Electron energy loss spectroscopy of van-der-Waals clusters

A method to measure electron energy loss spectra (EELS) of clusters with a high resolution (30 meV) has been developed and has been applied to some van der Waals clusters (Ar _n , Kr _n ). Structures have been found which relate to the excitation of atoms on the surface and inside the cluster. An influence of the cluster size on the spectra has been observed.     

Magic numbers in the mass distribution of the benzene+-argonn ions: evaporation dynamics and cluster structure

The intensity distribution of benzene⁺-Ar_n cluster ions formed by laser ionization of neutral clusters has been investigated: two main intensity anomalies (magic numbers atn=20 and 45) have been observed in the 15–60 size range. The evaporation dynamics of these species in the 2–50 microsecond time window following ionization has been studied using the electrostatic mirror of a reflectron time-of-flight mass spectrometer as a kinetic energy analyser capable to distinguish parent and daughter ions. The magic numbers are interpreted in terms of size dependent evaporation behaviors: beyondn=20, a sudden decrease of the evaporation energy is observed; in then=45–47 size range, the magic number is accounted for by the specific dynamics of then=46 and 47 clusters, in particular the possible loss of two argon atoms forn=47 within the experimental time window. These results and their implications on the cluster structure are discussed in the light of the evaporative ensemble model and compared to the evaporation characteristics of similar species, in particular the neat rare gas clusters.     

IR double resonance experiments with size selected clusters for identification of isomers

Infrared photodissociation spectra of (CH₃OH) _n clusters (n=2, 3 and 6) and the mixed dimer C₂H₄ · CH₃COCH₃ are presented. The clusters are generated in a supersonic jet expansion and size selected by scattering from a helium atomic beam combined with mass spectrometric detection. Continuous CO₂-lasers are used to vibrationally excite the molecules in the cluster leading to rapid dissociation of the complex. Various dissociation peaks that are found in single-laser dissociation spectra can be assigned unambigously in a pump-probe experiment with two lasers to either different isomers (acetone-ethene dimer) or splitted lines of one isomer (methanol hexamer). For size distributions, the method is able to select contributions of single masses which is demonstrated for mixtures of methanol dimers and trimers.     

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.     

Photoelectron spectroscopy of jet-cooled aluminium cluster anions

Aluminium cluster anions (Al _n ^? ) are produced by laser vaporization without additional ionization and cooled by supersonic expansion. Photoelectrons from mass-identified anion bunches (n=2...25) are detached by laser light (hv=3.68 eV) and undergo energy analysis in a magnetic bottle-type time-of-flight spectrometer. The measurements provide information about the electronic excitation energies from ionic ground states to neutral states of the clusters. In contrast to bulk aluminium these cluster photoelectron spectra partially have well-resolved bands which originate from low-lying excited bands. For small clusters, especially the aluminium dimer and trimer, quantum-chemical calculations will be compared to the measurements. The electron affinity size dependence of larger clusters shows conclusive evidence for “shell” effects.     

Cluster cations ejected from liquid metal ion source: alkali metals (Li,Na) and group IV elements (Si,Ge, Sn,Pb)

Cluster abundance of Li _n ⁺ (n≤19), Na _n ⁺ (n≤25), Si _n ^z+ (n≤8 forz=1, 3≤n≤7 forz=2), Ge _n ^z+ (n≤11 forz=1, 3≤n≤9 forz=2,n=4 forz=3), Sn _n ^z+ (n≤7 forz=1, 3≤n≤9 forz=2,n=4 forz=3) and Pb _n ^z+ (n≤6 forz=1, 5≤n≤7 forz=2) ejected from a liquid metal ion source has been investigated by mass spectrometry. The abundance spectra of alkali metal clusters showed distinct maxima and steps atn=3, 7, 9, 13 and 19 for Li, and atn=3, 5, 11, 13 and 19 for Na. Mass spectra of Si, Ge and Sn clusters were very similar each other, showing intensity drops aftern=4 and 6 (and alson=10 for Ge) for singly charged clusters. The magic numbers observed are discussed in terms of stability of charged clusters.     

Stability and structure of singly-charged xenon-argon clusters [Xe1Arn−1]+,n=3–27

Monte-Carlo calculations have been performed for positively charged xenon-argon clusters in the temperature range between 10K and 40K for cluster sizes up ton=27. The argon-argon interaction potential stems from empirical data, the Xe⁺-Ar potential is determined by ab initio MRD-CI calculations and a semi-empirical treatment of spin-orbit effects. Special stability is found for cluster sizesn=10, 13, 19 and less pronounced forn=23 and 25 fairly independent of the temperature. The geometrical structure of the clusters are given and the construction principle is discussed in light of the interactions among neutral argon atoms and the xenon ion — argon interaction. Comparison with measured mass spectra for mixed rare-gas clusters and [Xe_n]⁺ clusters is made and shows a consistent picture for the building principle.     

The kinetics of reactions of nickel clusters with hydrogen and deuterium

The kinetics of reactions of nickel clusters with hydrogen and deuterium are studied in a laser-vaporization cluster source coupled to a continuous-flow reactor. The abslute rate constants for the addition of the first H₂ (D₂) molecule to nickel clusters Ni _n (n=7→36 for H₂ andn=7→60 for D₂) have been measured. Rate constants are found to be only weakly dependent onn forn≧14, showing a gradual increase with size that scales approximately withn ^(2/3), i.e., the cluster geometrical cross section. Reaction probabilities for clusters in this size range are approximately 0.6 for H₂ and 0.3 for D₂. Belown=14, there is a stronger dependence of reactivity on size, with Ni₉ being far less reactive than any other cluster studied. These results are compared to bulk nickel studies, and a discussion of possible correlation of reactivity to cluster structure is presented.     

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.     

Study on structures and electron affinities of small potassium–silicon clusters Si n K (n = 2–8) and their anions with Gaussian-3 theory

The neutral Si _n K (n = 2–8) clusters and their anions have been systematically studied by means of the higher level of Gaussian-3 schemes. Equilibrium geometries and electron affinities have been calculated and are discussed for each considered size. For neutral Si _n K clusters, the ground state structure is found to be “attaching structure”, in which the K atom is bound to Si _n clusters. The most stable isomer for their anions, however, is found to be “substitutional structures”, which is derived from Si_(n+1) by replacing the Si atom with a K. The dissociation energies of K atom from the lowest energy structures of Si _n K have also been estimated to examine relative stabilities.     

Electronic shells and shells of atoms in metallic clusters

Intensity anomalies (magic numbers) have been observed in the mass spectra of sodium clusters containing up to 22 000 atoms. For small clusters (Na _n ,n≤1500) the anomalies appear to be due to the filling of electronic shells (groups of subshells having the same energy). The shells can be characterized rather well by a pseudoquantum-number, indicating the possible existence of a symmetry higher than spherical. The mass spectra of larger clusters (1500≤n≤22 000) are well explained by the completion of icosahedral or cuboctahedral shells of atoms. The fact that the two types of shells (electron and atom) occur in distinct and non-overlapping size intervals might indicate the existence of a “liquid” to “solid” transition in going from small to large clusters.     

Cage Structure Formation of Singly Doped Aluminum Cluster Cations Al<Subscript><Emphasis Type="BoldItalic">n</Emphasis></Subscript><Emphasis Type="BoldItalic">TM</Emphasis><Superscript>+</Superscript> (<Emphasis Type="BoldItalic">TM</Emphasis> = Ti,V, Cr)

Structural information on free transition metal doped aluminum clusters, Al _n TM ⁺ (TM = Ti, V, Cr), was obtained by studying their ability for argon physisorption. Systematic size (n = 5 – 35) and temperature (T = 145 – 300 K) dependent investigations reveal that bare Al _n ⁺ clusters are inert toward argon, while Al _n TM ⁺ clusters attach one argon atom up to a critical cluster size. This size is interpreted as the geometrical transition from surface-located dopant atoms to endohedrally doped aluminum clusters with the transition metal atom residing in an aluminum cage. The critical size, n _crit , is found to be surprisingly large, namely n _crit = 16 and n _crit = 19 – 21 for TM = V, Cr, and TM = Ti, respectively. Experimental cluster–argon bond dissociation energies have been derived as function of cluster size from equilibrium mass spectra and are in the 0.1–0.3 eV range.     

Stability,Infrared Spectra and Electronic Structures of (ZrO2)n(n=3-6)Clusters:DFT Study

The stability, infrared spectra and electronic structures of (ZrO₂)_n (n=3–6) clusters have been investigated by using density‐functional theory (DFT) at B3LYP/6‐31G* level. The lowest‐energy structures have been recognized by considering a number of structural isomers for each cluster size. It is found that the lowest‐energy (ZrO₂)₅ cluster is the most stable among the (ZrO₂)_n (n=3–6) clusters. The vibration spectra of Zr? O stretching motion from terminal oxygen atom locate between 900 and 1000 cm^?1, and the vibrational band of Zr? O? Zr? O four member ring is obtained at 600–700 cm^?1, which are in good agreement with the experimental results. Mulliken populations and NBO charges of (ZrO₂)_n clusters indicate that the charge transfers occur between 4d orbital of Zr atoms and 2p orbital of O atoms. HOMO‐LUMO gaps illustrate that chemical stabilities of the lowest‐energy (ZrO₂)_n (n=3–6) clusters display an even‐odd alternating pattern with increasing cluster size.     

Evolution of (GaAl) n Clusters and Chemisorptions of H2 on (GaAl) n Clusters

The growth behavior of (GaAl) _n (n = 1–12) and the chemisorptions of hydrogen on the ground state geometries have been studied with the three-parameter hybrid generalized gradient approximation due to Becke-Lee–Yang–Parr (B3LYP). The dissociation energy, the second-order energy differences, and the HOMO–LUMO gaps indicate that the magic numbers of the calculated (GaAl) _n clusters are n = 4 and 6. To my knowledge, this is the first time that a systematic study of chemisorptions of hydrogen on gallium aluminum clusters. The onefold top site of aluminum atom is identified to be the most favorable chemisorptions site for one hydrogen chemisorptions on most (GaAl) _n clusters. In general, dissociative chemisorptions of a hydrogen molecule on a top site of aluminum atom is found common for all sizes clusters considered here except for (GaAl) _n (n = 1–3) clusters. The stability of the (GaAl) _n H _m complexes shows that both large second-order difference and large fragmentation energies for (GaAl)₁₀H₂ and (GaAl)₁₁H₂ make these species behaving like magic clusters.     

Theoretical Investigation on the Interaction of C2H Radical with Small Gold Clusters Au n 0/− (n = 1–4)

A density-functional theory investigation on the interactions between C₂H radical and small gold clusters Au _n ^0/? (n = 1–4) has been performed. The calculated results predict that C₂H radical inclines to interact with small gold clusters Au _n ^0/? (n = 1–4) as an integrity in the most stable structures of C₂HAu _n ^0/? (n = 1–4). The Au _n ^0/? (n = 1–4) clusters retain their structural integrity as units in the ground states of C₂HAu _n ^0/? (n = 1–4). The stretching vibrational frequencies of C≡C and C–H in the ground states of C₂HAu _n ^? (n = 1–4) are decreased compared with those of the C₂H radical due to the interaction between the Au _n ^0/? clusters and C₂H radical. Smaller red shifts in the C≡C and C–H stretching bands of C₂HAu _n ^? occur with an increase in n. The photoelectron spectra of the most stable structures of C₂HAu _n ^? (n = 1–4) have been simulated to aid their future experimental characterizations. The current study provides further insight into the interaction between C₂H radicals and gold clusters, which may lead to exploitation of the high activity of gold nanocrystals.     

Cluster ion formation in laser mass spectrometry of substituted pyridines

Emission of cluster ions occurs during laser irradiation of substituted pyridines even at threshold laser power densities. The clusters generated include dimers and trimers, and appear in both positive-ion and negative-ion laser mass spectra. Fragments of cluster ions are observed and can be rationlized as losses of neutral molecules from (nM ± H)^±. Dissociation of clusters occurs primarily from substituents on the pyridine ring. Laser mass spectrometry of pyridoxine hydrochloride and pyridoxamine-dihydrochloride resulted in the emission of clusters analogous to those observed for nicotinic acid. In contrast to these results, secondary-ion and field-desorption mass spectra of salts contain the ions C_nA_n?1⁺ and C_nA_n+1^?, that were not detected in the laser mass spectra.     

Theoretical investigation on the geometries and electronic properties of cesium–silicon CsSi<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 2–12) clusters

The structures, relative stabilities, and electronic properties of pure Si _n and Cs-doped silicon clusters (n = 2–12) are systematically investigated using the density functional theory at the B3LYP level. The optimized structures indicated that the lowest-energy structures of CsSi _n are similar to those of pure Si _n clusters and prefer the 3-dimensional configuration for n = 3–12. The relative stabilities of CsSi _n clusters are analyzed based on the averaged binding energy, fragmentation energy, second-order energy difference, and HOMO–LUMO energy gap. It is found that CsSi₆ and CsSi₉ are the magic clusters, and the doping of Cs atom reduces the chemical stabilities of Si _n frame. The Mulliken population analysis pointed out that the charges in the corresponding CsSi _n clusters always transfer from Cs atom to Si _n host in the range of 0.80–0.91 electron. In addition, the partial density of states, infrared, and Raman spectra is discussed.     

The structure of C2H4 clusters from theoretical interaction potentials and vibrational predissociation data

Optimized geometries and binding energies are calculated for ethene (ethylene) dimers, trimers, and tetramers based on a pairwise additive dimer potential. From these results intermolecular frequencies and relative abundancies (catchment areas) of the different isomers are obtained and compared with the results of accurate measurements of the photodissociation upon absorption of one photon of a CO₂ laser in the region of thev ₇ monomer absorption band at 949 cm^?1. The clusters are size selected in a scattering experiment and show for a cluster size fromn=2 ton=6 a frequency maximum shifted by 3 cm^?1 to the blue compared with the monomer. The result is explained by the predominance of chains and chain-like structures of the clusters in the photodissociation process. The chains consist of cross-like dimer sub-units.     

Observation of doubly charged mercury cluster ions Hg n 2+, n=1-10 using secondary ion mass spectrometry

Mass spectra of doubly charged mercury clusters (m/z=30-1065) were investigated by secondary ion mass spectrometry. Positively charged ions were generated from an amalgam of mercury and silver by bombardment with a xenon ion beam and mass analysis by a grand-scale sector type mass spectrometer. Hg _n ²⁺, n=1-10 and Hg _n +, n =1- 5 were observed. Some doubly charged mercury clusters, (Hg _n ²⁺) survived at least for 0.1 ms.     

Search for the structures,stabilities, IR spectra,and thermodynamic properties of the asymmetric clusters (HClBN<Subscript>3</Subscript>)<Subscript> <Emphasis Type="Italic">n</Emphasis> </Subscript> (<Emphasis Type="Italic">n</Emphasis> = 1–6)

The density functional theory (DFT) method has been employed to systematically investigate the geometrical structures, stabilities, IR spectrum and thermodynamic properties of small asymmetric clusters (HClBN₃)_n (n = 1–6). When n ≥ 2, the optimized results suggest that the (BN_α)_2n cyclic structures with alternating boron and α-nitrogen atoms are observed in clusters. The influences of cluster size on the structures of clusters were discussed. The second-order difference in energies show that the (HClBN₃)₃ isomer is the most stable among the asymmetric clusters (HClBN₃)_n. Four main characteristic regions are obtained and assigned for the calculated IR spectra. A study of their thermodynamic properties suggests that monomer 1 forms clusters (2–6) thermodynamically favorable by the enthalpies at 298.2 K.