Fragmentation of neutral van der Waals clusters with visible laser light: a new variant of the Raman effect?

We have observed strong photodissociation (using visible laser light) of neutral van der Waals clusters (Ar, N₂, O₂, CO₂, SO₂, NH₃) produced by supersonic expansion and detected by electron ionization/mass spectrometer. Several tests were performed, all of them supporting this surprising discovery. We suggest that Raman induced photodissociation (RIP) is responsible for this phenomenon. This first observation of Raman induced photodissociation provides a new technique for the study of neutral van der Waals clusters.     

Ion attachment to Van der Waals clusters

Mixed ionized clusters have been produced in a supersonic nozzle beam experiment by attachment of stagnant cations (i.e. NO⁺ and Xe⁺) to neutral van der Waals clusters (i.e.Ar _n ) within a Nier type ion source. This new ionization technique leads to less fragmentation than electron impact ionization and the measured cluster distributions exhibit icosahedral shell and subshell closures which have not been detected in the case of electron impact of Ar _n -clusters ionization so far. Additionally, the obtained appearance energies and metastable fractions give insight into the production mechanism and the stability of the resulting ions.     

Van der Waals radii of metals from spectroscopic data

The lack of information about the van der Waals radii of metals can be compensated for by using the results of spectroscopic investigations of van der Waals molecules. It has been shown that the interatomic distances in these molecules obey an additive scheme if one allows for the polarization effects. The van der Waals radii of the alkali metals, Ag, Mg, Zn, Cd, Hg, B, Al, In, and Si, have been determined from the interatomic distances in their heteroatomic molecules with atoms of noble gases. Use of the obtained radii for crystal chemistry is discussed.Translated fromIzyestiya Akademil Nauk. Seriya Khimicheskaya, No. 8, pp. 1374–1378, August, 1994.     

Photoionization mass spectrometry of molecular clusters using synchrotron radiation

The photoionization and dissociative ionization of molecular aggregates using synchrotron radiation is reported. The main objective of the review is to consider the intracluster relaxation processes after ionization. For hydrogen-bonded systems proton transfer is dominant. For small clusters (n<4) appearance potentials, ionization potentials, absolute proton affinities, proton solvation energies and intermolecular bond energies in the ionic clusters are deduced. For van der Waals aggregates proton transfer can also be used to characterize the intermolecular bond in the ionic cluster. Aggregates of CH₄, SiH₄, CH₃F show proton transfer in contrast to simple aromatic compounds, which reveal no proton transfer. From the fragmentation pattern and appearance potentials relaxation by intracluster ion molecule reactions is discussed. In heterogeneous clusters intracluster Penning ionization is observed. The shift of the charge transfer resonances depends on the π-electron density in the aromatic system. The width and spectral position of these resonances are influenced by the cluster size.     

Anisotropy of the van der Waals Configuration of Atoms in Complex,Condensed, and Gas-Phase Molecules

Anisotropic van der Waals (vdW) radii of 5b–7bsubgroup elements were determined from structures of gas-phase van der Waals complexes and crystal molecular compounds. The anisotropy of the van der Waals configuration of atoms was shown to decrease when going from isolated molecules to the condensed state. Variations in intermolecular distances, which are usually explained in terms of the formation of hydrogen bonds, are substantially governed by the anisotropic effect.     

Adsorption of ions onto polymer surfaces and its influence on zeta potential and adhesion phenomena

 The adhesion behavior that governs many technologically and biologically relevant polymer properties can be investigated by zeta potential measurements with varied electrolyte concentration or pH. In a previous work [1] it was found that the difference of the adsorption free energies of Cl^- and K⁺ ions correlates with the adhesion force caused by van der Waals interactions, and that the decrease of adhesion strength by adsorption layers can be elucidated by zeta potential measurements. In order to confirm these interrelations, zeta potential measurements were combined with atomic force microscopy (AFM) measurements. Force–distance curves between poly(ether ether ketone) and fluorpolymers, respectively, and the Si₃N₄ tip of the AFM device in different electrolyte solutions were measured and analysed. The adsorption free energy of anions calculated from the Stern model correlates with their ability to prevent the adhesion between the polymer surface and the Si₃N₄ tip of the AFM device. These results demonstrate the influence of adsorption phenomena on the adhesion behavior of solids. The results obtained by AFM confirm the thesis that the electrical double layer of solid polymers in electrolyte solutions is governed by ion adsorption probably due to van der Waals interactions and that therefore van der Waals forces can be detected by zeta potential measurements. Received: 18 November 1997 Accepted: 19 January 1998     

On the transition from localized to delocalized electronic states in divalent-metal clusters

The transition from van der Waals to covalent bonding, which is expected to occur in divalent-metal clusters with increasing cluster size, is discussed. We propose a model which takes into account, within the same electronic theory, the three main competing contributions, namely the kinetic energy of the electrons, the Coulomb interactions between electrons, and thes ?p intraatomic transitions responsible for van der Waals like bonding. The model is solved by taking into account electron correlations using a generalized Gutzwiller approximation (slave boson method). The occurrence of electron localization is studied as a function of the interaction parameters and cluster size.     

Rb and Cs oligomers in different spin configurations on helium nanodroplets

The study of small clusters is intended to fill the knowledge gap between single atoms and bulk material. He nanodroplets are an ideal matrix for preparing and investigating clusters in a superfluid environment. Alkali-metal atoms are only bound very weakly to their surface by van der Waals forces. Due to the formation process, high-spin states of alkali-metal clusters on He nanodroplets are favorably observed, which is in contrast to the abundance in other preparation processes. Until now, the prevailing opinion was that stable clusters of the heavy alkali-metal atoms, rubidium (Rb) and cesium (Cs) on He nanodroplets, are limited to 5 and 3 atoms, respectively (Schulz et al., Phys. Rev. Lett. 2004, 92, 13401). Here, we present stable complexes of Rb(n)? and Cs(n)? consisting of up to n = 30 atoms, with the detection of large alkali-metal clusters being strongly enhanced by one-photon ionization. Our results also suggest that we monitored both high-spin and low-spin state clusters created on nanodroplets. The van der Waals bound high-spin alkali-metal clusters should show strong magnetic behavior, while low-spin states are predicted to exhibit metallic characteristics. Alkali-metal clusters prepared in these two configurations appear to be ideal candidates for investigating nanosized particles with ferromagnetic or metallic properties.     

长程范德华力导向作用下胶体凝聚的计算机模拟

采用计算机模拟方法研究了长程范德华力在胶体凝聚过程中的作用, 发现由于胶粒间的范德华力是长程力, 它对胶粒或团簇运动将产生导向作用. 与不考虑导向作用的扩散控制团簇凝聚(DLCA)模型比较, 这种导向作用不仅加速了胶体的凝聚过程, 而且形成了更致密、分形维数更大的结构体. 研究还发现, 长程范德华力导向作用对胶粒的初始浓度非常敏感, 不论是在凝聚物的结构还是凝聚速率方面, 只有在胶粒初始浓度较低时, 该导向作用效应才明显. 其可能的原因是,在胶粒初始浓度较高时, 由于胶粒布朗运动的平均自由程很短而且位阻效应大, 从而使导向作用效应未能反映出来.     

Calculation of the electronic properties of neutral and ionized divalent-metal clusters

The electronic properties of neutral and ionized divalent-metal clusters have been studied using a microscopic theory, which takes into account the interplay between van der Waals (vdW) and covalent bonding in the neutral clusters, and the competition between hole delocalization and polarization energy in the ionized clusters. By calculating the ground-state energies of neutral and ionized Hg _n clusters, we determine the size dependence of the bond character and the ionization potentialI _p (n). For neutral Hg _n clusters we obtain a transition from van der Waals to covalent behaviour at the critical sizen _c ～10–20 atoms. Results forI _p (Hg _n ) withn≤20 are in good agreement with experiments, and suggest that small Hg _n ⁺ clusters can be viewed as consisting of a positive trimer core Hg ₃ ⁺ surrounded byn?3 polarized neutral atoms.     

Energy transfer of highly vibrationally excited azulene. II. Photodissociation of azulene-Kr van der Waals clusters at 248 and 266 nm

Photodissociation of azulene-Kr van der Waals clusters at 266 and 248 nm was studied using velocity map ion imaging techniques with the time-sliced modification. Scattered azulene molecules produced from the dissociation of clusters were detected by one-photon vacuum ultraviolet ionization. Energy transfer distribution functions were obtained from the measurement of recoil energy distributions. The distribution functions can be described approximately by multiexponential functions. Fragment angular distributions were found to be isotropic. The energy transfer properties show significantly different behavior from those of bimolecular collisions. No supercollisions were observed under the signal-to-noise ratios S/N=400 and 100 at 266 and 248 nm, respectively. Comparisons with the energy transfer of bimolecular collisions in thermal systems and the crossed-beam experiment within detection limit are made.     

Electronic properties of divalent-metal clusters

The transition from van der Waals to metallic bonding expected to occur in divalent-metal clusters (e.g., Be _n , Mg _n , Hg _n ) as a function of cluster size is discussed. Theoretical results for several electronic properties reflecting this transition in Hg _n -clusters are briefly reviewed and compared with available experiments. The limitations of the present theory particularly concerning the role of correlations and van der Waals interactions are discussed and possible improvements are suggested.     

Van der Waals surface graphs and molecular shape

A van der Waals surface graph is the graph defined on a van der Waals surface by the intersections of the atomic van der Waals spheres. A van der Waals shape graph has a vertex for each atom with a visible face on the van der Waals surface, and edges between vertices representing atoms with adjacent faces on the van der Waals surface. These are discrete invariants of three‐dimensional molecular shape. Some basic properties of van der Waals surface graphs are studied, including their relationship with the Voronoi diagram of the atom centres, and a class of molecular embeddings is identified for which the dual of the van der Waals surface graph coincides with the van der Waals shape graph. This revised version was published online in July 2006 with corrections to the Cover Date.     

A systematic approach toward comparing electrospray ionization efficiencies of derivatized and non‐derivatized amino acids and biogenic amines

Ionization efficiency (IE) in mass spectrometry (MS) has been studied for many different compounds, and different IE scales have been constructed in order to quantitatively characterize IE. In the case of MS, derivatization has been used to increase the sensitivity of the method and to lower the limits of detection. However, the influence of derivatization on IE across different compounds and different derivatization reagents has not been thoroughly researched, so that practitioners do not have information on the IE‐enhancing abilities of different derivatization reagents. Moreover, measuring IE via direct infusion of compounds cannot be considered fully adequate. Since derivatized compounds are in complex mixtures, a chromatographic method is needed to separate these compounds to minimize potential matrix effects. In this work, an IE measurement system with a chromatographic column was developed for mainly amino acids and some biogenic amines. IE measurements with liquid chromatography electrospray ionization mass spectrometry (LC/ESI/MS) were carried out, and IE scales were constructed with a calibration curve for compounds with and without derivatization reagent diethyl ethoxymethylenemalonate. Additionally, eluent composition effects on ionization were investigated. Results showed that derivatization increases IE for most of the compounds (by average 0.9 and up to 2‐2.5 logIE units) and derivatized compounds have more similar logIE values than without derivatization. Mobile phase composition effects on ionization efficiencies were negligible. It was also noted that the use of chromatographic separation instead of flow injection mode slightly increases IE. In this work, for the first time, IE enhancement of derivatization reagents was quantified under real LC/ESI/MS conditions and obtained logIE values of derivatized compounds were linked with the existing scale.     

Spectroscopy of DABCO-rare-gas and DABCO-DABCO van der Waals complexes

The excited electronic origin bands of several DABCO containing van der Waals complexes have been observed via (1+1) resonance enhanced multi-photon ionization. Sharp resonances with widths of about 3 cm^–1 are seen for DABCO-Rg _n=1,2,3 (Rg is Ar, Kr or Xe), for the DABCO-DABCO dimer and for DABCO-DABCO-Ar. The origins of the rare-gas complexes are blue shifted with respect to the monomer origin. Broad features originating from DABCO-Rg _n complexes with highn, appear to higher energies than the complex origins, with widths of 120 cm^–1.     

Mode-selective broadening in low-frequency vibrational modes of trans-stilbene van der waals complexes

Both RE2PI and LIF excitation spectra have been taken of several van der Waals complexes of trans-stilbene, p-methyl-, and m-methyl-trans-stilbene. The t-stilbene spectra reveal a strongly mode-dependent broadening of the van der Waals complex transitions associated with the low-frequency vibrations 82 and 95 cm⁻¹ above the origin. The fwhm of these peaks is three times as broad as any other feature in the spectrum. The source of the broadening appears to be fast vibrational relaxation from these modes to the van der Waals modes of the complex.     

Nanoscale interfibrillar adhesion in UHMWPE fibers

The goal of this research is to quantify the fibrillar adhesive energy in ultra‐high molecular weight polyethylene fibers, characteristic of nanoscale fibril interactions. Quantification of these energies is vital to the understanding of fibrillar deformation mechanisms that have been shown to play an important role in fiber performance. This is achieved through the development and implementation of a nanosplitting technique developed through the use of AFM‐enabled nanoindentation. This technique allows the quantification of nanoscale adhesive energies through careful monitoring of load and unload curves as well as examination of the residual split through high‐resolution AFM images. Results indicate that the average nanoscale fibril adhesive energy is over 3 times larger than the energy expected from van der Waals interactions alone. This indicates that a significant degree of physical interactions exist between fibrils, beyond van der Waals interactions, in the form of tie‐molecules, fibrillar network junctions, and bridging lamellar crystals. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 391–401     

Atomic bonding of neutral and cationic Mn clusters

By using the spin-polarized DV-X-LCAO method, electronic states of neutral and cationic Mn _N clusters (N=25) are calculated to study atomic bonding of Mn clusters. For the neutral Mn₂ cluster, the equilibrium interatomic distance is much larger than that of the bulk crystal. The 3d-derived states are nearly half-filled, and the 4s-derived states are almost fully occupied,i.e. the electronic configuration is close to that of the isolated atom. These indicate that the Mn₂ cluster is bound by the van der Waals force. The same situation is true for the larger neutral clusters while the equilibrium interatomic distance becomes smaller and thes-d mixing becomes larger. For the cationic clusters, the behaviors tend to become metallic. The equilibrium interatomic distances are smaller and thes-d mixings are larger than those of the corresponding neutral clusters. However, the Mn ₂ ⁺ and Mn ₄ ⁺ clusters still remain the van der Waals characters. Contrary to these clusters, the Mn ₅ ⁺ cluster, whose interatomic distance is smaller than that of the bulk crystal, shows strong metallic bonding. These results seem to correspond to the magic number observed on the mass spectroscopy of cationic Mn clusters.     

Dependence of the Interaction Potentials of Ar-HF and N2-HF on HF Bond Length

We explore in detail the nature of the intermolecular interactions in two HF containing complexes, ArHF and N₂ HF, at vibrationally excited HF stretching states using both high overtone spectroscopic and ab initio computational methods. By using an infrared intracavity laser-induced fluorescence technique, second overtone spectra of the two HF complexes have been obtained for the HF stretches and their combination modes with low-frequency van der Waals vibrations. The two complexes show the same trend that both van der Waals bond strength and rotational constant increase smoothly with v of the HF stretch. The investigation of the intermolecular potential above minimum provides a rigorous test of ab initio calculations. In particular for the ab initio calculations using an efficient basis set incorporating bond functions, the technique reproduces reasonably well the anisotropy of the interaction potential of Ar and HF. It is found that the intermolecular potential depends strongly upon the HF bond length only at the linear Ar-H-F geometry.     

Long-Range van der Waals Interactions in Density Functional Theory

The early difficulties in accounting for long-range van der Waals interactions in the framework of density functional theory (DFT) have been overcome to a certain extent in recent works by several groups, and those interactions can be computed numerically. In this paper a derivation of the analytical form of the attractive van der Waals interaction between two neutral atoms with polarizabilities α₁ and α₂ at large distance R, namely E _int=−C ₆ α₁ α₂/R ⁶ is performed within the context of DFT. Use is made of the properties of the Coulomb correlation hole, and it is shown that nonlocal Coulomb correlations are responsible for long-range dispersion interactions.