Jorg De Haeck

Vibrational spectroscopy of neutral silicon clusters via far-IR-VUV two color ionization

Tunable far-infrared-vacuum-ultraviolet two color ionization is used to obtain vibrational spectra of neutral silicon clusters in the gas phase. Upon excitation with tunable infrared light prior to irradiation with UV photons we observe strong enhancements in the mass spectrometric signal of specific cluster sizes. This allowed the recording of the infrared absorption spectra of Si(6), Si(7), and Si(10). Structural assignments were made by comparison with calculated linear absorption spectra from quantum chemical theory.

Gas-phase structures of neutral silicon clusters

Vibrational spectra of neutral silicon clusters Si(n), in the size range of n = 6-10 and for n = 15, have been measured in the gas phase by two fundamentally different IR spectroscopic methods. Silicon clusters composed of 8, 9, and 15 atoms have been studied by IR multiple photon dissociation spectroscopy of a cluster-xenon complex, while clusters containing 6, 7, 9, and 10 atoms have been studied by a tunable IR-UV two-color ionization scheme. Comparison of both methods is possible for the Si(9) cluster. By using density functional theory, an identification of the experimentally observed neutral cluster structures is possible, and the effect of charge on the structure of neutrals and cations, which have been previously studied via IR multiple photon dissociation, can be investigated. Whereas the structures of small clusters are based on bipyramidal motifs, a trigonal prism as central unit is found in larger clusters. Bond weakening due to the loss of an electron leads to a major structural change between neutral and cationic Si(8).

Carbon Monoxide Adsorption on Silver Doped Gold Clusters

Well controlled gas phase experiments of the size and dopant dependent reactivity of gold clusters can shed light on the surprising discovery that nanometer sized gold particles are catalytically active. Most studies that investigate the reactivity of gold clusters in the gas phase focused on charged, small sized clusters. Here, reactivity measurements in a low-pressure reaction cell were performed to investigate carbon monoxide adsorption on neutral bare and silver doped gold clusters (Au(n)Ag(m); n = 10-45; m = 0, 1, 2) at 140 K. The size dependence of the reaction probabilities reflects the role of the electronic shells for the carbon monoxide adsorption, with closed electronic shell systems being the most reactive. In addition, the clusters reaction probability is reduced upon substitution of gold atoms for silver. Inclusion of a single silver atom causes significant changes in the reactivity only for a few cluster sizes, whereas there is a more general reduction in the reactivity with two silver atoms in the cluster. The experimental observations are qualitatively explained on the basis of a Blyholder model, which includes dopant induced features such as electron transfer from silver to gold, reduced s-d hybrization, and changes in the cluster geometry.

Stability and dissociation pathways of doped AunX+ clusters (X = Y, Er, Nb)

Size dependent stabilities, fragmentation pathways and dissociation energies of a series of gas phase cationic doped gold clusters, Au(n)X+ (3 < or = n < or = 20; X = Y, Er and Nb), and pure Au(n)+ clusters were investigated in photofragmentation experiments. Size dependent stability patterns were obtained and the branching between monomer and dimer evaporation was studied. For bare gold, the competing neutral monomer and dimer evaporation channels were found to be in agreement with earlier studies. For doped clusters, monomer evaporation is the most likely fragmentation channel with the exception of Au18Y+ and Au20Y+ for which gold dimer evaporation is also observed. Relations between the evaporative activation energies and both the experimental abundances and the fragment yield were derived based on unimolecular rate constants. The dissociation energies from this analysis show an odd-even staggering and enhanced stabilities for certain cluster sizes, in agreement with simple electronic shell model predictions.

Experimental Detection and Theoretical Characterization of Germanium-doped Lithium Clusters LinGe (n = 1 – 7)

We report a combined experimental and quantum chemical study of the small neutral and cationic germanium-doped lithium clusters Li(n)Ge(0,+) (n = 1-7). The clusters were detected by time-of-flight mass spectrometry after laser vaporization and ionization. The molecular geometries and electronic structures of the clusters were investigated using quantum chemical calculations at the DFT/B3LYP and CCSD(T) levels with the aug-cc-pVnZ basis sets. While Li3Ge(0,+) and Li4Ge+ prefer planar structures, the clusters from Li4Ge to Li7Ge and the corresponding cations (except Li4Ge+) exhibit nonplanar forms. Clusters having from 4 to 6 valence electrons prefer high spin structures, and low spin ground states are derived for the others because valence electron configurations are formed by filling the electron shells 1s/1p/2s/2p based on Paulis and Hunds rules. Odd-even alternation is observed for both neutral and cationic clusters. Because of the closed electronic shells, the 8- and 10-electron systems are more stable than the others, and the 8-electron species (Li4Ge, Li5Ge+) are more favored than the 10-electron ones (Li6Ge, Li7Ge+). This behavior for Ge is different from C in their doped Li clusters, which can be attributed to the difference in atomic radii. The averaged binding energy plot for neutrals tends to increase slowly with the increasing number of Li atoms, while the same plot for cations shows a maximum at Li5Ge+, which is in good agreement with the mass spectrometry experiment. Atom-in-molecules (AIM) analysis suggests that Li atoms do not bond to one another but through Ge or pseudoatoms, and an essentially ionic character can be attributed to the cluster chemical bonds. An interesting finding is that the larger clusters have the smallest adiabatic ionization energies known so far (IEa approximately 3.5 eV).

Singly and doubly lithium doped silicon clusters: Geometrical and electronic structures and ionization energies

The geometric structures of neutral and cationic Si(n)Li(m)(0/+) clusters with n = 2-11 and m = 1, 2 are investigated using combined experimental and computational methods. The adiabatic ionization energy and vertical ionization energy (VIE) of Si(n)Li(m) clusters are determined using quantum chemical methods (B3LYP/6-311+G(d), G3B3, and CCSD(T)/aug-cc-pVxZ with x = D,T), whereas experimental values are derived from threshold photoionization experiments in the 4.68-6.24 eV range. Among the investigated cluster sizes, only Si(6)Li(2), Si(7)Li, Si(10)Li, and Si(11)Li have ionization thresholds below 6.24 eV and could be measured accurately. The ionization threshold and VIE obtained from the experimental photoionization efficiency curves agree well with the computed values. The growth mechanism of the lithium doped silicon clusters follows some simple rules: (1) neutral singly doped Si(n)Li clusters favor the Li atom addition on an edge or a face of the structure of the corresponding Si(n)(-) anion, while the cationic Si(n)Li(+) binds with one Si atom of the bare Si(n) cluster or adds on one of its edges, and (2) for doubly doped Si(n)Li(2)(0/+) clusters, the neutrals have the shape of the Si(n+1) counterparts with an additional Li atom added on an edge or a face of it, while the cations have both Li atoms added on edges or faces of the Si(n)(-) clusters.

Growth Mechanism and Chemical Bonding in Scandium-Doped Copper Clusters: Experimental and Theoretical Study in Concert

Size matters! The electronic structure and size-dependent stability of neutral and cationic scandium-doped copper clusters have been investigated by mass spectrometric studies (for the cations) and also quantum chemical computations. The proposed reaction paths ultimately lead to the most stable Frank-Kasper-shaped Cu(16)Sc(+) cluster (shown here), which could be the germ of a new crystallization process.Electronic structure and size-dependent stability of scandium-doped copper cluster cations, Cu(n)Sc(+), were investigated by using a dual-target dual-laser vaporization production scheme followed by mass spectrometric studies and also quantum chemical computations in the density functional theory framework. The neutral species also were studied by using computational methods. Enhanced abundances and dissociation energies were measured in the case of Cu(n)Sc(+) for n=4, 6, 8, 10 and 16, the last of these identified as being extraordinary stable. Neutral clusters are stable with n=5, 7, 9 and 15, which are isoelectronic with respect to the number of the valence s electrons with the stable cationic clusters; hence a simple electron count determines cluster properties to a great extent. The Cu(17)Sc cluster was found to be a superatomic molecule, containing Cu(16)Sc(+) and Cu(-) units; however, the charge separation is not as pronounced as in the case of CuLi. Cu(15)Sc was found to be a stable cluster with a large dissociation energy and a closed electronic structure; hence this can be regarded as a superatom, analogous to the noble gases. The main factors determining the growth patterns of these clusters are the central position of the scandium atom and the successive filling of the shell orbitals. For smaller clusters, the reaction paths appear to diverge yielding various products; however all paths ultimately lead to the most stable Frank-Kasper shaped Cu(16)Sc cluster, which in turn can be the germ of the crystallization process.