Isolated atoms,molecules and clusters

The basic constituents of matter, namely, atoms molecules and small clusters can be isolated in a solid rare-gas-matrix (RGM) and be characterized by Mössbauer Spectroscopy (MS). Experiments can be conducted both by the absorption method using a co-evaporation method of Mössbauer isotopes or by emission spectroscopy where source nuclei are isolated in the RGM. Experimental aspects of both techniques are outlined. Examples of applications of RGMI-MS are described, namely,⁵⁷Fe clustering with noble-metals (Cu, Ag, Au); unusual electronic states of Fe following⁵⁷Co e.c. decay; species formed by the bonding of Fe⁰, in its ground state, to small molecules are compared with those formed by Fe in its various excited states; and finally, the characterization of atomic I⁰ and Te⁰ and Te^?1 embedded in a solid argon matrix.     

氢气分子在沸石中的吸附模拟研究

采用巨正则蒙特卡罗方法模拟了氢气在沸石中的吸附行为,并采用Dubinin-Astakhov微孔分析方法,分析了沸石结构对储氢量大小的影响,总结了影响储氢量大小的物理因素,该工作有利于指导合理的设计与合成储存材料,为改善材料储氢能力提供了有力的理论支撑. 关键词： 储氢 吸附等温线 巨正则蒙特卡罗     

spin-lattice relaxation in cobalt-containing aluminophosphate molecular sieves

Phosphorus spin-lattice relaxation was studied in aluminophosphate molecular sieves containing various concentrations of either framework or non-framework cobalt. The behaviour of nuclear magnetisation in the presence of these paramagnetic centres was described successfully in the limit of no spin-diffusion. The diffusionless regime was strongly indicated with non-exponential magnetisation recovery and was therefore easy to recognise. According to the model, spin-lattice relaxation rates depend on the square of cobalt concentration. Measured relaxation rates agreed well with calculations if effective cobalt concentration was considered rather than the average one. The latter was obtained by bulk elemental analysis, while the former was extracted from cobalt concentration depth-profiles measured with Auger electron spectroscopy. These measurements indicated that in impregnated samples containing non-framework cobalt there could be much more cobalt near the crystal surface than within the crystal. Because high cobalt concentration can lead to an invisible phosphorus, only nuclei deep within the crystal contribute to the NMR signal. In such a case, the effective concentration is simply the concentration of cobalt far from the crystal surface. In our case, two impregnated samples with different bulk cobalt concentrations exhibited equal relaxation rates. Previously, such a case was misinterpreted as a case, in which nuclear spin-lattice relaxation was independent of cobalt concentration. AES measurements, however, revealed, that although average concentrations of the two samples were different by a factor of two, their effective concentrations were equal and thus in complete agreement with observed relaxation rates.     

HRTEM investigation of mesoporous molecular sieves

High resolution transmission electron microscopic (HRTEM) studies of some typical mesoporous molecular sieves, such as MCM-41, SBA-2 and STAC-1, etc. are presented. Since the materials consist of amorphous silica and their unit cell dimensions are in a range of 1.5 to over 10 nm, the conventional X-ray diffraction method gives us very limited information about the detailed structures. On the other hand, HRTEM has been found to be the most powerful technique to detect the local structures and to image metal clusters inside the channels of these materials.     

Double rotation27Al NMR in molecular sieves

Although high-resolution NMR spectra can be obtained in solids, the use of²⁷Al NMR to investigate the structure of aluminosilicate and aluminophosphate molecular sieves has been severely limited because anisotropic second-order quadrupolar interactions, responsible for spectral broadening, cannot be eliminated by conventional magic angle spinning (MAS) or multiple pulse techniques. Here we give the principles of the double rotation (DOR) NMR technique which can remove not only the first-order broadenings but also the second-order broadenings in the NMR spectra of quadrupolar nuclei in solids. High-resolution²⁷Al NMR using DOR is capable of resolving discrete framework aluminum sites in aluminophosphate molecular sieves, permitting quantitative investigation of site-specific adsorbate-host interactions, and of discriminating different aluminum species in zeolites.     

Magic numbers of atoms in surface-supported planar clusters

Surface-supported planar clusters can sprout active research and create numerous applications in the realm of nanotechnology. Exploitation of these clusters will be more extended if their properties on a supported substrate are thoroughly apprehended, and if they can be fabricated in a controllable way. Here we report finding the magic numbers in two-dimensional Ag clusters grown on Pb quantum islands. We demonstrate, with the images and energy spectra of atomic precision, the transition from electronic origin to a geometric one within the same system. Applying the magic nature, we can also produce a large array of planar clusters with well-defined sizes and shapes.     

Critical dynamics in clusters of noble gas atoms

The multi-fragmentation dynamics of noble gas atomic clusters is considered for different statistically distributed deposited energies. The conditions giving rise to the development of criticality in the cluster evolution are revealed from an analysis of the signals in the fragment mass distribution. The time dependence of the observables related to critical exponents is studied. It is demonstrated that in a certain regime the cluster exhibits a behavior which can be identified as the precursor of a second-order liquid-gas phase transition. Received 1st September 1998 and Received in final form 14 January 1999     

Dynamics of water in molecular sieves by dielectric spectroscopy

We present recent dielectric data on the dynamics of water confined in molecular sieves with pore sizes 5 and 10 A. The dielectric measurements in the frequency and temperature ranges 10(-2)-10(6) Hz and 120-300 K show three relaxation processes for both samples. In the case of the 10 A pore the slowest process shows an Arrhenius temperature dependence at low temperatures (<220 K), while at high temperatures the relaxation appears to follow a more Vogel-Fulcher-Tammann (VFT) like behaviour. The relaxation time for this process is 100 s at about 170 K. The second slowest process is at low temperatures very similar to the main process of (bulk-like) water in a fully hydrated clay, but also this process seems to exhibit some kind of dynamical transition, in this case at T approximately 185 K. All the three processes in the 5 A pore exhibit Arrhenius temperature dependence, and two of them are considerably slower than the main relaxation in the hydrated clay. Thus, dynamics of bulk-like water is only observed in the 10 A molecular sieves, and most of the water molecules in both 5 and 10 A pores have considerably slower dielectric relaxation than has been observed for water confined in clay, most likely due to strong interactions with the considerably more hydrophilic inner surfaces of molecular sieves.     

Dynamics of water in molecular sieves by dielectric spectroscopy

A short review of our thermodynamic and structural work on Ar, N₂, CO, O₂, and the n-alkane C₁₉H₄₀ in mesoporous glasses is presented. The experimental information comes from X-ray diffraction, vapour pressure isotherms, optical transmission, heat capacity, and dielectric measurements. The filling in the pore centre formed by capillary condensation is distinguished from the adsorbed layers on the pore walls and is compared to the bulk state. Special attention is paid to metastable states and to the rearrangement of partial fillings that show up as a consequence of phase transitions.Received: 1 January 2003, Published online: 21 October 2003PACS: 61.43.Gt Powders, porous materials - 78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures - 75.75. + a Magnetic properties of nanostructures - 77.22.-d Dielectric properties of solids and liquids     

Charge transfer and dissociation in collisions of metal clusters with atoms

We present a combined theoretical and experimental study of charge transfer and dissociation in collisions of slow Li31(2+) clusters with Cs atoms. We provide a direct quantitative comparison between theory and experiment and show that good agreement is found only when the exact experimental time of flight and initial cluster temperature are taken into account in the theoretical modeling. We demonstrate the validity of the simple physical image that consists in explaining evaporation as resulting from a collisional energy deposit due to cluster electronic excitation during charge transfer.     

碱金属钠原子修饰硅原子团簇的结构及储氢性能研究

提出碱金属钠原子修饰笼形Si_6团簇的结构模型,采用密度泛函理论(DFT)研究钠原子修饰笼形Si_6团簇的结构及储氢性能.研究结果表明,氢分子与笼形Si_6团簇表面相互作用很弱,氢分子在其表面容易脱附.采用钠原子修饰笼形Si_6团簇后可有效避免氢分子的脱附,并且钠原子在笼形Si_6团簇的表面不发生团聚,有利于氢分子在其表面吸附和循环利用.研究发现在两个钠原子修饰笼形Si_6团簇的结构中,每个钠原子可以有效吸附六个氢分子.计算得到Na2Si_6团簇结构储氢的质量分数高达10.08 wt%,且氢分子的平均吸附能约为0.837 kcal/mol.可见,实现钠原子修饰笼形Si_6团簇结构在常温常压条件下储氢是有可能的.     

Fluorescence ofp-N,N-dimethylaminobenzonitrile incorporated in pores of molecular sieves

The donor-acceptor-substituted aromatic moleculep-N, N-dimethylaminobenzonitrile (DMABN) is known to show a characteristic dual twisted intramolecular charge transfer (TICT) fluorescence with a pronounced solvent dependence. Despite the two-band structure of the fluorescence from DMABN incorporated within pores of the molecular sieves AlPO-5, SAPO-5, ZSM-5, and silicalite, there is no indication for TICT-state emission.     

Spontaneous alloying of tin atoms into nanometer-sized gold clusters and phase stability in the resultant alloy clusters

Alloying behavior and phase stability has been studied in situ by transmission electron microscopy using clusters in the Au-Sn system. When tin atoms are vapor-deposited onto nm-sized gold clusters, rapid dissolution of tin atoms into gold clusters takes place and as a result Au-rich solid solution, amorphous-like Au-Sn alloy and AuSn compound clusters are formed depending upon the concentration of tin. The remarkable enhancement of solubility has been observed in Au-rich solid solution and AuSn compound. It becomes more difficult to form two phases in the interior of individual clusters even if the composition of alloy clusters falls in the two-phase region in the phase diagram for the bulk alloy and as a result amorphous-like phase is stabilized in nm-sized Au-Sn alloy clusters. Received 2 August 1999 and Received in final form 8 November 1999     

Magnetic properties of iron loaded MCM-48 molecular sieves

Mesoporous molecular sieves of MCM-48 type were loaded with iron by the wet impregnation method, using Fe(III) nitrate or Fe(II) sulfate aqueous solutions as Fe sources, to obtain a magnetic porous composite. The iron loaded materials were characterized by XRD, N₂ adsorption and DRUV-vis and compared with the Si-MCM-48 host. Their magnetic properties were studied by measuring the hysteresis loops up to 1.5 T at different temperatures (5-300 K) and by magnetization vs. temperature curves following the conventional zero field cooling (ZFC) and field cooling (FC) protocols. Materials with high structure regularity and surface area are obtained, which exhibit a mixed paramagnetic and superparamagnetic behavior, arising in isolated iron ions inserted in the host framework, and in small iron oxide clusters or nanoparticles forming inside the pores, respectively. Larger hematite particles (8-13 nm) grown on the external surface provide a quite small ferromagnetic contribution to the hysteresis loop.     

One-dimensional atomic transport by clusters of self-interstitial atoms in iron and copper

Atomic-scale computer simulation has been used to study the thermally activated atomic transport of self-interstitial atoms (SIAs) in the form of planar clusters in pure Cu and f-Fe. There is strong evidence that such clusters are commonly formed in metals during irradiation with high-energy particles and play an important role in accumulation and spatial distribution of surviving defects. An extensive study of the mobility of SIA clusters containing two to 331 interstitials has been carried out using the molecular dynamics simulation technique for the temperature range from 180 to 1200 K. The results obtained show that clusters larger than three to four SIAs are one-dimensionally mobile in both Cu and Fe. Large clusters of more than 100 SIAs in Cu and 300 SIAs in Fe have significantly reduced mobility. The problem of describing one-dimensional (1D) motion in three-dimensional space is discussed. An attempt is made to describe the mobility of SIA clusters within the approximation of 1D diffusion. For clusters in both metals the effective migration energy of 1D diffusion as estimated via the jump frequency of the cluster centre of mass is found to be independent of the number of SIAs in the clusters, although the cluster jump frequency decreases with increasing cluster size. Mechanisms of 1D mobility of interstitial clusters are discussed.