Electron impact ionization of thymine clusters embedded in superfluid helium droplets

Embedding molecules in helium clusters has become a powerful technique for the preparation of cold targets for spectroscopy experiments, as well as for the assembly of complex, fragile molecular species. We have recently developed a helium cluster source and a pick-up cell to produce neutral beams of doped helium droplets, to be used as targets in studies on electron collisions with molecules of biological relevance. In the present work we present the results of a series of experiments on electron-impact ionization of helium clusters doped with thymine and 1-methylthymine, where several interesting phenomena were observed, i.e., (i) electron impact ionization of molecular clusters inside the helium droplets leads predominantly to protonated clusters; (ii) the appearance energies are close to the ionization threshold of the helium atom but ionization efficiency curves in addition extend down by several eV; (iii) ionized molecular clusters can undergo metastable decay via the loss of one neutral monomer.     

强子衰变对高能反应末态超子极化的影响

在分析不同情形超子衰变的基础上, 给出超子衰变对超子产生率和末态超子极化的贡献的一般计算公式. 用Monte-Carlo数值计算研究强子衰变对高能反应末态超子极化的影响,结果发现, 强子衰变对Λ超子的贡献很大这一结论具有普遍性, 与模型无关; 不同模型给出的单极化pp碰撞产生的大横动量Λ超子极化的差别, 很可能相当大一部分来源于强子衰变. 考虑到衰变对极化贡献的计算中有较大不确定性, 检验不同模型, 最好看Σ^±,Ξ⁰,Ξ^－的极化.     

Calculated cross sections for electron elastic and inelastic scattering from DNA and RNA bases

Electron scattering cross sections from DNA/RNA bases are calculated in the 10 eV–10 keV energy range by combining screening-corrected aditivity-rule (SCAR) with ab-initio quasifree inelastic atomic potential. Similar calculations compare very favourably with available experimental data for benzene, C₆F₆ and tetrahydrofuran molecules. For DNA/RNA bases, where no experimental data is available, present results are compared with other theoretical values.     

QCD sum rule study of the masses of light tetraquark scalar mesons

We study the low-lying scalar mesons of light u, d, s flavors in the QCD sum rule. Having all possible combinations of tetraquark currents in the local form, QCD sum rule analysis has been carefully performed. We found that using the appropriate tetraquark currents, the masses of σ, κ, f₀ and a₀ mesons appear in the region of 0.6–1 GeV with the expected ordering. The results are compared with that of the conventional currents, where the masses are considerably larger.     

高电荷态离子引起的富勒烯离子的碎裂

离子在与富勒烯的相互作用过程中会导致C60分子的激发。处于低激发态的C60r 离子通过发射中性C2分子或带电的轻团簇碎片Cn 等非对称碎裂方式来耗散激发能,但如果激发能很高,笼形的C60r 离子可能会彻底崩溃,而发生多重碎裂。C60r 离子的碎裂过程与其电荷态r及分裂势垒密切相关。低电荷态的C60r (r≤3)离子蒸发一个C2分子需要克服10.3 eV左右的势垒。随着电荷态的升高,发射带电的Cn 会变得越来越容易,并逐渐过渡到多重碎裂过程。另一方面,C60r 离子的碎裂机制还与激发方式有关,在直接正碰过程中,将C60分子当作固体薄靶来处理,通过分析不同价态的C60r 离子的碎片谱,发现母核的初始电荷态决定碎裂方式,由此获得一个可以表征激发能大小的可观测量——发射电子个数。     

Stability and fragmentation processes of highly charged sodium clusters

Highly charged sodium clusters produced in collisions between neutral clusters and multiply charged ions are formed within a large range of temperatures and fissilities, and identified by means of a high-resolution reflectron-type time-of-flight mass spectrometer ( m/m 14000). The limit of stability of these charged clusters is experimentally investigated, and the time-of-flight spectra are compared with theoretical spectra based on Monte-Carlo simulations. The results indicate that the maximum fissility (X) of stable clusters is approaching the Rayleigh limit (X = 1) for larger clusters sizes. It is mainly limited by the initial neutral cluster temperature ( T 100 K) and the energy transfer in the ionizing collision. In addition, the comparison between the measured and simulated spectra suggests for high cluster charges a multi-fragmentation process, in which most of charge is emitted, creating low charged residual cluster ions.     

New approaches to stored cluster ions

Ion traps are wall-less containers which allow the extended storage of selected species. During the storage various interaction steps may be repeatedly applied. To this end no further hardware has to be added - in contrast to beam experiments. In this progress report two examples of recent developments are presented: the experiments have been performed with metal clusters stored in a Penning (ion cyclotron resonance) trap. A new experimental scheme has been developed which allows precision measurements of the dissociation energies of polyatomic species. It has been triggered by investigations on the delayed photodissociation of stored metal clusters. However, the technique is also readily available for application to a broad variety of different species and it is not even restricted to trapping experiments. The second development is more closely connected with ion storage in Penning traps: by application of an electron bath singly charged anionic clusters can be converted into multiply charged species. Subsequently, they are charge selected and investigated with respect to their reaction upon excitation. In particular, preliminary results indicate that dianionic metal clusters emit two electrons upon photoexcitation whereas the singly charged species show dissociation.     

Highly branched stimuli responsive poly[(N-isopropyl acrylamide)-co-(1,2-propandiol-3-methacrylate)]s with protein binding functionality

Highly branched poly(NIPAM) have been prepared using the technique of reversible addition-fragmentation chain transfer (RAFT) polymerisation using a chain transfer agent that allows the incorporation of imidazole functionality in the polymer chain-ends. The lower critical solution temperature (LCST) of the polymers can be controlled by the amount of hydrophobe and GMA incorporated during copolymerisation procedures. These thermally responsive "smart" polymers were used to purify a His-tagged BRCA-1 protein fragment by affinity precipitation. [Diagram: see text]     

On numerical improvement of Gauss–Legendre quadrature rules

Among all integration rules with n points, it is well-known that n-point Gauss–Legendre quadrature rule∫₋₁¹f(x) dx∑_i=1ⁿw_if(x_i)has the highest possible precision degree and is analytically exact for polynomials of degree at most 2n−1, where nodes x_i are zeros of Legendre polynomial P_n(x), and w_i's are corresponding weights.In this paper we are going to estimate numerical values of nodes x_i and weights w_i so that the absolute error of introduced quadrature rule is less than a preassigned tolerance ε₀, say ε₀=10⁻⁸, for monomial functionsf(x)=x^j, j=0,1,…,2n+1.(Two monomials more than precision degree of Gauss–Legendre quadrature rules.) We also consider some conditions under which the new rules act, numerically, more accurate than the corresponding Gauss–Legendre rules. Some examples are given to show the numerical superiority of presented rules.