Reversible bond formation in a gold-atom-organic-molecule complex as a molecular switch

We report on the formation of a metal-molecule complex that can be used as a molecular switch. Using a cryogenic scanning tunneling microscope, a covalent bond was formed reversibly between a gold atom and a perylene-3,4,9,10-tetracarboxylic dianhydride molecule supported by a thin insulating film. The bonded and the nonbonded state of the complex were found to be associated with different charge states, and the switching between the two states was accompanied by a considerable change in the tunneling current. Atomic force microscopy molecular imaging was employed to determine precisely the atomic structure of the complex, and the experimental results were corroborated by density functional theory calculations.     

The formation of positronium in molecular substances

The theories regarding the formation of positronium in molecular substances are discussed. A modified theory based upon the “spur” theory is described. Theoretical derivation and experimental results suggest that the fraction of the long lifetime component due too-positronium annihilation has a complex origin. Initially, the electron of the positron-electron pair is likely to be separated from the positron by medium molecules even when the total kinetic energy of the pair is less than the potential energy between them. The formation of positronium depends on the mobilities of the positron and electron, the slowing down process and the potential energy between them. Positronium is formed in less than 10 ps. Then the positronium produced may react with the radicals created in the “spur” due to the positron. Only the fraction ofo-positronium able to escape the “spur” will have a long lifetime. Therefore, positronium formation is highly related to the fast reactions in the “spur” during the first 100 ps.     

Intramolecular vibrations and noise effects on pattern formation in a molecular helix

Modulational instability in a biexciton molecular chain is addressed. We show that the model can be reduced to a set of three coupled equations: two nonlinear Schr?dinger equations and a Boussinesq equation. The linear stability analysis of continuous wave solutions of the coupled systems is performed and the growth rate of instability is found numerically. Simulations of the full discrete systems reveal some behaviors of modulational instability, since wave patterns are observed for the excitons and the phonon spectrum. We also take the effect of thermal fluctuations into account and we numerically study both the stability and the instability of the plane waves under 300 K. The plane wave is found to be stable under modulation, but displays a gradual increase of the wave amplitudes. Under modulation, the same behaviors are observed and wave patterns are found to resist thermal fluctuations, which is in agreement with earlier research on localized structure stability under thermal noise.     

Resonant formation of ddµ molecular ions

Calculations of the ddµ formation rate were performed beyond the dipole approximation. In the temperature rangeT=25–150 K the results are in good agreement with the Vienna-PSI fusion rate data. The calculated spin flip rate is also closer to experiment than in the dipole approximation. New experiments for the verification of the finite width theory are discussed.     

Muon molecular formation and transfer rate in solid hydrogen-deuterium mixtures

In an experiment at TRIUMF to study muon-catalyzed fusion and associated atomic and molecular effects, negative muons were stopped in a solid protium hydrogen layer containing a small amount of deuterium. Most of the resulting µp atoms disappeared by formation of ppµ molecules or by muon transfer to a deuteron. The µd can drift almost freely through the hydrogen layer due to the Ramsauer-Townsend effect and may even leave the layer. If a thin neon layer is frozen atop the hydrogen, the exiting muonic atoms will very rapidly release their muon to a neon atom. The analysis of the time structure of the neon X-rays is used to determine the rates of the slower processes involved in the evolution of the µp. This analysis has been performed with the help of Monte Carlo calculations, which simulate the kinetics of both µp and µd atoms in the hydrogen mixtures.     

Predicting secondary ion formation in molecular dynamics simulations of sputtering

We present a model to incorporate the excitation and ionization of sputtered particles into molecular dynamics simulations of ion bombardment-induced collision cascades in metals. The kinetic excitation of the solid is described by electronic friction experienced by all moving particles and electron promotion in close atomic collisions. Transport of the resulting excitation energy is treated by a nonlinear diffusion equation. The resulting space- and time-dependent electron temperature is then introduced into a rate equation model describing the ionization of ejected particles. This way, secondary ion formation is described by assigning an individual ionization probability to every sputtered atom. Averaging over the entire flux, this allows the prediction of measurable quantities like integral or spectral ionization probabilities as well as velocity spectra of secondary ions.     

In situ characterisation of thin-film formation in molecular low-temperature plasmas

Thin films can be formed in low-temperature plasma in two ways: as a result of material modification usually in a discharge of non-organic gas and in the course of a deposition process in a discharge of organic vapour. Thin-film formation requires in situ control of plasma processing in order to get information on the growth mechanisms and control the deposition process. Here are shown some effective in situ techniques of thin-film characterisation: various kinds of infrared spectroscopy (infrared reflection absorption spectroscopy, attenuated total reflection spectroscopy, etc.), ellipsometry and microgravimetry. We discuss the application of these methods to polymer modification as well as plasma polymerisation. The applied techniques give very good spatial resolution, up to a few nm. The limitations of method or equipment on time resolution can be compensated by the appropriate experimental arrangement. Received: 4 October 2000 / Accepted: 12 December 2000 / Published online: 3 April 2001     

Time-of-flight measurement of resonant molecular formation in muon-catalyzed dt fusion

Preliminary results are reported for an experiment at TRIUMF where a time-of-flight technique was tested for measuring the energy dependence of the rate for muon-catalyzed dt fusion. Muonic tritium atoms were created following transfer of negative muons from muonic protium in a layer of solid hydrogen (protium) containing a small fraction of tritium. The atoms escaped from the solid layer via the Ramsauer-Townsend mechanism, traversed a drift region of 18 mm, and then struck an adjacent layer of deuterium, where the muonic atom could form a molecular system. The time of detection of a fusion product (neutron or alpha) following muon arrival is dependent upon the energy of the muonic tritium atom as it traverses the drift region. By comparison of the time distribution of fusion events with a prediction based on the theoretical energy dependence of the rate, the strength of resonant formation can in principle be determined. The results extracted so far are discussed and the limitations of the method are examined.     

Percolation approach for atomic and molecular cluster formation

We apply a percolation approach for the theoretical analysis of mass spectra of molecular microclusters obtained by adiabatic expansion technique. The evolution of the shape of the experimental size distributions as function of stagnation pressure and stagnation temperature are theoretically reproduced by varying the percolation parameter. Remaining discrepancies between theory and experiment are discussed. In addition, the even-odd alternation as well as the “magic” shell structure within metallic, secondary ion mass spectra are investigated by introducing statistical weights for the cluster formation probabilities. Shell correction energies of atomic clusters as function of cluster size are deduced from the experimental data.     

Catalytic action of Ni atoms in the formation of carbon nanotubes: a molecular dynamics study

Catalytic action of Ni atoms in the growth of single-wall carbon nanotubes is investigated using tight-binding molecular dynamics and ab initio methods. Our results demonstrate this to be a two step process in which the Ni atom first creates and stabilizes defects in nanotubes. The subsequent incorporation of incoming carbon atoms anneals the Ni-stabilized defects freeing the Ni atom to repeat the catalytic process.     

共轭高聚物双分子链中激子生成的动力学研究

通过对耦合共轭高分子双链中激子生成的动力学研究,讨论了受到光激发后,激子在分子内或分子间生成的制约因素.激子的生成除了受分子链间耦合强度的制约,还受到光激发模式的影响.结果表明,激子既可以在分子内生成,也可以在耦合分子间生成.增大分子之间的相互作用强度,可以增大在分子之间生成激子的概率.     

New results in the theory of muonic atom formation in molecular hydrogen

Muonic atom formation in molecular hydrogen proceeds in two stages. In the first stage, the mu-molecular complex (abµe)^* is formed due to Coulomb capture of a muon by a hydrogen molecule (abee), and, in the second stage, the decay of the complex leads to exotic-atom formation. We consider various channels for the decay of the complex. The main competition channels are direct dissociation and Auger decay. The primary distribution of muonic atoms over quantum states and kinetic energy has been obtained taking into account the competition of the decay channels.     

大质量分子云核和恒星形成活动

用国家台紫金山天文台13.7米望远镜的三谱线系统对23个有线翼特征的大质量恒星形成区进行了观测.获得了12个云核,发现其中5个具有外向流,2个有系统的速度变化,1个可能有塌缩,这些核的质量分布在9.4*10e2到2.2*10e5个太阳质量,氢分子密度从每立方厘米3.4*10e2到1.2*10e4.外向流的质量大于3.5个太阳质量,动能高于9*10e37焦耳,比低质量年轻星体附近的外向流大很多,每个核往往包含着几十个2mass源,其色指数和亮度各不相同,但最红的那个总是在IRAS源的误差椭圆内,可能就是IRAS源的对应体.     

Friction through dynamical formation and rupture of molecular bonds

We introduce a model for friction in a system of two rigid plates connected by bonds (springs) and experiencing an external drive. The macroscopic frictional properties of the system are shown to be directly related to the rupture and formation dynamics of the microscopic bonds. Different regimes of motion are characterized by different rates of rupture and formation relative to the driving velocity. In particular, the stick-slip regime is shown to correspond to a cooperative rupture of the bonds. Moreover, the notion of static friction is shown to be dependent on the experimental conditions and time scales. The overall behavior can be described in terms of two Deborah numbers.     

Influence of urea on polyvinyl alcohol molecular superstructure formation

Whiskers up to 1 cm in length were grown in polyvinyl alcohol (PVA) and urea solution. Raman and IR spectra discover an interaction between PVA and urea molecules. Optical and electronic microscopy data show that urea influences on PVA molecular superstructure formation. PVA whiskers prepared in urea solution can be used for organic semiconductors production which properties are determined by arrangement of polymer macromolecules.