Photoionization of argon, krypton and xenon clusters in the inner valence shell region

Photoionization of rare gas clusters in the innervalence shell region has been investigated using threshold photoelectron and photoion spectrometers and synchrotron radiation. Two classes of states are found to play an important role: (A) valence states, correlated to dissociation limits involving an ion with a hole in its innervalence ns shell, (B) Rydberg states correlated to dissociation limits involving an ion with a hole in its outervalence np shell plus an excited neutral atom. In dimers, class A states are “bright”, that is, accessible by photoionization, and serve as an entrance step to form the class B “dark” states; this character fades as the size of the cluster increases. In the dimer, the “Mulliken” valence state is found to present a shallow potential well housing a few vibrational levels; it is predissociated by the class B Rydberg states. During the predissociation a remarkable energy transfer process is observed from the excited ion that loses its innershell electron to its neutral partner. Received: 10 February 1998 / Revised: 17 July 1998 / Accepted: 31 July 1998     

使用时间切片离子速度成像技术研究CF2Cl2光解动力学

本文使用时间切片离子速度成像技术结合共振增强多光子电离技术研究了CF₂Cl₂分子在235 nm附近的光解动力学. 通过测量CF₂Cl₂分子在235 nm附近单光子解离产生的氯原子影像,直接得到了解离产物的速度分布和角分布. 激发态氯原子的速度分布包含高动能组分和低动能组分,分别对应³Q₀电子态的直接解离和由于内转换引起的基态预解离. 基态氯原子的速度分布也包含高动能组分和低动能组分,分别对应³Q₀与¹Q₁电子态的预解离和由于内转换引起的基态预解离. 自由基解离通道被确认,二次解离通道和三体解离通道被排除.     

S3解离中的非绝热过程

采用量子力学从头计算方法研究了硫的三聚物S_３低激发态解离过程.构造了S_３分子的解离极限.对态态间跃迁极矩的计算表明，在势能曲面的交叉区域态态间存在相互转换的非绝热过程，从而确定了在S_３解离为S₂的复杂过程中，非绝热预解离是重要的解离通道. 关键词： ３')" href="#">S_３ 从头算 预解离 非绝热     

Ionic interactions and collision dynamics in cold traps: rotational quenching of OH<Superscript>−</Superscript>(<Superscript>1</Superscript>Σ<Superscript>+</Superscript>) by Rb(<Superscript>2</Superscript>S)

A newly computed potential energy surface, which describes the forces at play between the OH⁻ (X ¹Σ⁺) anionic molecule (treated as a rigid rotor at its equilibrium geometry) and the Rb(²S) atomic gas was obtained from fully ab initio methods, yielding highly correlated electronic wavefunctions for the interacting partners. It is in turn employed to calculate their rotationally quenching collision cross sections at ultralow energies and for different initial rotational states of the molecular anion. The results suggest that these strongly interacting partners are among the most efficient systems in providing very large internal de-excitation rates whenever the collision regime of an ultracold trap can occur for the partners. The collision encounters at such vanishing values of translational energies are seen to be strongly controlled by the behaviour of the real part of the scattering length as a function of the initial rotational state: the latter indicates, in fact, the presence of several virtual states close to the dissociation threshold of the complex.     

AuB2 in comparison to superconducting MgB2-an ab initio study

The noble metal diboride AuB₂, a potential candidate for superconductor, is studied by an ab initio method in comparison to the superconducting MgB₂. The results, described in terms of equilibrium lattice constants, bulk modulus, pressure derivative of bulk modulus and their in- and out-of-plane linear values, volume coefficient of T_c, density of states, band structure, show some similarity as well as dissimilarity between the behaviour of the two compounds. The implications for the behaviour are discussed.     

Ab initio study of structural,electronic, and elastic properties of M<Subscript>2</Subscript>SbP (M = Ti,Zr, and Hf)

This study predicts the structural behaviour of selected M₂SbP compounds with the same structure as MAX phases. Zero pressure results of the lattice parameters, equilibrium volume, and the internal parameter Z_M are calculated with an error less than 3%. Band structure, total and partial density of states were calculated and show the metallic character of these phases. Moreover we observed strong hybridising states; M d–P p, and M d–Sb p. The pressure dependence of the volume, and the lattice parameters were studied. The stiffness of M–P, and M–Sb bonds was discussed in term of relative length change under hydrostatic pressure. Hf₂SbP present the highest bulk modulus and the unidirectional elastic modulus C₃₃ is slightly greater than C₁₁.     

Nonradiative relaxation processes in molecular crystals

Internal conversion is the dominant relaxation channel from higher lying excited states in molecular crystals and involves the transfer of energy from the electronic system to the lattice. In this work, we present results from simulations of the nonradiative relaxation process with an emphasis on both intra- and interband transitions. We find the internal conversion process to be strongly nonadiabatic and the associated relaxation time in the case of large energy excitations to be limited by the transitions made between states of different bands.     

Photoinduced dissociation reactions of silver fluoride cluster ions

Photoinduced dissociation in the ultraviolet region has been investigated for Ag _nF _n-1 ⁺ cluster ions. Photodissociation spectrum of Ag₂F⁺ in the energy of 3.8–5.6 eV exhibits several sharp bands corresponding to the transition to electronically excited states. In this dissociation, only the Ag₂ ⁺ ion was observed as a fragment ion. Theoretical calculation indicates that the parent Ag₂F⁺ ion has a linear Ag-F-Ag equilibrium geometries in the ground and excited states. Since conformational changes by excitation of bending vibration are necessary for the fragmentation of an F atom, this indicates that production of Ag₂ ⁺ from Ag₂F⁺ is a result of internal conversion and following conformational changes.     

Time-dependent fragment distributions detected via pump-probe ionisation: a theoretical approach

We show how in molecular predissociation a method combining ultrafast pump-probe techniques with a measurement of the relative recoil velocity can map time-dependent neutral fragment distributions into the ionic continuum. With an appropriate probe pulse exciting a resonant transition (such as (1+1) Resonance Enhanced Multiphoton Ionisation, or excitation of ZEKE states), the temporal evolution of fragment distributions can in principle be measured. Numerical simulations on NaI predissociation are compared to a simple approximate mapping interpretation. The results are discussed in terms of the interplay between temporal and energetic resolution with respect to current experimental limitations. Received 13 October 2000 and Received in final form 8 December 2000     

Molecular picture of excited states and fragmentation paths of the Na 5 F 4 cluster

The stability against fragmentation and possible relaxation of the lowest excited states of the Na₅F₄ cluster (representative of cubic non stoechiometric clusters with an excess sodium atom, also called sodium-tail) is investigated by means of one-electron pseudopotential calculations with particular reference to photoabsorption processes from the ground state. Whereas the equilibrium configuration of the ground state has C_3v symmetry, the doubly degenerate 1²E excited state is affected by a conical intersection and a Jahn-Teller effect associated with the rotation of the sodium tail around the C₃-axis. This yields a “Mexican hat" topology for the lowest sheet with three equivalent C_s minima. Alternatively the 2²A₁ state has a minimum retaining the C_3v symmetry. The dissociation paths of the cluster along the C₃-axis into respectively Na₄F_{4 +} Na and Na₄F_{3 +} NaF are also investigated. Among the former paths, the excited states are found adiabatically stable with respect to the products. However in the A₁ symmetry, fragmentation into NaF exhibits an interesting avoided crossing between configurations correlated respectively with Na₄F₃ ^{+ +} NaF^- and Na₄F_{3 +} NaF. Such interaction, similar to the well-known charge exchange processes in elementary molecules might induce non adiabatic predissociation of the 2²A₁ state. This mechanism is invoked to explain the differences between R2PI and depletion spectra, correlated with the dissociation or relaxation of the excited states. Received 24 March 2000 and Received in final form 11 July 2000     

The formation of O(3s3S0) and N(3s4P) by photodissociation of NO

The dissociation of NO molecules to excited atomic states by dispersed synchrotron radiation (700 Å – 800 Å) is investigated by observing vuv-fluorescence (1050 Å – 1400 Å) of the fragments. The atomic levels are populated by predissociation of high-lying molecular states.     

A rare nuclear decay process: The internal conversion between bound atomic states

We shall report on the recently observed dependence of the lifetime of the first excited state in ¹²⁵Te on the ionic charge state. Then we shall give an interpretation of the dependence of the half-life in terms of a new type of nuclear internal conversion without emission of the electron into the continuum of electron energies. We have named this process internal conversion between bound atomic states or BIC. The resonant character of the BIC will be established and the main parameters governing the decay process will be discussed [1–3]. Finally the results of a recent experiment performed at the GANIL accelerator attempting to measure directly the value of the internal conversion coefficient associated with BIC in ¹²⁵Te ions with charge states ranging between 44+ and 48+ will be given. In conclusion we shall discuss the relation between the BIC and nuclear excitation by electron transition, NEET, in the excitation of some nuclear isomeric states.     

Indole photodissociation in the gas phase

Kinetic characteristics of the growth in delayed luminescence intensity of indole in the gas phase have been measured with excitation of the molecules in the spectral region 250–287 nm. The luminescence build-up kinetics show two components with characteristic lifetimes of about 1 and 10 μs that are associated with the formation of free radicals due to N–H bond dissociation. Two basic mechanisms for photodissociation of indole in the gas phase have been identified based on an analysis of the dependence of the kinetic characteristics for these components on the excitation radiation wavelength/intensity and the vapor pressures of indole and foreign gases. The fast component results from the dissociation of an N–H bond of vibrationally excited molecules in the ground state that is populated through internal conversion. The slow component is associated with the generation of free radicals because of annihilation of two triplet indole molecules. A dependence of the quantum yield of intersystem crossing on the excitation wavelength in the spectral region 260–287 nm has been found. It is shown that the fluorescence quantum yield drop with a decrease of the excitation radiation wavelength is caused by an increase in the internal conversion quantum yield to the electronic ground state.     

Unimolecular decay of hydrogen-bonded cluster ions

The unimolecular decay of the hydrogen-bonded phenol(H₂O)₄ ⁺ cluster ion was studied both experimentally and theoretically. The ion is formed by resonant two-color two-photon ionization of the neutral cluster. If the second color exceeds the dissociation threshold of the ion, fragmentation occurs on a microsecond time scale. By analyzing the time-of-flight (TOF) signal of the metastable ions at different excess energies the dependence of the fragmentation rate on the excess energy can be deduced. These experimental rates are compared to microcanonical decay constants k(E) calculated from Rice–Ramsperger–Kassel–Marcus (RRKM) theory based on ab initio data solely. Received: 8 March 2000 / Revised version: 28 April 2000 / Published online: 5 October 2000     

Domain wall delocalization,dynamics and fluctuations in an exclusion process with two internal states

We investigate the delocalization transition appearing in an exclusion process with two internal states, respectively on two parallel lanes. At the transition, delocalized domain walls form in the density profiles of both internal states, in agreement with a mean-field approach. Remarkably, the topology of the system’s phase diagram allows for the delocalization of a (localized) domain wall when approaching the transition. We quantify the domain wall’s delocalization close to the transition by analytic results obtained within the framework of the domain wall picture. Power law dependences of the domain wall width on the distance to the delocalization transition as well as on the system size are uncovered, they agree with numerical results.     

A rational method for probing macromolecules dissociation: the antibody-hapten system

The unbinding process of a protein-ligand complex of major biological interest was investigated by means of a computational approach at atomistic classical mechanical level. An energy minimisation-based technique was used to determine the dissociation paths of the system by probing only a relevant set of generalized coordinates. The complex problem was reduced to a low-dimensional scanning along a selected distance between the protein and the ligand. Orientational coordinates of the escaping fragment (the ligand) were also assessed in order to further characterise the unbinding. Solvent effects were accounted for by means of the Poisson–Boltzmann continuum model. The corresponding dissociation time was derived from the calculated barrier height, in compliance with the experimentally reported Arrhenius-like behaviour. The computed results are in good agreement with the available experimental data.     

苯乙炔分子电子激发态超快动力学研究

采用飞秒时间分辨质谱技术结合飞秒时间分辨光电子影像技术研究了苯乙炔分子电子激发态超快非绝热弛豫动力学.用235 nm光作为泵浦光,将苯乙炔分子激发到第二激发态S2,用400 nm光探测激发态的演化过程.时间分辨的母体离子的变化曲线用指数和高斯函数卷积得到不同的两个组分,一个是超快衰减组分,时间常数为116 fs,一个是慢速组分,时间常数为106 ps.通过分析时间分辨的光电子影像得到光电子动能分布,结合时间分辨光电子能谱数据发现,时间常数为116 fs的快速组分反映了S2态向S1态的内转换过程.实验还表明S1态通过内转换被布局后向T1态的系间窜跃过程为重要的衰减通道.本工作为苯乙炔分子S2态非绝热弛豫动力学提供了较清晰的物理图像.     

Cs2分子的光解离和碰撞能量转移

利用He-Cd+激光器的441.6 nm线光解Cs2分子,使Cs(6D)态得到布居,在Cs2密度为1～9×1015 cm-3范围内,测量原子荧光对分子荧光强度比,得到碰撞转移率系数对解离率之比为9.6×10-18 cm3.测定Cs(6D3/2)对6D5/2荧光强度分支比,得到62D态精细结构能级解离率之比为0.41.从翼激发得到的精细结构转移截面与从共振激发得到的截面结果相符.     

低振动激发HeI2分子振动预离解寿命和终转动态分布

用含时黄金规则波包法，对ＨｅＩ２分子在低初始振动激发（ｖ＜１２）态下振动预离解动力学作了全维量子力学计算。所预言的总衰变宽度和寿命与谱线宽和皮秒时间分解的实时态－态测量外推数据符合得相当好。计算的总衰变宽度对初始振动态ｖ是敏感的并呈现一种非线性关系。结果表明低振动激发ＨｅＩ２分子衰变模式仍应是量子力学的。除终态相互作用对决定终转动分布有重要作用以外，首次发现，低振动激发态的初始特性也能显著影响终转动态分布。用Ｉ２的转动常数对ｖ的关系合理地解释了这个独特现象     

Franck-Condon analysis of predissociations in the lower valence states of the NH radical

The origins of the predissociations observed by Smith, Brzozowski, and Erman for the A³Πand c¹Π states of NH have been investigated via a Franck-Condon analysis. It is shown that the measured variation of the predissociation probability is is proportional to the Franck-Condon densities between the bound rotational levels of the A and c states and the vibrational continuum of a ⁵Σ^- state arising from the ground state asympote. Similar calculations for the X³Σ^- ground state vibrational continuum, the only other state with a lower dissociation limit, showed that state to be ineffective in producing the observed predissociation.