Probing electronic states of Ne2+ and Ar2+ by measuring kinetic-energy-release distributions

Dissociative decay of metastable, electronically excited neon and argon dimer ions produces fragment ions with strikingly dissimilar kinetic-energy-release distributions. The distributions have been modeled based on ab initio calculations of potential energy curves. The unusual bimodal distribution observed for dissociation of Ne2+ arises from competition between radiative and nonradiative decay of the long-lived II(1/2)(u) state. For Ar2+, however, electronic predissociation is insignificant.     

Self-sum- and -difference-frequency mixing in GdAl(3)(BO(3))(4):Nd(3+) for generation of tunable ultraviolet and infrared radiation

We exploited Nd(3+) laser emission at 1061.9 nm ((4)F(3/2)?(4)I(11/2) channel) in a self-chi((2)) active GdAl(3)(BO(3))(4):Nd(3+) laser crystal. UV radiation was obtained from a 1.8% yield from self-sum-frequency mixing: 1/pump + 1/1061.9 = 1/UV, during pumping in the Nd(3+) (4)G(5/2)-(2)G(7/2) levels near 588 nm. The UV tunability had a range of 378-382 nm. We have demonstrated, for the first time to our knowledge, generation of coherent IR radiation from a self-difference-frequency mixing laser: 1/pump - 1/1061.9 = 1/IR. We got a 0.5% yield and tunability in the 1305-1365-nm range.     

四氟化碳分子电离解离过程中的异构化动力学

采用离子动量成像谱仪研究了能量为1.0 ke V的电子束碰撞条件下CF₄分子的解离动力学.实验上,对解离离子的三维动量进行了成像测量,通过离子飞行时间关联谱识别了CF₄²⁺异构化生成F₂⁺分子的两个通道:F₂⁺+CF₂⁺与CF⁺+F₂⁺+F,得到了两个通道的离子动能及动能释放分布.对于其中的三体解离通道,我们进一步采用Dalitz图与Newton图等三体动力学分析方法对解离碎片的动量关联进行了分析.该通道以两个带电离子的背对背出射为主,中性的F原子作为旁观者只得到极小的反冲动量.     

Br_2分子在360—610 nm的光解离动力学研究

利用包含转动自由度在内的含时薛定谔方程研究了Br_2分子在波长范围为360—610 nm的光解离动力学.通过计算得到了Br_2分子在四个特征波长处的切片解离影像,并经过分析得到了与切片解离影像相对应的动能分布;计算了Br_2分子在波长范围为360—610 nm内总的动能分布,以及从A,B和C三个电子态解离的碎片各自所对应的动能分布;计算了A,B和C三个电子态各自的解离概率以及碎片产物的分支比Γ(Br~*/(Br+Br~*))随波长的变化.     

Effects of strain on the dissociation dynamics of O2 on Si(001)

There are two distinctive channels in the dissociation reaction of O2 on Si(001)-(2 x 1): a trapping-mediated channel and a direct-activated channel. Externally applied tensile strain along the <110> direction on the (001) surface is found to suppress the dissociation via a direct-activated channel and to enhance that via a trapping-mediated channel in the temperature range between 200 and 300 K. It has been demonstrated that the dissociation dynamics involving elementary processes such as inelastic scattering and trapping, desorption and/or dissociation from a trapping precursor, and direct dissociation are sensitively influenced by the strain to change the branching ratio of the dissociation reaction.     

Multiphoton double ionization of Ar in intense extreme ultraviolet laser fields studied by shot-by-shot photoelectron spectroscopy

Photoelectron spectroscopy has been performed to study the multiphoton double ionization of Ar in an intense extreme ultraviolet laser field (hν ～ 21 eV, ～ 5 TW/cm2), by using a free electron laser (FEL). Three distinct peaks identified in the observed photoelectron spectra clearly show that the double ionization proceeds sequentially via the formation of Ar(+): Ar+hν→Ar (+) + e? and Ar2(+) + 2hν→Ar(+) + e?. Shot-by-shot recording of the photoelectron spectra allows simultaneous monitoring of FEL spectrum and the multiphoton process for each FEL pulse, revealing that the two-photon ionization from Ar(+) is significantly enhanced by intermediate resonances in Ar(+).     

Exploring relativistic many-body recoil effects in highly charged ions

The relativistic recoil effect has been the object of experimental investigations using highly charged ions at the Heidelberg electron beam ion trap. Its scaling with the nuclear charge Z boosts its contribution to a measurable level in the magnetic-dipole (M1) transitions of B- and Be-like Ar ions. The isotope shifts of 36Ar versus 40Ar have been detected with sub-ppm accuracy, and the recoil effect contribution was extracted from the 1s(2)2s(2)2p 2P(1/2) - 2P(3/2) transition in Ar13+ and the 1s(2)2s2p 3P1-3P2 transition in Ar14+. The experimental isotope shifts of 0.00123(6) nm (Ar13+) and 0.00120(10) nm (Ar14+) are in agreement with our present predictions of 0.00123(5) nm (Ar13+) and 0.00122(5) nm (Ar14+) based on the total relativistic recoil operator, confirming that a thorough understanding of correlated relativistic electron dynamics is necessary even in a region of intermediate nuclear charges.     

Coulomb stability limit of highly charged Cq+60 fullerenes

We perform density functional theory calculations of Cq+60 and Cq+58 fullerenes as well as of transition states related to dissociation of Cq+60 into Cq-s(+)58 and C(s+)2 for charges q=0-14. For q< or =8, the lowest fission barrier corresponds to C+2 emission, whereas, for higher q values, the lowest barrier corresponds to the emission of two atomic fragments [2C]s+ with s> or =2. We conclude that the Coulomb stability limit corresponds to q=14.     

Symmetry breakdown in ground state dissociation of HD+

Experimental studies of the dissociation of the electronic ground state of HD+ following ionization of HD by fast proton impact indicate that the H++D(1s) dissociation channel is more likely than the H(1s)+D+ dissociation channel by about 7%. This isotopic symmetry breakdown is due to the finite nuclear mass correction to the Born-Oppenheimer approximation which makes the 1ssigma state 3.7 meV lower than the 2psigma state at the dissociation limit. The measured fractions of the two dissociation channels are in agreement with coupled-channels calculations of 1ssigma to 2psigma transitions.     

强场中初始振动态对H2+解离过程的影响

本文通过数值求解非玻恩-奥本海默近似下的一维含时薛定谔方程模拟了强场中初始振动态对H_2~+解离过程的影响.结果表明:不同初始振动态H_2~+的对应的核动能谱结构不同.低振动态时解离过程主要由阈上解离通道贡献,振动态提高时会增加一个键的软化通道.     

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.     

利用速度成像技术研究碘乙烷多光子电离解离动力学

卤代烷烃会破坏臭氧层,而碘乙烷（C₂H₅I）是卤代烷烃中重要代表物质之一.采用离子速度成像技术、飞秒激光技术和飞行时间质谱技术,探究了C₂H₅I的多光子电离解离动力学.通过分析C₂H₅I在强场作用下多光子电离解离得到的解离通道、碎片的动能、角度分布和各向异性参数等信息来研究碘乙烷离子（C₂H₅I⁺）C–I键裂解机理.根据飞行时间质谱实验,C₂H₅I在飞秒激光脉冲作用下发生多光子电离解离得到的碎片有C₂H₅⁺,I⁺,CH₂I⁺,C₂H₂⁺,C₂H₃⁺,C₂H₄⁺等.与C–I键相关的碎片为C₂H₅⁺和I⁺,解离机制分别对应于C₂H₅I⁺→C₂H₅⁺+I和C₂H₅I⁺→C₂H₅+I⁺.同时,采用离子速度成像技术研究C₂H₅I⁺的C–I键裂解产生的C₂H₅⁺和I⁺的速度影像,得出两者的速度分布和动能分布,分析结果表明C–I键裂解产生C₂H₅⁺和I⁺的过程都存在高能通道和低能通道.进一步分析解离碎片离子的角度分布发现C₂H₅⁺解离时各向异性参数接近于0,可能对应于慢速的振动预解离过程.I⁺在解离时各向异性参数较高,可能源于排斥势能面上的快速解离过程.最后采用密度泛函理论计算了C₂H₅I分子电离前后构型变化、离子态的能级强度及谐振强度,对C₂H₅I⁺的解离机制做了更进一步的分析和讨论.     

CH3CH2+N(4S)反应机理的理论研究

利用abinitio方法对CH3CH2+N(4S)反应进行了理论研究,在MP2/6-311+G(d,p)水平上优化得到了反应途径上的反应物、中间体、过渡态和产物的几何构型和谐振频率,并在QCISD(T)/6-311+G(d,p)水平上进行单点能计算.计算结果表明,CH2CH2+3NH和H2CN+CH3是此反应主要产物,CH3CHN+H是此反应次要产物.产物CH2CH2+3NH主要来自直接氢抽提反应通道,H2CN+CH3来自加成-解离反应通道,CH3CHN+H来自加成-解离反应通道.     

Magnetization in permalloy thin films

Thin films of permalloy (Ni₈₀Fe₂₀) were prepared using an Ar+N₂ mixture with magnetron sputtering technique at ambient temperature. The film prepared with only Ar gas shows reflections corresponding to the permalloy phase in X-ray diffraction (XRD) pattern. The addition of nitrogen during sputtering results in broadening of the peaks in XRD pattern, which finally leads to an amorphous phase. The M-H loop for the sample prepared with only Ar gas is matching well with the values obtained for the permalloy. For the samples prepared with increased nitrogen partial pressure the magnetic moment decreased rapidly and the values of coercivity increased. The polarized neutron reflectivity measurements (PNR) were performed in the sample prepared with only Ar gas and with nitrogen partial pressure of 5 and 10%. It was found that the spin-up and spin-down reflectivities show exactly similar reflectivity for the sample prepared with Ar gas alone, while PNR measurements on 5 and 10% sample show splitting in the spin-up and spin-down reflectivity.      

Direct ultraviolet writing of channel waveguides in congruent lithium niobate single crystals

We report the fabrication of optical channel waveguides in congruent lithium niobate single crystals by direct writing with continuous-wave ultraviolet frequency-doubled Ar+ laser radiation (244 nm). The properties and performance of such waveguides are investigated, and first results are presented.     

Coupled cluster calculations of the ground state potential and interaction induced electric properties of the mixed dimers of helium,neon and argon

Using augmented polarized correlation consistent basis sets extended with midbond functions, we evaluate the ground state interaction potential and the induced electric dipole polarizabilities and first and second hyperpolarizabilities of the He–Ar, Ne–Ar and He–Ne van der Waals complexes. For the calculation of the potential we resort to the coupled cluster singles and doubles (CCSD) model corrected for triple excitations, CCSD(T), whereas properties are evaluated with CCSD response theory. As a check of the quality of the potential, the rovibrational spectrum and the gas second virial coefficients are evaluated. The rovibrational spectra improve previously available theoretical results, although the dissociation energies are probably still slightly underestimated. For the gas second virial coefficients the agreement with experiment is satisfactory. The frequency dependence of the interaction (hyper)polarizabilities is analysed and a comparison with previous results on the mixed dimers and the pure gases is made.     

Double and triple ionization of silver clusters by electron impact

The production of silver cluster cations Ag(n)(2+) (for several selected sizes in the range n = 39-119) and Ag(n)(3+) (for n = 58, 61, 67) by electron impact ionization of neutral precursors has been studied. The scaling of appearance energies with cluster radius follows the metallic droplet model but, curiously, with a slope which is estimated to be quite different from the literature values for single ionization, Ag(n)(+), as well as for the appearance of smaller Ag(n)(2+) ions. It is also found that as the electron energy increases, the yield of high-charge cations grows faster than that of singly-charged Ag(n)(+). This behavior is consistent with the power-law dependence of post-threshold ionization. The mechanisms involved in multiple ionization phenomena in clusters of noble metals are not yet fully understood and call for further experimental and theoretical examination.     

甲胺分子的紫外光解离动力学实验研究

在280—287.5 nm区域内,通过实验测定共振增强多光子电离-时间飞行质谱、碎片离子的分质量激发谱以及光强指数等对甲胺分子的光解离通道进行了研究.实验结果证实甲胺分子在单光子能量范围内存在一个电子排斥态,主要的光解离过程为甲胺分子共振吸收1个光子到达该电子排斥态后解离成中性碎片,然后是中性碎片经多光子共振电离形成碎片离子和碎片离子的进一步解离.     

Mechanism of relaxation enhancement of spin labels in membranes by paramagnetic ion salts: dependence on 3d and 4f ions and on the anions

Progressive saturation EPR measurements and EPR linewidth determinations have been performed on spin-labeled lipids in fluid phospholipid bilayer membranes to elucidate the mechanisms of relaxation enhancement by different paramagnetic ion salts. Such paramagnetic relaxation agents are widely used for structural EPR studies in biological systems, particularly with membranes. Metal ions of the 3d and 4f series were used as their chloride, sulfate, and perchlorate salts. For a given anion, the efficiency of relaxation enhancement is in the order Mn(2+) > or = Cu(2+) > Ni(2+) > Co(2+) approximately Dy(3+). A pronounced dependence of the paramagnetic relaxation enhancement on the anion is found in the order ClO(-)(4) > Cl(-) > SO(2-)(4). This is in the order of the octanol partition coefficients multiplied by spin exchange rate constants that were determined for the different paramagnetic salts in methanol. Detailed studies coupled with theoretical estimates reveal that, for the chlorides and perchlorates of Ni(2+) (and Co(2+)), the relaxation enhancements are dominated by Heisenberg spin exchange interactions with paramagnetic ions dissolved in fluid membranes. The dependence on membrane composition of the relaxation enhancement by intramembrane Heisenberg exchange indicates that the diffusion of the ions within the membrane takes place via water-filled defects. For the corresponding Cu(2+) salts, additional relaxation enhancements arise from dipolar interactions with ions within the membrane. For the case of Mn(2+) salts, static dipolar interactions with paramagnetic ions in the aqueous phase also make a further appreciable contribution to the spin-label relaxation enhancement. On this basis, different paramagnetic agents may be chosen to optimize sensitivity to different structurally correlated interactions. These results therefore will aid further spin-label EPR studies in structural biology.     

甲醛的态选择性多光子电离研究

我们在XeCl准分子激光多光子电离甲醛分子的研究中,获得了甲醛经过2+1和2+2光子共振激发过程而产生的母体离子和高能离解通道的离子CO⁺。在电子轰击和单光子电离中占优势的低能阈离解通道离子HCO⁺,由于态选择性激发的结果而未出现。 关键词：