Isolated molecules in metals

In this paper, some results obtained on the formation of isolated molecules of composition SnO_x in silver and SnF _x in copper are reviewed. Hyperfine interaction and ion beam interaction techniques were used for the identification of these molecules.     

Biomass-derived oxygenate reforming on Pt(111): A demonstration of surface science using d-glucose and its model surrogate glycolaldehyde

Molecules derived from cellulosic biomass, such as glucose, represent an important renewable feedstock for the production of hydrogen and hydrocarbon-based fuels and chemicals. Development of efficient catalysts for their reformation into useful products is needed; however, this requires a detailed understanding of their adsorption and reaction on catalytically active transition metal surfaces. In this paper we demonstrate that the standard surface science techniques routinely used to characterize the reaction of small molecules on metals are also amenable for use in studying the adsorption and reaction of complex biomass-derivatives on single crystal metal surfaces. In particular, Temperature Programmed Desorption (TPD) and High Resolution Electron Energy Loss Spectroscopy (HREELS) combined with Density Functional Theory (DFT) calculations were used to elucidate the adsorption configuration of d-glucose and glycolaldehye on Pt(111). Both molecules were found to adsorb in an η₁ aldehyde configuration partially validating the use of simple, functionally-equivalent model compounds for surface studies of cellulosic oxygenates.     

FIELD IONIZATION OF MOLECULES IN AN INTENSE LASER FIELD

In order to predict the field ionization probabilities, the accurate ab initio electrostatic potential of molecules has to be calculated. However, the calculation of the full ab initio electrostatic potential of molecules is complicated, even impossible for some larger molecules with low symmetry. Here, we present a semi-empirical model to treat the field ionization of molecules in an intense laser field. In this model, a modified Coulomb potential is used to take the place of the complicated ab initio electrostatic potential of molecules. The analytic equations of the Keldysh adiabatic parameter using the Coulomb potential and the modified Coulomb potential have first been given. Using our semi-empirical model, we have calculated the field ionization probabilities and the Keldysh adiabatic parameters of O₂, N₂, SO₂, C₂H₄, CH₃CN and C₆H₆ in an intense laser field. The results agree excellently with the calculations using the ab initio electrostatic potential of molecules. As the modified parameter for the Coulomb potential can be found from experimental measurements, the field ionization mechanism of molecules can be immediately predicted with our semi-empirical model.     

Molecular dynamics in solid n-nonadecane: Experiments and computer simulations

We have undertaken a collaborative study on the molecular dynamics and the phase transition mechanisms in low-dimensional molecular crystals by combining incoherent quasielastic neutron scattering, deuterium NMR and computer simulation techniques. In this paper, we summarize the method used for deriving a dynamical model for the reorientations of n-nonadecane molecules in the rotator phase R _I of the crystal. This model has been applied successfully to interpret experimental and computational data.     

Contribution of proton NMR relaxation to the investigation of molecular dynamics in columnar mesophases of discotic and polycatenar molecules

We present in this work a review concerning wide frequency rangeT ₁ proton NMR relaxation studies performed in compounds exhibiting columnar mesophases, namely the Col_ho mesophase in the case of a liquid crystal of discotic molecules and the ø_h mesophase in the case of a liquid crystal of biforked molecules. These NMR relaxation studies were performed combining conventional and fast field cycling NMR techniques in a frequency range between 100 Hz and 300 MHz. The possibility of probing such a large frequency range has provided a way to effectively distinguish the influence, on theT ₁ relaxation profiles, of the different molecular movements observed in this type of mesophases. In addition, we present a comparison between the molecular dynamics in columnar (ø_h) and lamellar (SmC) mesophases exhibited by the same biforked compound.     

Temperature dependence of Rb2 molecule formation rate constant in a magneto-optical trap

In this paper, we report the measurement of Rb₂ molecule formation rate constant due to a two body process in a magneto-optical trap as a function of the sample temperature. The ground state molecules are detected by two-photon ionization, through the intermediate a ³Σ _u ⁺ → 2³Π _g molecular band. Our results show that the Rb₂ molecules formed in the MOT could be due to a wave shape resonance, which enhances the molecule formation rate. This effect may be used to enhance the molecule production; and therefore it maybe important to future experiments involving production and trapping of cold ground state molecules.     

The effect of concentration of <Emphasis Type="Bold">H</Emphasis> <Subscript> <Emphasis Type="Bold">2</Emphasis> </Subscript> physisorption on the current–voltage characteristic of armchair BN nanotubes in CNT–BNNT–CNT set

In this research, we have studied physisorption of hydrogen molecules on armchair boron nitride (BN) nanotube (3,3) using density functional methods and its effect on the current–voltage (I–V) characteristic of the nanotube as a function of concentration using Green’s function techniques. The adsorption geometries and energies, charge transfer and electron transport are calculated. It is found that H₂ physisorption can suppress the I–V characteristic of the BN nanotube, but it has no effect on the band gap of the nanotube. As the H₂ concentration increases, under the same applied bias voltage, the current through the BN nanotube first increases and then begins to decline. The current–voltage characteristic indicates that H₂ molecules can be detected by a BN-based sensor.     

NMR molecular characterization of lubricating base oils: Correlation with their performance

The NMR technique was used to characterize some base oils of different nature (mineral, saturated polyolefins, hydrocracked and from waxes hydroisomerization) at the molecular level. Base oils are used in the formulation of lubricants with the addition of suitable additives which improve the final products performance. The studied base oils are used particularly in the formulation of engine oils. In this paper we have compared two “NMR editing” techniques which allow one to quantify the CH_n (n = 0?3) fractions of an “average” base oil molecule: 1) the gated spin echo (GASPE) and 2) the distortionless enhancement by polarization transfer (DEPT). We found that the quantitative data obtained from GASPE have a better accuracy than those from DEPT and so we chose GASPE to study the molecular characteristics of the aliphatic moieties of the base oils. On the basis of the GASPE data and of the average molecular mass obtained via VPO (Vapor Pressure Osmometry), the average number of branches and aliphatic rings were calculated. Moreover, on the basis of GASPE the distribution of side chain lengths and the positions of the methyl groups along the straight chain (methyls are the only groups for which the positions are directly obtained from NMR spectra) were obtained. A confirmation of the reliability of the NMR approach was achieved by comparing the branching degree, found with the NMR approach, of some base oils with their heats of fusion measured with the Differential Scanning Calorimetry (DSC). The heats of fusion are expected to be directly related to the length of straight CH₂ sequence and therefore to be lower for more branched molecules. We found that for base oils with similar aliphatic moieties, the heats of fusion are lower for more branched base oil molecules with a nearly linear correlation. The exception to this rule is represented by the two olefinic oils. Besides, we found that the fraction of the methyls on aliphatic chains longer than three carbons, determined with GASPE, correlates very well with the pour points values, with the only exception of the mineral oil.     

Time-dependent delayed electron spectra: a direct measurement of total decay rate as a function of internal energy

We present a general method to measure internal energy dependant decay rate in the case of systems emitting thermal electrons. Our approach is based on the measurement of the time-dependent kinetic energy spectra of delayed electrons using time-resolved velocity map imaging spectrometry. We illustrate this method in the case of C₆₀ molecules. Indeed electron spectra for C₆₀ have been studied in great details in the past few years, allowing a complete analysis of the observed features. Moreover, C₆₀ offers the opportunity to study competing decay mechanisms demonstrating that the technique may have broader applicability to other molecules. Using a model that includes all contributing decay channels relevant in our time delay range (namely delayed ionization and dissociation by C₂ emission) we are able to derive quantitative information on the decay channels of the molecule. In the situation considered here, the time-dependent electron temperature extracted from the kinetic energy spectra is used to determine more precisely the rate constant for the dominant process, namely neutral C₂ dissociation channel. In other words, the measurement of the cooling of an ensemble of C₆₀ molecules as a function of time delay after heating provides a direct and quantitative access to its decay dynamics. This method may be used to map out the total rate for complex decay mechanisms.     

Laser spectroscopy and optical pumping of alkali atoms in superfluid helium

In this paper, our recent works on the alkali atoms in superfluid helim (HeII) are reported. At first we mentions the laser-sputtering method for implantation, which is simple but is very efficient to produce various kinds of neutral atoms and molecules in HeII. Secondly, we report on the laser spectroscopy of alkali atoms in HeII. Optical excitation and emission spectra are found to be roughly explained by a spherical atomic bubble model, but the spectra corresponding to the D₂ lines indicate the quadrupole oscillation of the bubble shape. Optical pumping by a circularly polarized laser beam is found to produce perfect polarization, for both electron and nuclear spins. Using the rf-optical double resonance techniques, the magnetic and hyperfine resonances are observed. It is discussed also about the phenomena which have observed in the experiments done so far but have not been fully explained.     

Fabrication of large-scale ultra-smooth metal surfaces by a replica technique

The IR double-resonance techniques IR/R2PI (infrared/resonant 2-photon ionization), IR/PIRI (infrared-photo-induced Rydberg ionization) and IR-photodissociation spectroscopy are valuable tools to investigate structure, vibrations, and dynamical processes of neutral and ionic hydrogen-bonded clusters containing aromatic molecules. In this paper we report on the application of the IR double-resonance techniques to determine the NH and OH stretching vibrations of 4-aminophenol and 4-aminophenol(H₂O)₁, both in the neutral (S₀) and ionic (D₀) ground state. All vibrational frequencies obtained for 4-aminophenol and the cluster are compared with the values obtained from ab initio and DFT calculations. In the S₀ state, a trans-linear arrangement of 4-aminophenol(H₂O)₁ is obtained containing an O-H·· O hydrogen bond. In the D₀ state an overlay of two spectra can be observed resulting from the trans-linear structure and a second structure which contains a N-H·· O hydrogen bond. The observation of these two structures within the ion is an interesting example of a rearrangement reaction in the ionic state. Received: 3 November 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

小分子在真空紫外波段从量子态到量子态的光解离动力学

本文主要描述小分子在真空紫外波段(VUV,6-20 eV)光解离动力学的最新实验和理论研究进展.得益于基于商业化激光器的真空紫外光源技术,以及离子速度成像、高分辨氢原子-里德堡态标记-飞行时间测量和VUV-VUV泵浦-探测等方法的发展,研究人员现在可以对很多小分子在真空紫外波段的光解离动力学进行量子态到量子态层面的测量和研究,本文重点综述H_2(D_2,HD),CO,N_2,NO,O_2,H_2O(D_2O,HOD),CO_2,N_2O以及一些多原子分子在真空紫外波段光解离动力学的最新研究进展.这些小分子在真空紫外波段的光解离在天体化学以及大气化学中有着非常重要的应用.分子吸收一个VUV光子以后,通常会被直接激发到比较高的电子激发态,解离过程会涉及到多个电子态势能面之间的复杂非绝热相互作用.在实验上对解离截面等参数进行从量子态到量子态层面的精细测量对于深入了解这些复杂的势能面之间的相互作用有非常重要的意义.最近建成的大连相干光源是目前世界上唯一一台在真空紫外波段工作的自由电子激光,具有脉冲能量高、扫描范围宽(50～150 nm)等优越的性能,它的建成必将会大大促进小分子真空紫外光解离研究的发展.     

Manipulation of atoms and molecules with a low temperature scanning tunneling microscope

Apart from its ability to image surfaces with atomic resolution the scanning tunneling microscope has evolved as a tool to manipulate single atoms and molecules. In this paper we present several examples of atomic manipulation and the formation of nanostructures with this technique. As examples for lateral manipulation, i.e. the sliding of atoms and molecules along the surface, we show CO, C₂H₄ and Pb on a stepped copper surface, the Cu(211). As examples for vertical manipulation, i.e. the transfer of atoms and molecules between the tip and the surface, we discuss Xe and C₃H₆. The design of our low temperature scanning tunneling microscope is shortly described.     

Data and modeling of negative ion transport in gases of interest for production of integrated circuits and nanotechnologies

We review techniques to prepare, evaluate and apply sets of cross section and transport data for negative ions that are required for the modeling of collisional non-equilibrium plasmas used for processing of microelectronic circuits. We collect and discuss the transport coefficients and cross section sets.We have compiled data for negative ions in CF₄ and CF₄-related negative ions in rare gases. In addition, we consider data for F⁻ and CF₃⁻ in rare gases. Furthermore, we analyze the cross sections of halogen negative ions in rare gases and other molecules. This is followed by the data for SF₆ related ions in SF₆ and in rare gases. The cross section for scattering of O⁻ in O₂ has been derived from the transport data and used to make calculations of the transport properties. Finally we give a brief discussion of the availability of the data for H⁻ ions in H₂. We have derived cross sections in several cases but the basic aim is to show the basic features of transport coefficients. In particular we discuss the need to represent properly some details such as the non-conservative nature of transport coefficients and the anisotropy of diffusion. Application of approximate theories and representations of cross sections are also discussed.     

Absorption lines of HLi2 by operator-ket expansion

The aim of this paper is twofold. We wish to explain and test an analytical method for calculating the matrix elements of linear operators, that simplifies the numerical calculation to mere bookkeeping. The method has been described in Huber, D., 1985, Int. J. quant. Chem., 27, 245 [1] and applied there to the general Watson-rotation-vibration hamiltonian for non-linear molecules. In this paper we want to show that the method can also be used in more complex cases. As an example we present an application to the calculation of the first electronic transition in Li₂H. The second aim is to report a calculation of the absorption spectrum of Li₂H and the discussion of its rotation-vibration states.     

Graphene-based nitrogen dioxide gas sensors

In this study, we demonstrated that graphene could selectively absorb/desorb NO_x molecules at room temperature. Chemical doping with NO₂ molecules changed the conductivity of the graphene layers, which was quantified by monitoring the current–voltage characteristics at various NO₂ gas concentrations. The adsorption rate was found to be more rapid than the desorption rate, which can be attributed to the reaction occurred on the surface of the graphene layer. The sensitivity was 9% when an ambient of 100 ppm NO₂ was used. Graphene-based gas sensors showed fast response, good reversibility, selectivity and high sensitivity. Optimization of the sensor design and integration with UV-LEDs and Silicon microelectronics will open the door for the development of nano-sized gas sensors that are extremely sensitive.     

Electronic transport calculation of adsorbate NO<Subscript>2</Subscript> and NO molecules on graphene using Maximally Localized Wannier functions

In this paper we have investigated the adsorption of the gas molecules (NO₂, NO) on graphene, using first-principles methods. For full geometric relaxation of the molecules in the vicinity of a graphene sheet, we obtain the adsorption geometry, adsorption energies, charge transfer and density of states (DOS). We can identify which of the adsorbate molecules is acting as donor or acceptor. We find that the conductance of graphene at the Fermi level decreases with adsorbing NO₂ molecules and increases with adsorbing NO molecules.     

分子动力学模拟压水反应堆中联氨对水的影响

本文采用分子动力学方法模拟在常温常压下(1 atm,298 K)和在压水堆环境下(155 atm,626 K),水分子数为256,联氨(N₂H₄)分子数为0,25,50,75等不同数目时,水和联氨粒子系统的动力性质和微观结构.同时探讨了联氨分子的引入对水中溶解氧的影响.从模拟结果可知,在常温常压下,当联氨的分子数为0,25,50,75时,粒子系统的均方位移会随联氨分子数的增加而增加;联氨分子数为0与为25,50,75比较时会少一个数量级;压水堆环境下,联氨分子数 关键词： 分子动力学 压水堆 联氨     

Coulomb blockade sensors based on nanostructured mesoporous silicon

Mesoporous silicon (mesoPS) is a nanosponge where Si nanocrystals are interconnected forming a disordered 3D array. The electronic characteristics of this material are particularly interesting, due to some intriguing effects, such as a huge increase of conductivity, reversible insulator-to-metal transition and n- or p-type doping of the nanocrystals, exhibited in presence of donor or acceptor molecules like NH₃ and NO₂. Here we report on the observation of a sharp conductance gap, which can be ascribed to Coulomb blockade phenomena. Moreover, we show that the width of the gap can be tuned by NO₂ molecules, so that the fabrication of highly sensitive threshold sensors is possible. Our results suggest that electrochemical etching of heavily doped Si can be used as a simple self-assembly technique for the production of Si nanocrystal arrays and for the fabrication of sensitive nanosensors.