employ aqueous molecular clusters. The reactive properties of individual oxygens can be determined separately using these aqueous clusters in spectroscopic studies. These molecules are sufficiently large (1–5 nm) that they overlap in size with the smallest colloids, yet the bond lengths and atom positions can be determined unequivocally from X-ray structural studies. In this paper we present research on a 2-nm cluster that provides a particular useful example. These molecules, unlike surface structures that are inferred from bulk structures, allow direct comparison of experimental data with molecular simulations.     

电离效率对激光电离团簇的高价离子产物的影响

纳秒激光与团簇相互作用产生高价离子逐渐成为分子物理界的热点之一, 为了深入研究团簇电离的本质, 本文以分子结构相似、元素组成相同的苯、环己烯和环己烷的分子团簇为对象, 利用飞行时间质谱研究了其与5 ns的532 nm激光相互作用时电离产物的价态和强度分布. 结果表明: 这三种化合物多光子电离效率苯>环己烯>环己烷, 但其高价离子的价态和比值苯是最低的, 环己烷的碳离子最高价态为4价, C³⁺和C²⁺的比值为1.1; 环己烯电离产物C³⁺和C²⁺ 的比值降低为0.6; 苯团簇的最高价态只有3价, C³⁺和C²⁺的比值约为0.4. 引起这种现象的原因可以归结于高的多光子电离效率会导致团簇多位点的电离, 引起团簇在电子加热到发生碰撞电离之前发生解离, 减小了团簇的尺寸, 进而减少了离子发生碰撞电离产生高价离子的反应时间, 最终阻碍了高价态离子的产生.     

基于极紫外自由电子激光的中性团簇红外光谱

团簇在能源催化和大气雾霾等诸多化学过程中广泛存在,团簇表征与性能研究对诠释化学反应机理至关重要. 然而,中性团簇由于缺乏电荷、难于探测,实验研究非常困难. 鉴于上述情况,发展了基于极紫外自由电子激光的中性团簇红外光谱实验方法,用于质量选择中性团簇的高灵敏探测、结构表征和性能研究. 红外-极紫外衰减和红外+极紫外增强光谱实验方法已被应用于一些中性水团簇和一些中性金属羰基化合物的研究. 由于极紫外自由电子激光的波长范围涵盖了绝大多数中性团簇的第一电离势,这一独特的实验方法为开展各类中性团簇红外谱学和结构的研究打开了大门. 本文综述了这些红外光谱实验方法及其在中性团簇研究中的应用.     

Cluster formation in binary fluids with competing short-range and long-range interactions

Abstract

The equilibrium phase behaviour of a model binary fluid is investigated through Monte Carlo simulations and by developing a molecular thermodynamic model. Both fluid components interact through a hard core with short-range attractions (SA), but one of the components exhibits an additional long-range repulsion (SA+LR). We find that phase behaviour for this system is controlled by the cross-interaction between the two types of particles as well as their chemical potentials. For a weak cross-interaction, the system displays behaviour that is a composite of the behaviour of the individual components, i.e. the SA component can display bulk vapour/liquid phase separation, while the SALR component can display giant micelle-like clusters for a suitable combination of SA and LR interactions. For a strong cross-interaction, qualitatively different behaviour is observed, with the resulting clusters typically composed of a more equal mixture of SA and SALR particles. Moreover, these mixed clusters can exist even when the SA component by itself would be undersaturated or supercritical, and/or when the SALR component by itself would not form giant clusters. These insights should help to identify the mechanisms for clustering in experimental systems where giant equilibrium clusters are observed.     

甲醇－乙醚团簇的多光子电离和从头算

在355 nm波长下用激光电离飞行时间质谱装置研究了氢键团簇甲醇/乙醚混合团簇的多光子电离,飞行时间质谱仪观测到一系列质子化团簇,为了探讨质子化团簇的形成机理,在B3LYP/6-31 G(d,p)//B3LYP/6-311 G(d,p)基组水平上计算了CH30H-(C2H5)2O离子和中性团簇的稳定几何结构和解离通道和解离能,发现存在一个质子转移过程,解离产物主要为CH3O和[(C2H5)2O]H .     

Structure and charge transfer dynamics of uranyl ions in boron oxide and borosilicate glasses

Laser spectroscopic experiments, molecular dynamics simulation, and charge transfer-lattice interaction modeling have been conducted for studying the electronic and structural properties of the uranyl ion UO ₂ ²⁺ in boron oxide and borosilicate glasses. The charge transfer electronic and vibrational energy levels for uranyl ions in the glass matrices were obtained from laser excitation and fluorescence spectra of UO ₂ ²⁺ . A model structure for uranyl ions in the glass matrices was established using the method of molecular dynamics (MD) simulation in comparison with the results of extended x-ray absorption fine structure (EXAFS) for U⁶⁺ ions in the glasses we studied. The formation and stabilization of uranyl clusters in glass matrices are interpreted in terms of charge transfer-lattice interactions on the basis of self-consistent charge transfer accompanied by lattice distortion. The latter is in the framework of the simultaneous action of pseudo-Jahn-Teller and pseudo-Jahn-Teller analog effects on charge transfers between oxygen and uranium ions.     

中性和阳离子丁酮团簇的结构及稳定性的理论研究

使用密度泛函B3LYP方法,在6-31G*和6-311+G**基组水平上计算中性和阳离子丁酮团簇(CH_3COC_2H_5)_n和(CH_3COC_2H_5)_n~+(n 7)的稳定结构,并比较不同尺寸团簇之间的相对稳定性.中性和阳离子丁酮团簇的结构具有相似性:n=3—7时,组成团簇的丁酮的平均几何参数基本相同,单环结构最稳定;随着团簇尺寸的增加,双环结构的稳定性逐渐上升.通过平均结合能、一阶差分能、HOMO-LUMO能隙等计算分析可知:在所研究的各种尺寸团簇中,(CH_3COC_2H_5)_3是最稳定的中性团簇,与实验中的最强峰对应;(CH_3COC_2H_5)_4~+是最稳定的阳离子团簇.通过电离能计算得到丁酮分子的垂直电离能为9.535 eV与实验值相符,同时证明中性和阳离子丁酮二元团簇的结构变化较大.研究结果为实验中丁酮团簇碎片离子的形成机理提供一定的理论依据,并且为进一步研究酮类分子团簇的生长规律提供有价值的信息.     

Accurate frequency control of a mode-locked laser diode by reference-light injection

Injection locking is employed to narrow the spectrum linewidth of individual modes of an active mode-locked laser diode (ML-LD). The multiple modes of the injection-locked ML-LD lie over 3.0 THz with 25-GHz spacing at 30 dB down; their linewidth and stability, ~25 MHz, are nearly the same as those of the master laser. We also demonstrate the accurate frequency measurement of laser diodes stabilized to molecular (H(13)C (15)N ,(13)C(2)H (2)) absorption lines by use of the injection-locked ML-LD.     

非线性荧光光谱的神经网络分析及其应用

大功率超快脉冲激光和气体相互作用可产生非线性荧光光谱,不同的气体分子具有不同的非线性荧光光谱。因而这种光谱可以作为物质的指纹模式加以识别分类,进而获知气体的成分。由于不同气体分子的光谱在同一波段上有很大的交叉重叠,用传统的光谱分析方法分析存在困难,采用神经网络方法分析上述非线性荧光光谱,利用经过预处理的荧光光谱数据作为模式样本,其中一部分样本作为学习样本对级联神经网络进行训练,用训练好的网络对所有样本进行实时识别,学习样本和测试样本的的正确识别率均可达100％,结果表明此方法可实时判断混合气体的组分。     

Modelling chemical kinetics of small clusters after nanosecond laser ablation

Nanoclusters of various materials have recently been obtained by laser ablation. Strong evaporation of a condensed phase caused by laser irradiation is well known to generate an overcooled vapour. Further expansion thereof increases the oversaturation degree and facilitates homogeneous nucleation and cluster growth. To investigate homogeneous nucleation at very high expansion rates attained at nanosecond laser ablation, kinetic equations are applied describing all the possible gas-phase chemical reactions of dissociation and coalescing between small clusters. Additional cooling due to thermal emission by clusters is taken into account. Twenty smallest carbon molecules are considered. The model is applied to nanosecond laser ablation of graphite in vacuum. The resulted vapour molecular composition is characterised by dominating molecules C₃ and C₅ and an exponential drop of heavier clusters concentrations with their mass. The growth of heavier clusters is controlled by the balance between liberating the latent heat of their formation and the energy losses by expansion and thermal emission.     

Terahertz probe of individual subwavelength objects in a water environment

Terahertz (THz) spectroscopy and imaging have been heralded for some time as potentially revolutionary techniques for biomedical applications. Label‐free detection of molecules and recognition of molecular events are often mentioned as the most exciting possibilities. A crucial practical goal, however, is the ability to perform such measurements on tiny amounts of biological fluids or even on individual organic structures. Living cells, for instance, have diameters at most of some tens of micrometers, i.e. at least λ/10 even for few‐THz radiation. Furthermore, all analyses relevant for a biological perspective must be performed in a water environment, which presents a strong absorption across the whole THz spectral range, severely limiting the penetration of the electromagnetic field. Here, it is shown how both issues can be overcome with a lab‐on‐a‐chip approach based on a microfluidic platform coupled to a plasmonic antenna. Using a quantum cascade laser as THz illumination source, liquid volumes down to the picoliter range are probed, and direct operation on individual 10‐µm diameter microparticles flowing in water is shown. The present demonstration opens the way to the development of THz biosensing of individual living cells and small probe volumes.     

锌基-有机金属大环荧光探针对谷胱甘肽生物分子的识别

利用1H NMR,ESI-MS,UV-Vis,荧光光谱等测试手段研究了基于锌基-有机金属三元大环探针M-1对生物分子谷胱甘肽(GSH)的识别与传感。并且,通过研究识别过程中M-1与组成谷胱甘肽的氨基酸(半胱氨酸、谷氨酸、甘氨酸)的作用关系,确立了M-1对GSH的识别机理。结果表明,化合物M-1在H₂O/DMF(1∶9, φ)溶液中形成了稳定的[3+3]大环结构;紫外滴定光谱表明,向M-1中加入GSH后303 nm处吸收峰强度增加,380 nm处吸收峰强度减弱,在330 nm处出现了一个等吸收点,紫外滴定和ESI-MS质谱证实了M-1能够1∶1包合GSH,平衡常数(log K_GSH)为4.62±0.15。1H NMR表明谷胱甘肽在M-1中的构型为组成谷胱甘肽的谷氨酸通过羧基与金属中心之间的静电作用深深地进入M-1空穴内部。荧光光谱表明,向M-1中加入GSH时,以330 nm的光激发,发射波长从510 nm红移至540nm,荧光强度增加1倍;加入半胱氨酸、谷氨酸时,荧光强度分别增加0.4倍和0.2倍;而加入甘氨酸时,荧光没有变化。综合上述结果证明了M-1空穴的限域作用及其底部三元环上的氨基和GSH上的巯基(半胱氨酸)间的氢键作用使M-1的电子构型发生转变,进而引起紫外光谱和荧光光谱发生变化,实现了大环化合物M-1对生物分子谷胱甘肽的可视化、高灵敏度检测,检测限达到3.0×10-6 mol·L-1。     

A direct approach for the reduction of diatomic spectra to molecular constants for the construction of RKR potentials

A nonlinear fitting procedure is presented that employs all measured line positions and iteratively compares their values with those calculated from numerically diagonalized model Hamiltonians with adjustable molecular constants. Case (a) wavefunctions with definite parity are introduced as a convenient basis set, and the effects of spin-orbit, spin-spin, spin-rotation, and centrifugal distortion interactions neglected in the Born-Oppenheimer separation are included simultaneously using the Van Vleck transformation. The spectroscopic constants found by this procedure represent the minimum-variance, unbiased set and maintain, to a high degree of accuracy, the separate mechanical and magnetic meanings of the molecular constants. Arguments are presented that such spectroscopic constants with mechanical meaning allow the most accurate construction of potential energy functions using the Rydberg-Klein-Rees (RKR) procedure. Residual mechanical and magnetic ambiguities, such as engendered by Λ-type doubling, are discussed.     

Dynamics of single-wall carbon nanotube synthesis by laser vaporization

The key spatial and temporal scales for single-wall carbon nanotube (SWNT) synthesis by laser vaporization at high temperatures are investigated with laser-induced luminescence imaging and spectroscopy. Graphite/(Ni, Co) targets are ablated under typical synthesis conditions with a Nd:YAG laser at 1000 °C in a 2-in. quartz tube reactor in flowing 500-Torr Ar. The plume of ejected material is followed for several seconds after ablation using combined imaging and spectroscopy of Co atoms, C₂ and C₃ molecules, and clusters. The ablation plume expands in stages during the first 200 7s after ablation and displays a self-focusing behavior. Interaction of the plume with the background gas forms a vortex ring which segregates and confines the vaporized material within a ~1-cm³ volume for several seconds. Using time-resolved spectroscopy and spectroscopic imaging, the time for conversion of atomic and molecular species to clusters was measured for both carbon (200 7s) and cobalt (2 ms) at 1000 °C. This rapid conversion of carbon to nanoparticles, combined with transmission electron microscopy analysis of the collected deposits, indicate that nanotube growth occurs over several seconds in a plume of mixed nanoparticles. By adjusting the time spent by the plume within the high-temperature zone using these in situ diagnostics, single-walled nanotubes of controlled (~100 nm) length were grown and the first estimate of a growth rate on single laser shots (0.2 7m/s) was obtained.     

飞行时间质谱峰值的漂移

飞行时间质谱仪(time-of-flight mass spectrometer, TOF-MS)在分子团簇的激光电离/解离动力学研究中广泛使用。文章报道了在用脉冲355 nm的YAG激光进行水/甲醇二元团簇的多光子电离研究中,发现在电离激光相对于脉冲分子束的不同延时下,即激光作用于脉冲束的不同位置,飞行时间质谱仪测得的离子的质谱峰值发生漂移。在激光作用于脉冲束的中段时,离子的信号最强,同时离子的峰值漂移达到最大。分析认为：这种峰值漂移不是因为新质量数谱峰的出现,而是离子在穿越质谱仪的离子引出区和加速区极板时发生部分离子吸附,引起极板间电压的起伏造成的。离子在电场起伏下的数值模拟与实验中观测到的离子峰值漂移规律一致。     

Differentiation of Positional Isomers of Propyl Alcohols Using Filament-Induced Fluorescence

We experimentally demonstrate the recognition of positional isomers of propyl alcohol vapor through nonlinear fluorescence induced by high-intensity femtosecond laser filaments in air.By measuring characteristic fluorescence of n-propyl and isopropyl alcohol vapors produced by femtosecond filament excitation,it is found that they show identical spectra,that is,those from molecular bands of CH,C_2,NH,OH and CN,while the relative intensities are different.By comparing the ratios of the CH and C_2 signals,the two propyl alcohol isomers are differentiated.The different signal intensities are ascribed to different ionization potentials of the two isomer molecules,leading to different production efficiencies of fluorescing fragments.     

The HITRAN 2008 molecular spectroscopic database

This paper describes the status of the 2008 edition of the HITRAN molecular spectroscopic database. The new edition is the first official public release since the 2004 edition, although a number of crucial updates had been made available online since 2004. The HITRAN compilation consists of several components that serve as input for radiative-transfer calculation codes: individual line parameters for the microwave through visible spectra of molecules in the gas phase; absorption cross-sections for molecules having dense spectral features, i.e. spectra in which the individual lines are not resolved; individual line parameters and absorption cross-sections for bands in the ultraviolet; refractive indices of aerosols, tables and files of general properties associated with the database; and database management software. The line-by-line portion of the database contains spectroscopic parameters for 42 molecules including many of their isotopologues.