Marko Haertelt

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.

Structure determination of neutral MgO clusters—hexagonal nanotubes and cages

Structural information for neutral magnesium oxide clusters has been obtained by a comparison of their experimental vibrational spectra with predictions from theory. (MgO)(n) clusters with n = 3-16 have been studied in the gas phase with a tunable IR-UV two-color ionization scheme and size-selective infrared spectra have been measured. These IR spectra are compared to the calculated spectra of the global minimum structures predicted by a hybrid ab initio genetic algorithm. The comparison shows clear evidence that clusters of the composition (MgO)(3k) (k = 1-5) form hexagonal tubes, which confirm previous theoretical predictions. For the intermediate sizes (n≠ 3k) cage-like structures containing hexagonal (MgO)(3) rings are identified. Except for the cubic (MgO)(4) no evidence for bulk like structures is found.

Structural diversity and flexibility of mgo gas-phase clusters

Magnesium oxide (MgO) is a prototype material of (simple) metal oxides. The NaCl-type structure of bulk MgO is the only phase observed in experiments up to a pressure of 227 GPa. 2] This indicates that MgO has an inherent structural stability, which can be expected to persist when passing from the bulk solid to molecular clusters. Indeed, mass spectra of (MgO)n + and (MgO)nMg + cluster ions along with calculations using rigid ion and polarizable ion shell model potentials indicate compact cubic structures similar to fragments of the MgO crystal lattice, with the most abundant clusters based on a (MgO)3 subunit. [4] The spectra and cluster compositions observed in IR resonance-enhanced multiphoton ionization experiments on large neutral (MgO)n clusters (n 15) have also given indications for cubic structures. Up to now, computational studies have almost exclusively investigated neutral MgO clusters, without direct comparison to experiment, 6–8, 10–17] despite the fact that most experiments were performed on cationic clusters. The main conclusion from these studies has been that the most stable structures for a given value of n are cubelike, except for (MgO)3n clusters, for which rings and stacks of rings are preferred. The geometric structures of the cationic MgO clusters have been assumed to be the same as for neutral ones (vertical ionization approximation), except for small hypermagnesium ions. So far, no systematic theoretical studies of the stoichiometric cationic clusters have been reported. Herein we demonstrate that, in contrast to the bulk material, neutral and cationic gas-phase clusters of MgO display unusual structural diversity and flexibility. Not only are the structures of the clusters in most cases noncubic, but the neutral and charged ones also differ. The atomic structures of cationic stoichiometric (MgO)n + (n = 2–7) clusters were determined by combining quantum chemical calculations with infrared multiple photon dissociation (IRMPD) experiments. In particular, global structure optimizations using density functional theory (DFT) have been performed on all the cluster sizes. Although several of the geometric structures reported here (but not all of them) have been found before with neutral 7, 10–17] and anionic clusters by different computational techniques, our calculations reveal unequivocally the global minima of all these configurations. Cationic clusters and their weakly bound complexes with Ar and O2 have been investigated experimentally in a molecular beam. Changes in this cluster distribution induced by the interaction with tunable infrared radiation were used to obtain the cluster-size-specific IR-MPD spectra. Figure 1 shows the global minimum structures of neutral (MgO)n and cationic (MgO)n + clusters with n = 2–7; for other low-energy isomers see Figures 1S and 2S in the Supporting Information. Figures 2 and 3 show a comparison between the experimental IR-MPD spectra and the calculated linear IR

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).

Probing the structures of gas-phase rhodium cluster cations by far-infrared spectroscopy

The geometric structures of small cationic rhodium clusters Rh(n)(+) (n = 6-12) are investigated by comparison of experimental far-infrared multiple photon dissociation spectra with spectra calculated using density functional theory. The clusters are found to favor structures based on octahedral and tetrahedral motifs for most of the sizes considered, in contrast to previous theoretical predictions that rhodium clusters should favor cubic motifs. Our findings highlight the need for further development of theoretical and computational methods to treat these high-spin transition metal clusters.

Infrared induced reactivity on the surface of isolated size-selected clusters: dissociation of N2O on rhodium clusters.

Multiple photon infrared excitation of size-selected Rh(6)N(2)O(+) clusters drives surface chemistry resulting in partially oxidized clusters.

Infrared-Induced Reactivity of N2O on Small Gas-Phase Rhodium Clusters

Far- and mid-infrared multiple photon dissociation spectroscopy has been employed to study both the structure and surface reactivity of isolated cationic rhodium clusters with surface-adsorbed nitrous oxide, Rh(n)N(2)O(+) (n = 4-8). Comparison of experimental spectra recorded using the argon atom tagging method with those calculated using density functional theory (DFT) reveals that the nitrous oxide is molecularly bound on the rhodium cluster via the terminal N-atom. Binding is thought to occur exclusively on atop sites with the rhodium clusters adopting close-packed structures. In related, but conceptually different experiments, infrared pumping of the vibrational modes corresponding with the normal modes of the adsorbed N(2)O has been observed to result in the decomposition of the N(2)O moiety and the production of oxide clusters. This cluster surface chemistry is observed for all cluster sizes studied except for n = 5. Plausible N(2)O decomposition mechanisms are given based on DFT calculations using exchange-correlation functionals. Similar experiments pumping the Rh-O stretch in Rh(n)ON(2)O(+) complexes, on which the same chemistry is observed, confirm the thermal nature of this reaction.

Communications: The structure of Rh(8) (+) in the gas phase.

The geometric structure of the Rh(8) (+) cation is investigated using a combination of far-infrared multiple photon dissociation spectroscopy and density functional theory (DFT) calculations. The energetic ordering of the different structural motifs is found to depend sensitively on the choice of pure or hybrid exchange functionals. Comparison of experimental and calculated spectra suggests the cluster to have a close-packed, bicapped octahedral structure, in contrast to recent predictions of a cubic structure for the neutral cluster. Our findings demonstrate the importance of including some exact exchange contributions in the DFT calculations, via hybrid functionals, when applied to rhodium clusters, and cast doubt on the application of pure functionals for late transition metal clusters in general.

Infrared Spectroscopy and Binding Geometries of Oxygen Atoms Bound to Cationic Tantalum Clusters

The binding of isolated oxygen atoms to small tantalum clusters is investigated using vibrational spectroscopy. Infrared spectra of Ta(n)O(0,1,2)(+) (n = 6-11) are reported in the range of 90-1100 cm(-1), comprising both the internal cluster modes and the adatoms vibrations. The vibrational spectra are compared to the results of DFT calculations for n = 6-8, which show that the oxygen atoms bind preferentially as 2-fold bridging adatoms. Addition of one or two O atoms induces only minor distortions of the underlying metal cluster core.

The structures of neutral transition metal doped silicon clusters, SinX (n = 6−9; X = V, Mn)

We present a combined experimental and theoretical investigation of small neutral vanadium and manganese doped silicon clusters Si(n)X (n = 6-9, X = V, Mn). These species are studied by infrared multiple photon dissociation and mass spectrometry. Structural identification is achieved by comparison of the experimental data with computed infrared spectra of low-lying isomers using density functional theory at the B3P86∕6-311+G(d) level. The assigned structures of the neutral vanadium and manganese doped silicon clusters are compared with their cationic counterparts. In general, the neutral and cationic Si(n)V(0,+) and Si(n)Mn(0,+) clusters have similar structures, although the position of the capping atoms depends for certain sizes on the charge state. The influence of the charge state on the electronic properties of the clusters is also investigated by analysis of the density of states, the shapes of the molecular orbitals, and NBO charge analysis of the dopant atom.