Molecular clusters spectroscopy

Molecular clusters are weakly bound aggregates of stable molecules. Such clusters can be produced easily using supersonic expansion, and have been extensively studied by both electronic and vibrational spectroscopy [146,... 

As described above, classical infrared spectroscopy using grating spectrometers and gas cells provided some valuable infonnation in the early days of cluster spectroscopy, but is of limited scope. However, tire advent of tunable infrared lasers in tire 1980s opened up tire field and made rotationally resolved infrared spectra accessible for a wide range of species. As for microwave spectroscopy, tunable infrared laser spectroscopy has been applied botli in gas cells and in molecular beams. In a gas cell, tire increased sensitivity of laser spectroscopy makes it possible to work at much lower pressures, so tliat strong monomer absorjDtions are less troublesome. 

Guidi, T. (2012) in Neutron Spectroscopy of Molecular Nanomagnets in Molecular Cluster Magnets (ed R.E.P. Winpenny), World Scientific. 

The elemental composition, oxidation state, and coordination environment of species on surfaces can be determined by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques. Both techniques have a penetration depth of 5-20 atomic layers. Especially XPS is commonly used in characterization of electrocatalysts. One common example is the identification and quantification of surface functional groups such as nitrogen species found on carbon-based catalysts.26-29 Secondary Ion Mass spectrometry (SIMS) and Ion Scattering Spectroscopy are alternatives which are more surface sensitive. They can provide information about the surface composition as well as the chemical bonding information from molecular clusters and have been used in characterization of cathode electrodes.30,31 They can also be used for depth profiling purposes. The quantification of the information, however, is rather difficult.32... 

T. Haber, U. Schmitt, and M. A. Suhm, FTIR spectroscopy of molecular clusters in pulsed supersonic slit jet expansions. Phys. Chem. Chem. Phys. 1, 5573 5582 (1999). 

M. Quack and M. A. Suhm, Spectroscopy and quantum dynamics of hydrogen fluoride clusters, in Molecular Clusters, Advances in Molecular Vibrations and Collision Dynamics, Vol. Ill, J. M. Bowman and Z. Bacic, eds., JAI Press, London, 1998, pp. 205 248. 

Fs DFWM spectroscopy was successfully applied for the structural study of molecular clusters in both, a gas cell at room temperature and in a supersonic expansion at reduced temperature. 

In spite of the widespread recognition of the theoretical inadequacies of classical nucleation theories, attempts to formulate more realistic theories have met with limited success, in part because nucleation rate measurements are notoriously difficult to make. Consequently, the available data base with which to evaluate various theories is inadequate. Molecular level approaches would seem to hold promise of providing more rigorously acceptable theories without resorting to the use of uncertain bulk properties in treating clusters that are intrinsically molecular. Furthermore, new experimental techniques, such as molecular beams and cluster spectroscopy, make the properties of small clusters amenable to investigation at the molecular level. 

Various experimental methods used to investigate the H-bonded clusters in gas phase are described in the earlier reviews [150-152]. Since molecular clusters are produced in supersonic beams in the gas phase under collision free conditions, they are free from perturbation of many-body interactions. The spectroscopic characterization of these clusters has less complexity. Hence, high level quantum chemical calculations on these clusters can be directly compared with the experimental values. Due to advent of laser-based techniques, it is currently possible to study the size and mass selective molecular clusters produced in supersonic beam. The combination of high resolution spectroscopy along with the mass and size selective strategies has enabled the scientific community to look at the intrinsic features of H-bonding. Principles behind the method of size selection, beam spectroscopy, and experimental setup have also been thoroughly described in an earlier thematic issue in chemical review [105, 150-152]. 

Dynamics. Cluster dynamics constitutes a rich held, which focused on nuclear dynamics on the time scale of nuclear motion—for example, dissociahon dynamics [181], transihon state spectroscopy [177, 181, 182], and vibrahonal energy redistribuhon [182]. Recent developments pertained to cluster electron dynamics [183], which involved electron-hole coherence of Wannier excitons and exciton wavepacket dynamics in semiconductor clusters and quantum dots [183], ultrafast electron-surface scattering in metallic clusters [184], and the dissipahon of plasmons into compression nuclear modes in metal clusters [185]. Another interesting facet of electron dynamics focused on nanoplasma formation and response in extremely highly ionized molecular clusters coupled to an... 

The leading nonadditive term in the many-body expansion of a potential is the three-body interaction. Similarly like dimers, trimers (and larger clusters) can be selectively studied by molecular beam spectroscopy. A number of such trimers have been the subjects of investigations. Among them are the Rg2-diatom trimers mentioned above, with the most extensive data available for Ar2-HF [64]. Both empirical [29,30] and ab initio [33] nonadditive potentials have been obtained for this system. A large number of spectral data are available also for the water trimer [65,66]. An accurate three-body potential for water has recently been developed [34]. 

Ions have greater masses than electrons thus their transfer of energy to surface species is much more efficient. Ions incident on surfaces can break chemical bonds and eject atoms, molecules, or molecular clusters. Most of these species are neutral, but the ion impact may also ionize a fraction of these ejected particles. The detection of these ions, called secondary-ion mass spectroscopy (SIMS), is an important technique of surface-composition analysis. Ion bombardment is used frequently to remove unwanted molecular or atomic layers of impurities in order to clean a surface efficiently. Ion sputtering is also used to deposit thin films of the bombarded material... 

L. E DiMauro and P. Agostini Infrared Spectroscopy of Size Selected Molecular Clusters, U. Buck Femtosecond Spectroscopy of Molecules and Clusters, T. Baumer and G. Gerber Calculation of Electron Scattering on Hydrogenic Targets, /. Bray and A. T. Stelbovics Relativistic Calculations of Transition Amplitudes in the Helium Isoelectronic Sequence, W R. Johnson, D.R. Plante, and J. Sapirstein... 

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