Free sodium-water clusters: photoionisation studies in a pulsed molecular beam source

Neutral Na·(H₂O) _n clusters are studied by near-uv one-photon ionisation and time-of-flight mass spectroscopy. The clusters are formed in a “pickup source” by injection of a beam of Na atoms into the expansion zone of a pulsed nozzle beam of water vapour seeded into an argon carrier gas. The performance of this novel technique for studying cold aggregates of potentially reactive species is discussed in detail. The photoion efficiency (PIE) spectrum of the monomer near its ionisation threshold (4.379(2) eV) shows a rich structure. Vibrational frequencies of the ion can be deduced and some indication of molecular Rydberg states is seen. Ionisation potentials for larger clusters and the binding energies of the neutral clusters up ton=5 are reported.     

Spectroscopy of free sodium-ammonia clusters

Neutral sodium ammonia clusters are formed in apickup source by injecting a beam of sodium atoms into the expansion zone of a pulsed nozzle beam of neat ammonia gas. The clusters are studied by one-photon ionisation in the range of 266 nm to 500 nm with pulsed lasers and Time-of-Flight mass spectroscopy. Na(NH₃) _n cluster ion signals up ton=35 are observed. The ionisation potentials of complexes up ton=9 are reported.     

Fragmentation dynamics of ammonia cluster ions after single photon ionisation

A reflecting time of flight mass spectrometer (RETOF) is used to study unimolecular and collision induced fragmentation of ammonia cluster ions. Synchrotron radiation from the BESSY electron storage ring is used in a range of photon energies from 9.08 up to 17.7 eV for single photon ionisation of neutral clusters in a supersonic beam. The threshold photoelectron photoion coincidence technique (TPEPICO) is used to define the energy initially deposited into the cluster ions. Metastable unimolecular decay (µs range) is studied using the RETOF's capacity for energy analysis. Under collision free conditions the by far most prominent metastable process is the evaporation of one neutral NH₃ monomer from protonated clusters (NH₃) _{n ? 2}NH ₄ ⁺ . Abundance of homogeneous vs. protonated cluster ions and of metastable fragments are reported as a function of photon energy and cluster size up ton=10.     

Exciton absorption and intramolecular Penning ionisation in small molecular clusters I: (N2O)n and C6H5CN·(N2O)n

Photoionisation efficiency (PIE)-spectra are measured for small (N₂O)_n-clusters,n=2...8, using synchrotron radiation from the electron storage ring BESSY at Berlin. Ionisation thresholds are reported. Resonances in the wavelength-range 97...100 nm forn>2 are attributed to exciton absorption with N₂O-Rydberg levels, series I and II parentage. This interpretation is supported by measurements of the PIE of mixed benzonitrile · (N₂O)_n clusters where benzonitrile serves as a monitor for photon absorption in N₂O at energies above the ionisation limit of benzonitrile.     

Photoabsorption of sulfur dioxide cluster anions

Photodissociation and photodetachment of negatively charged sulfur dioxide clusters (SO₂) _n ^? ,n=2–11, were investigated in the wavelength range from 458 to 660 nm. Electrons obtained from the interaction of photons with clusters were found to be produced in two photon processes forn≥3. Hence their detachment threshold energy is increased by at least 0.7 eV with respect to the dimer. Wavelength dependent depletion spectra indicate that the clusters are composed of a dimer anion chromophore solvated by neutral molecules. The spectral position of the absorption band is maintained and the shape evolves continuously with cluster size. However, a narrowing of the band with increasing cluster size is observed.     

The metastable formation of di-ethylchloronium ions from ethylchloride dimer ions in a seeded molecular beam

Cluster ions of ethylchloride and their dissociation products have been produced in a supersonic expansion of ethylchloride seeded in Ar and energy selected by the threshold photoelectron photoion coincidence (TPEPICO) method. The peak widths of the ion time of flight distribution indicate that all of the clusters are produced by dissociative photoionization of higher order clusters. Thus, trimer ions dissociate to form dimer ions and an ethylchloride monomer. This dimer ion was found to be metastable with respect to the formation of the di-ethylchloronium ion and a chlorine atom. The measured dissociation rate as a function of the dimer ion internal energy was compared to the calculated rates based on the statistical RRKM/QET theory. Good agreement was found when the dimer adiabatic IP was assumed to be 10.2 eV. The Cl loss from the ethylchloride dimer ion is associated with a reverse activation energy of about 0.32 eV.     

Enrichment and segregation in alkali heteroclusters

A spherical average pseudopotential method (SAPS) is used to investigate some properties of compound alkali clusters. The effect that the substitution of a Sodium atom by a Lithium atom in a Na _n cluster has on the stabilities and geometries is studied forn≤21. We have found that substitution is always energetically possible. On the other hand equiatomic Na _n Cs _n clusters are considered in the size rangen≤16. We find a strong segregation effect of the Cesium atoms towards the cluster surface. This agrees with what happens in liquid Na _x Cs_1?x alloys.     

Electron attachment to HCl clusters

Negatively charged cluster ions of hydrogen chloride are formed by electron attachment to HCl clusters, which are produced in a seeded supersonic beam traversing a sustained gas discharge. Cluster ions of (HCl) _n ^? , withn=2, and tentatively withn=3 and 4 are observed. Cluster ions like Cl _n ^? , Cl _n ^? (HCl) _m , and withAr attached to them are also seen. The relevance to radiation chemistry of HCl if briefly discussed. Atoms evaporating from the hot, thoriated tungsten filament of the glow discharge lead to clusters such as Th _n ^? and its oxides.     

On the photoionization and fragmentation of ammonia clusters using TPEPICO

Measurements of the appearance potentials of ammonia cluster ions (NH₃) _n ⁺ and (NH₃) _n ?2NH ₄ ⁺ using the threshold photoelectron photoion coincidence (TPEPICO) time-of-flight method are performed. The results agree well with other available data, however, solvation energies derived from these and from thermochemical data show significant discrepancies. The energetics for ammonia clusters are elucidated from various points of view.     

Structural and dynamical properties of magnesium microclusters

We present systematic Density Functional Theory-Local Density Approximation computations for neutral Magnesium clusters Mg _n withn≤13. For the smaller sizes the ground state structure is optimized starting from selected symmetries and allowing for relaxation, Jahn-Teller distorsion and spin polarization. For the larger sizes we perform a simulated annealing based on the ab-initio Molecular Dynamics. By the same method, we study the thermal and dynamical properties of Mg₁₀ and Mg₁₆. The general picture emerging from these computations shows that already atn ≈10 these clusters have acquired many characteristic features of metallic Magnesium.     

First-principle calculations ofK-edge X-ray absorption spectra for small iron and nickel clusters

In this study we present theoretical X-ray absorption near-edge spectra (XANES) evaluated for Fe₂, Ni₂, and Ni₃ using a modified multiple-scattered wave procedure. The calculations were performed for different internuclear distances of the dimers and by varying the geometry from linear to equilateral triangular for Ni₃. The calculated absorption spectra for both Fe₂ and Ni₂ predict a broad resonant structure at the threshold and weak 1s transitions to unoccupied valence orbitals ?5→0 eV giving a shoulder at the threshold. The resonance structure at the threshold of Ni₃ appeared to be sensitive to the geometry of the clusters.     

Alignment of Ar+ (2p −1 2 P 3/2) ions after electron impact ionisation in the range 1 keV to 268 eV

We have measured the alignmentA ₂₀ of Ar⁺(2p ^{?1 2} P _3/2) ions after electron impact ionization in the range of primary electron energyE ₀=1000...268 eV (range of excess energyE ₁=750...19.5 eV) via the anisotropic angular distribution ofL ₃?M _2,3 M _2,3(¹ S ₀) Auger electrons. On decomposing the Auger spectra into their components special care was taken by including the effect of the postcollision interaction on the shape of Auger lines. The present alignment values forE ₀≧350 eV agree well with previously existing experimental values of DuBois and Rodbro and with theoretical DWBA results of Berezkho and Kabachnik, but forE ₀<350 eV they deviate systematically from the DWBA values. For the lowest impact energyE ₀, which is only 19.5 eV above threshold, we obtainedA ₂₀=+0.09(16). This value clearly indicates that in the ionisation process near threshold the two low-energy electrons escape not only with a two-electron partial waveL=0, according to Wannier's original assumption, but also with partial wavesL>0.     

Photodissociation of sulfur dioxide cluster anions

Photodissociation of negatively charged sulfur dioxide clusters (SO₂) _n ^? , 2≦n≦11, was investigated in the wavelength range between 458 nm and 600 nm using a tandem mass spectrometer. The spectral position of the absorption band remains unchanged, however it exhibits narrowing with increasing cluster size. The smooth evolution of the spectra shows that the clusters are composed of a dimer anion core surrounded by neutral molecules. The analysis of the fragmentation products reveals that the absorption of a photon is followed by statistical evaporation of neutrals with a mean energy loss of 0.28±0.05 eV per evaporated monomer in the large cluster limit.     

Photoionization studies of free sodium ammonia clusters

Free sodium ammonia clusters Na(NH₃) _n up ton=45 were generated in a pickup source by injecting a beam of neutral sodium atoms into the expansion zone of a piezo driven pulsed nozzle. The clusters thus formed are studied by one-photon ionisation in the region of 266 nm to 520 nm, time-of-flight mass spectrometry as well as photoelectron spectrometry. Ionisation thresholds for clusters up ton=18 and dissociation energies for the neutral Na(NH₃) _n up ton=6 are reported.     

Molecular beam study of the system Kr + HCl

Angular distributions of HCl scattered off Kr have been measured at collision energiesE ranging from 66.3 to 208.7 meV. The curves exhibit a clearly resolved damped rainbow structure. In addition time-of-flight distributions of scattered HCl were measured indicating significant energy transfer. To a part of these data (E=99 meV) the parameters of the M5 potential energy surface of Hutson and Howard [3] were fitted using the infinite order sudden approximation (IOSA) and quasi classical trajectories (QCT). A comparison of QCT and IOSA results suggests applicability of the IOSA to the present system atE=99 meV. We find a well depth of ?31.8 meV (compared to Hutson and Howard's value of ?26.5 meV) at the collinear Kr ... HCl conformation and strong indication for the existence of a secondary potential minimum at the collinear Kr ... ClH conformation. The analysis of the angular distributions based on a one channel scattering calculation leads to an isotropic potential similar to the full potential energy surface cut along the 90°-conformation rather than to the spherical average or spherical limit of the surface.     

Computational Studies on the Mo-Doped Gold Nanoclusters Au<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>Mo(<Emphasis Type="Italic">n</Emphasis> = 1–10): Structures,Stabilities and Magnetic Properties

The geometric structures, relative stabilities, magnetic properties of Mo-doped gold clusters Au _n Mo(n = 1–10) have been investigated at the PBE1PBE/def2TZVP level of theory. The results show that molybdenum doping has a significant effect on the geometric structures and electronic properties of Au _n Mo(n = 1–10) clusters. For the lowest energy structures of Au _n Mo(n = 1–10), the two dimensional to three dimensional transition occurs at cluster size n ≥ 8, and their relative stabilities exhibit odd–even oscillation with the change of Au atom number. It is found that charge in corresponding Au _n Mo clusters transfers from Mo atom to Au _n host in the size range n = 1–7, whereas the charge in opposition direction in the size range n = 8–10. In addition, the magnetic properties of Au _n Mo clusters are enhanced after doping single Mo atom into the corresponding gold clusters. Our results are valuable for the design of magnetic material.     

Microscopic solvation: spectroscopic results vs. Monte Carlo simulations

Van-der-Waals clusters of carbazole (representing the ‘solute’) with up to 40 nitrogen or methane solvent molecules were characterized using two-color resonant two-photon ionization spectroscopy. Features in these spectra (redshift, homogeneous and heterogeneous broadening, etc.) are interpreted as being caused by various static and dynamic effects of the solvent shell surrounding the aromatic substrate. For a better understanding of such effects, Monte Carlo simulations of the clusters were performed: Statics: Using a Monte Carlo simulated annealing minimization procedure, minimum energy structures (local, probably global) were found for the various cluster species. Using a simple empirical additivity rule, spectral shifts are rationalized from these structures.Dynamics: Starting from these minimum configurations, canonical ensemble simulations were carried out in a temperature range from 0 to 35 K. Severalorder-disorder transitions were identified including:

1. orientational isomerization or ‘melting’
2. surface isomerization or decoupling
3. rigid → fluxional transitions
4. full cluster ‘melting’

We present some of our experimental results on the systems carbazole · (N₂) _n and carbazole · (CH₄) _n together with the corresponding simulation data.     

Compared electronic structures of negative ionsM p C n − : II. Transition metals in Hückel theory

SIMS experiments on transition metal carbides produce negative cluster ionsM _p C _n ^? (n < 10) where the transition metalM is Ti, Zr, V, Cr, W, Fe and Ni withp=1 and V withp=2. TheM _p C _n ^? ions show very pronounced alternations in their emission intensities I(M _p C _n ^? ) versusn with strong maxima for evenn whateverM. Since such phenomena are due to the stability properties of the clusters themselves (correspondence rule), it means that the negative ions are the most stable ones for evenn. It is thus possible to get the general outlines of their electronic structures from the Pitzer and Clementi model (sp hybridization in Hückel approximation): the clusters are assumed to be linear chains of “cumulene”-type :=C=..C=C=M and the alternations in the relative stabilities of these chains are due to the fact that the HOMO (highest occupied molecular orbital) of the clusters lies in a double degenerate π level band. Now HOMO may be either full (or almost full) or half-filled (or nearly half-filled), and an aggregate with a complete (or almost complete) HOMO is more stable than an aggregate with a half-filled HOMO. Consequently, the number of π electrons is governing the parity effect in the stability alternations. However, this number is depending on the number of σ electrons of the chain and on the existence of onedδ level (due to theM atom) which is either empty for deficientd electron elements (Ti, Zr, V, Cr, W) or filled for richd electron elements (half-filled for Fe or full up for Ni). As theMC _n ^? chain must have a full (or nearly full) HOMO ifn is even, it is then possible to infer a likely electronic configurations of these clusters, and hence to determine their relative stabilities, and to verify that “even” clusters are more stable than “odd” ones. Thus such Hückel model results can be used as a guide for more sophisticated calculations (ab initio, etc...).     

Synthesis,structures, sorption and magnetic properties of coordination polymers based on 3d metal pivalates and polydentate pyridine-type ligands

The reactions of 3d metal pivalates with pyridine-containing ligands of different structures afforded the 1D coordination polymers [Co₂(Piv)₄(dpe)₂] _n , [Ni(Piv)₂(dpe)(EtOH)₂] _n , [Cu₂(Piv)₄(dpe)] _n , [Cu(Piv)₂(dpe)] _n , [Ni(Piv)₂(4-ptz)(EtOH)₂] _n , and [Cu₂(Piv)₄(4-ptz)· ·mSolv] _n (Solv is EtOH, m = 2; Solv is C₆H₆, m = 1; Piv^? is pivalate, dpe is trans-1,2-bis(4-pyridyl)ethylene, 4-ptz is 2,4,6-tris(4-pyridyl)-1,3,5-triazine), as well as the 3D coordination polymer [{Cu₂(Piv)₄}₃(3-ptz)₂] _n (3-ptz is 2,4,6-tris(3-pyridyl)-1,3,5-triazine). The sorption and magnetic properties of a series of the synthesized compounds and magnetic properties of the earlier characterized coordination polymer [Mn₂(O₂CC₆H₅)₄(dpe)₂·dpe] _n were studied. It was shown that the desolvation of the complexes [Ni(Piv)₂(4-ptz)(EtOH)₂] _n and [Cu₂(Piv)₄-(4-ptz)·2EtOH] _n resulted in the formation of the crystal structures, in which the pores are accessible to nitrogen and hydrogen at 78 K (S _BET are up to 92 m² g^?1). The temperature dependences of the molar magnetic susceptibility for [Co₂(Piv)₄(dpe)₂] _n , [Mn₂(O₂CC₆H₅)₄-(dpe)₂·dpe] _n , [Ni(Piv)₂(dpe)(EtOH)₂] _n , [Ni(Piv)₂(4-ptz)(EtOH)₂] _n , and [Cu₂(Piv)₄-(4-ptz)·2EtOH] _n are described in terms of models taking into account the zero-field splitting and exchange interactions or isotropic exchange Hamiltonians.     

A cathode for solid polymer electrolyte fuel cells: Designing the optimal structure of the active layer

The complete computer simulation of the cathodic active layer with solid polymer electrolyte (Nafion) is carried out. The active layer structure can be described by 8 parameters. In designing the optimal structure, it is shown that to provide the high overall characteristics of the cathode and save the catalyst, 0.5 of the active layer volume should be set aside for the support grains (agglomerates of carbon particles covered with platinum and containing Nafion incorporations and microvoids). Protons and oxygen molecules must be supplied to the active layer by means of peculiar combined percolation clusters. The latter consist of a combination of support grains with either Nafion grains (to produce “protonic” clusters) or grains-voids (to afford “gas” clusters). The volume fractions of Nafion grains and grain-voids are assumed to be 0.25 and 0.25. The computer simulation of the support grain structure is also carried out. Their composition, i.e., the volume fractions of the carbon component (g _e), Nafion (g _ii), and microvoids (g _gg), is varied. The support grains play the key role in the active layer functioning. It is impossible to organize three full-value percolation clusters (electronic, protonic, and gas); hence, one has to have one or two combined clusters in the active layer. Thus the double load fells on the support grains. Their optimal structure should not only sustain the transport of protons and electrons in the active layer but also create the best conditions for the electrochemical process in each grain. The maximum current I _max (realized upon reaching the optimal active layer thicknesses Δ*) is calculated. The dependences of I _max and Δ* on the main parameters characterizing the support grains (g _e and g _ii) are analyzed. Here, two goals are sought: (1) to obtain the high currents, (2) to provide the low consumption of platinum per power unit. To solve the first problem, one has to work with high values of g _e. The second problem requires the opposite: the values of g _e must be minimal possible.