Studies of intermolecular interactions by matrix isolation vibrational spectroscopy: Self-association of water

The mid- and far-infrared spectra of water in argon and nitrogen matrices have been reinvestigated. The water molecule rotates in an argon matrix, but in a nitrogen matrix two low-frequency bands are attributed to libration of the water monomer about its A and C rotational axes. Combinations of these modes with the stretching and bending modes of the water monomer were observed. A detailed concentration-dependence study enabled bands to be attributed to dimer, trimer, and tetramer or higher multimers. The dimer and trimer were both found to have open chain structures, whereas tetramer or higher multimer species appear to be predominantly cyclic. Intermolecular modes, observed for dimer and trimer in the far-infrared spectra, are tentatively assigned.     

具有电子推拉结构的多支化分子的光物理特性的研究

利用稳态光谱和飞秒时间分辨荧光亏蚀的技术,研究了不同溶剂中一系列有分子内电荷转移特性的分子的结构与光物理性质的关系,研究体系为三苯胺作为电子给体,2,1,3-苯并噻二唑作为受体的单支分子及其对应的两支和三支分子. 并结合TD-DFT计算进一步解释了实验中所观察到的现象. 三个分子相似的吸收和荧光光谱以及强的溶剂依赖光谱特性表明两支与三支分子激发态与单支分子相似,表明激发态都定域在其中一支上. 激发时多支分子内发生多维电荷转移,然后快速地定域到某一支上发射. 另一方面多支分子相对于单支分子吸收和发射光谱的红     

Adsorption of water monomer and clusters on platinum(111) terrace and related steps and kinks: I. Configurations,energies, and hydrogen bonding

Adsorption and rotation of water monomer, dimer, and trimer on the (111) terrace, (221) and (322) stepped, and (763) and (854) kinked surfaces of platinum were studied by density functional theory calculations using the PW91 approximation to the energy functional. On the (111) terrace, water monomer and the donor molecule of the dimer and trimer adsorb at atop sites. The per-molecule adsorption energies of the monomer, dimer, and trimer are 0.30, 0.45, and 0.48 eV, respectively. Rotation of monomers, dimers, and trimers on the terrace is facile with energy barriers of 0.02 eV or less. Adsorption on steps and kinks is stronger than on the terrace, as evidenced by monomer adsorption energies of 0.46 to 0.55 eV. On the (221) stepped surface the zigzag extended configuration is most stable with a per-molecule adsorption energy of 0.57 eV. On the (322) stepped surface the dimer, two configurations of the trimer, and the zigzag configuration have similar adsorption energies of 0.55 ± 0.02 eV. Hydrogen bonding is strongest in the dimer and trimer adsorbed on the terrace, with respective energies of 0.30 and 0.27 eV, and accounts for their increased adsorption energies relative to the monomer. Hydrogen bonding is weak to moderate for adsorption at steps, with energies of 0.04 to 0.15 eV, as the much stronger water–metal interactions inhibit adsorption geometries favorable to hydrogen bonding. Correlations of hydrogen bond angles and energies with hydrogen bond lengths are presented. On the basis of these DFT/PW91 results, a model for water cluster formation on the Pt(111) surface can be formulated where kink sites nucleate chains along the top of step edges, consistent with the experimental findings of Morgenstern et al., Phys. Rev. Lett., 77 (1996) 703.     

Interaction of low energy electrons with formic acid monomer,dimer and trimer

Configurations of the formic acid monomer, dimer and trimer are presented. We have calculated the total energies for these structures. We have considered also the negatively charged clusters and discussed the possible pathways of interaction of low energy electrons with monomers and clusters. Calculations have been executed using 6-311G** and 6-311++G** basis sets. The electron affinities of monomer and clusters are presented. Influence of choice of the molecular basis set is also discussed. Only such systematic studies allow to explain values observed experimentally, for example in electron attachment experiments on formic acid [1].     

Infrared observation of OC-C2H2, OC-(C2H2)2 and their isotopologues

ABSTRACT

The fundamental band for the OC-C₂H₂ dimer and two combination bands involving the intermolecular bending modes ν₉ and ν₈ in the carbon monoxide CO stretch region are re-examined. Spectra are obtained using a pulsed supersonic slit jet expansion probed with a mode-hop free tuneable infrared quantum cascade laser. Analogous bands for OC-C₂D₂ and the fundamental for OC–DCCH as an impurity are also observed and analysed. A much weaker band in the same spectral region is assigned to a new mixed trimer, CO-(C₂H₂)₂. The trimer band is composed uniquely of a-type transitions, establishing that the CO monomer is nearly aligned with the a-inertial axis. The observed rotational constants agree well with ab initio calculations and a small inertial defect value indicates that the trimer is planar. The structure is a compromise between the T-shaped structure of free acetylene dimer and the linear geometry of free OC-C₂H_2. A similar band for the fully deuterated isotopologue CO-(C₂D₂)₂ confirms our assignment.     

Kinetic Monte Carlo simulations of Pd deposition and island growth on MgO(1 0 0)

The deposition and ripening of Pd atoms on the MgO(1 0 0) surface are modeled using kinetic Monte Carlo simulations. The density of Pd islands is obtained by simulating the deposition of 0.1 ML in 3 min. Two sets of kinetic parameters are tested and compared with experiment over a 200-800 K temperature range. One model is based upon parameters obtained by fitting rate equations to experimental data and assuming the Pd monomer is the only diffusing species. The other is based upon transition rates obtained from density functional theory calculations which show that small Pd clusters are also mobile. In both models, oxygen vacancy defects on the MgO surface provide strong traps for Pd monomers and serve as nucleation sites for islands. Kinetic Monte Carlo simulations show that both models reproduce the experimentally observed island density versus temperature, despite large differences in the energetics and different diffusion mechanisms. The low temperature Pd island formation at defects is attributed to fast monomer diffusion to defects in the rate-equation-based model, whereas in the DFT-based model, small clusters form already on terraces and diffuse to defects. In the DFT-based model, the strong dimer and trimer binding energies at charged oxygen vacancy defects prevent island ripening below the experimentally observed onset temperature of 600 K.     

Square lattice calculations for the general dimer and trimer problems using the Kikuchi method

Square lattice models for the general dimer and trimer problems are considered. A secondary lattice for the dimer problem is constructed, and a double square Kikuchi calculation produces an athermal molecular freedom within 2% of the known exact result. With interactions, departures from ideality of mixing entropy and vapour pressure are calculated at different temperatures, and an asymmetrical phase separation occurs at a reduced temperature 0.08. The general trimer problem is considered in a single square Kikuchi calculation. A single interaction is included. The molecular freedom and thermodynamic non-ideality of rigid linear, rigid angled and flexible trimers are compared. In the rigid linear case a phase separation occurs at reduced temperature 0.04. The molecular freedom of rigid linear trimers is calculated using higher Kikuchi approximations, and the result agrees well with previous estimates.     

Complexes d'autoassociation du cyclohexanol. I - modele thermodynamique des solutions diluees.

Activity coefficients for cyclohexanol in diluted isooctane solutions at 25°C and 45°C are obtained from vapour pressure measurements, x_u = 10^?3 to 5.10^?2 (x_u mole fraction of cyclohexanol) ; enthalpies of mixing for the same composition range are obtained at 25°C. The results are interpreted in terms of the model of ideal association where open dimer and trimer, open and cyclic other i-mers are considered in a six parameters model : two standard enthalpies for formation of the H-bonds (Δ$\text{H}$_2o for the dimer and h for the stepwise formation for other bonds), four equilibrium constants (K_2o and K_3o for the dimer and trimer formation from the monomer, K_c for the cyclisation of an open i-mer and k for the stepwise formation of an open, or cyclic, i-mer higher than the trimer). The cyclic form of the tetramer is shown to be the predominent one.     

Water adsorption on the Be(0001) surface: from monomer to trimer adsorption

In this paper, the density functional theory has been used to perform a comparative theoretical study of water monomer, dimer, trimer, and bilayer adsorptions on the Be(0001) surface. In our calculations, the adsorbed water molecules are energetically favoured adsorbed on the atop sites, and the dimer adsorption is found to be the most stable with a peak adsorption energy of ～437 meV. Further analyses have revealed that the essential bonding interaction between the water monomer and the metal substrate is the hybridization of the water 3a₁-like molecular orbital with the (s, p_z) orbitals of the surface beryllium atoms. While in the case of the water dimer adsorption, the 1b₁-like orbital of the H₂O molecule plays a dominant role.     

Small atomic clusters on metal, semiconductor, and graphene: A model approach

The analytical expressions for the densities of states of adsorbed monomer, dimer, and trimer have been derived using the simplest models for the densities of states of metal, semiconductor, and single-sheet graphene. It has been shown that all specific features in the densities of states of small clusters are predominantly determined by the main peculiarities of the density of states of the substrate rather than by the adsorbed clusters themselves.     

Pd diffusion on MgO(1 0 0): The role of defects and small cluster mobility

Density functional theory is used to explore the energy landscape of Pd atoms adsorbed on the terrace of MgO(1 0 0) and at oxygen vacancy sites. Saddle point finding methods reveal that small Pd clusters diffuse on the terrace in interesting ways. The monomer and dimer diffuse via single atom hops between oxygen sites with barriers of 0.34 eV and 0.43 eV respectively. The trimer and tetramer, however, form 3D clusters by overcoming a 2D-3D transition barrier of less than 60 meV. The trimer diffuses along the surface either by a walking or flipping motion, with comparable barriers of ca. 0.5 eV. The tetramer rolls along the terrace with a lower barrier of 0.42 eV. Soft rotational modes at the saddle point lead to an anomalously high prefactor of 1.3 × 10¹⁴ s⁻¹ for tetramer diffusion. This prefactor is two order of magnitude higher than for monomer diffusion, making the tetramer the fastest diffusing species on the terrace at all temperatures for which diffusion is active (above 200 K). Neutral oxygen vacancy sites are found to bind Pd monomers with a 2.63 eV stronger binding energy than the terrace. A second Pd atom, however, binds to this trapped monomer with a smaller energy of 0.56 eV, so that dimers at defects dissociate on a time scale of milliseconds at room temperature. Larger clusters bind more strongly at defects. Trimers and tetramers dissociate from monomer-bound-defects at elevated temperatures of ca. 600 K. These species are also mobile on the terrace, suggesting they are important for the ripening observed at ?600 K during Pd vapor deposition on MgO(1 0 0) by Haas et al. [G. Haas, A. Menck, H. Brune, J.V. Barth, J.A. Venables, K. Kern, Phys. Rev. B 61 (2000) 11105].     

Adsorption of water monomer and clusters on platinum(111) terrace and related steps and kinks II. Surface diffusion

Surface diffusion of water monomer, dimer, and trimer on the (111) terrace, (221) and (322) stepped, and (763) and (854) kinked surfaces of platinum was studied by density functional theory using the PW91 approximation to the energy functional. Monomer diffusion on the terrace is facile, with an activation barrier of 0.20 eV, while dimer and trimer diffusions are restricted due to their high activation barriers of 0.43 and 0.48 eV, respectively. During monomer diffusion on the terrace the O–Pt distance increases by 0.54 Å, about 23% of the initial distance of 2.34 Å. The calculated rate of monomer diffusion hops is in good agreement with the onset temperature of diffusion measurements of Daschbach et al., J. Chem. Phys., 120 (2004) 1516. Alternative monomer diffusion pathways, in which the molecule rolls or flips, were also found. These pathways have diffusion barriers of 0.22 eV. During dimer diffusion on the terrace, the donor molecule rises 0.4 Å at the saddle point, while the acceptor rises by only 0.03 Å. Monomer diffusion up to steps and kinks, with activation barriers of 0.11–0.13 eV, facilitate chain formation on top of step edges. The energy landscape of monomer diffusion from terrace to step to kink sites is downhill with a maximum activation barrier of 0.26 eV. A model for water adsorption is presented in which monomer diffusion leads to concurrent formation of terrace clusters and population of steps/kinks, the latter consistent with the STM measurements of Morgenstern et al., Phys. Rev. Lett., 77 (1996) 703.     

Properties of Universal Bosonic Tetramers

The system of four identical bosons is studied using momentum–space equations for the four-particle transition operators. Positions, widths and existence limits of universal unstable tetramers are determined with high accuracy. Their effect on the atom–trimer and dimer–dimer scattering observables is discussed. We show that a universal shallow tetramer intersects the atom–trimer threshold twice leading to resonant effects in ultracold atom–trimer collisions.     

Excimer formation of pyrene as a probe to investigate the recombination of geminate pairs: the effect of charge hopping process on fluorescence and ODESR spectra in X-irradiated ethylene propylene rubber doped with pyrene

The fluorescence detected ESR (ODESR) spectrum and X-induced and UV-excited fluorescence spectra were successfully obtained for ethylene propylene rubber doped with pyrene under various conditions. From the analysis of the ODESR intensity and its line-width, it was found that charge hopping between the solute pyrenes is the main mechanism for geminate recombination at the pyrene content larger than 0.3 wt%. At elevated temperatures, the excimer fluorescence relative to the monomer fluorescence induced by X-ray excitation is considerably stronger than that by UV-excitation, especially at high concentrations. On the other hand, at lower temperatures below ca. 280 K this difference could not be observed even at high concentration of 3.0 wt%. These observations indicate that the dimer cation formation, from which the excimer fluorescence is efficiently emitted, is accelerated by the hopping of cation-radical center to the clustering pyrenes at high temperatures and high concentrations. While only the electron hopping takes place at low temperatures, thus whether a charge recombination results in excimer formation or not is determined by the cationic trapping site, i.e. if several pyrenes are clustering there, excimer may be formed regardless of the excitation source. Although rapid translational motion is restricted in ethylene propylene rubber, the effective formation of excimer or dimer cation at high temperatures indicates that rotational diffusion and small distance translational diffusion are allowed within the fluorescence lifetime at those temperatures.     

Single-crystal structural study of the pressure-temperature-induced dimerization of C<Subscript>60</Subscript>

We present a study by Raman spectroscopy and X-ray diffraction/diffuse scattering of C₆₀ single-crystals treated at high-pressure and high-temperature. This allowed us to obtain structural information on the C₆₀ dimer state which can be considered as an intermediate state in the polymerization process. In the 1-6 GPa pressure range the crystals are primarily formed of dimers with additional minor fractions of monomers, 1D and 2D polymers, as shown by the analysis of the Raman spectra. The dimers are disordered within an average cubic lattice derived from that of the monomer. Single-crystal diffraction patterns reveal a characteristic diffuse scattering intensity distribution which has been simulated by calculating the diffuse scattering produced by dimer and trimer model structures. Satisfactory agreement is obtained for random positional and orientational disorder of the C₆₀-C₆₀ dimers although a small concentration of similarly disordered trimers is likely. In a first approximation the dimer/trimer disorder can be considered as random but various inter-dimer correlations are probably present, as discussed.Received: 3 October 2003, Published online: 19 February 2004PACS: 61.48. + c Fullerenes and fullerene-related materials - 61.43.Bn Structural modeling: serial-addition models, computer simulation - 78.30.Na Fullerenes and related materialsT. Wågberg: Present address: Groupe de Dynamique des Phases Condensées, Université Montpellier II, Place E. Bataillon, CC026, 34095 Montpellier, France     

金纳米环双体尺寸和耦合效应对表面等离子体共振特性的影响

采用离散偶极子近似方法系统地研究了金纳米环双体的消光光谱及其电场分布. 计算结果表明, 金纳米环双体在耦合作用下的共振消光峰对应着不同振动模式, 改变金纳米环双体的排列方式、 间距和尺寸大小, 其表面等离子体共振消光峰发生红移或蓝移. 因此可以通过对金纳米环双体结构参数和排列方式的设定, 调节其表面等离子体共振消光峰的位置. 电场分布表明, 水平排列的金纳米环双体较单个金纳米环产生更强的局部表面增强电场. 适当的小间距, 较大的内外半径的金纳米环水平阵列更适合做表面增强拉曼散射的衬底, 在生物分子检测等领域具有潜在的应用.     

IR absorption spectrum (4200–3100 cm) of H2O and (H2O)2 in CCl4. Estimates of the equilibrium constant and evidence that the atmospheric water absorption continuum is due to the water dimer

IR absorption spectra, 4200–3100 cm⁻¹, of water in CCl₄ solutions are presented. It is shown that for saturated solutions significant amounts of water are present as dimer (ca. 2%). The IR spectra of the monomer and dimer are retrieved. The integrated absorption coefficients of the monomer absorption are significantly enhanced relative to the gas phase values. The dimer spectrum consists of 5 bands, of which 4 were expected from data from cold beams and cold matrices. The origin of the “extra” band is discussed. In addition it is argued that the dimer absorption bands intensities must be enhanced relative to the gas phase values. Based on recent calculations of band strengths, and observed frequency shifts relative to the gas phase, the intensity enhancement factors are estimated as well as the monomer/dimer equilibrium constant in CCl₄ solution at T=296 K (K_c=1.29 mol⁻¹ L). It is noted that the observed dimer spectrum has a striking resemblance with the water vapour continuum determined by Burch in 1985 which was recently remeasured by Paynter et al. and it is concluded that the atmospheric water absorption continuum in the investigated spectral region must be due to water dimer. Based on the newly published spectral data a revised value of the gas phase equilibrium constant is suggested (K_p=0.035 atm⁻¹ at T=296 K) as well as a value for the standard enthalpy of formation, ΔH⁰=15.4 kJ mol⁻¹.     

Effect of separation on second-order hyperpolarizability of two silver nanoclusters

We present a study on dielectric response of Ag₁₄ nanocluster dimer using first principles methods. The interaction energy, (hyper)polarizability of dimer are predicted under various separations of the two identical Ag₁₄ nanoclusters. The appropriate separation makes the second-order hyperpolarizability of Ag₁₄ dimer is about 70 times larger than the Ag₁₄ monomer. The increase of polarizability and hyperpolarizability is ascribed to reconfiguration of molecular electronic state from Ag cluster monomer to dimer. Nonlinear response is more susceptible to the steady intermediate state compared to linear response. The crucial transitions contributed to hyperpolarizability are assigned to be from highest occupied molecular orbitals to the lowest unoccupied molecular orbital (HOMOs-LUMO) of nanocluster dimer. Binding character of LUMO plays an important role in determining nonlinear optical properties.     

Theoretical study of photoabsorption cross-section of water cluster anions: the size and isomer dependences

Photoabsorption cross-sections of water dimer and trimer anions are calculated with an ab initio molecular orbital method. Because the electron detachment energy of these small water cluster anions is less than 0.3 eV, all of the photoabsorption spectra in the near-infrared and visible region are due to the bound-free transitions. The theoretical spectra of two isomers of dimer anion and three isomers of trimer anion are compared, which suggest the possibility of the identification of the isomer structure by the photoabsorption spectra. The convergence on the basis sets is also examined by adding diffuse functions.     

Mössbauer effect in iron picolinate-dimer and monomer

A study of the quadrupole doublets of the dimer |Fe(pic)₂(OH)|₂ and of the monomer |Fe(pic)₂(OH₂)Cl|, which are asymmetric when observed at room temperature, has been made. For the dimer it has been proposed that this asymmetry was due mainly to effects of spin-spin relaxation between the two bridged iron atoms, antiferromagnetically coupled. Observation of the spectrum at a temperature of 4.2 K in the presence of an external magnetic field up to 18 KG, leads to identification of the sign of the electric field gradient and the relative direction of the fluctuating magnetic field. The observed small area asymmetry has been attributed the Karyagin-Goldanskii effect. For the monomer, the area asymmetry was attribtued to the Karyagin-Goldanskii effect, and the asymmetry of the line widths was explained in terms of a relaxation mechanism between the Kramers doublets.