Gas-Phase Analysis Techniques

Four of the most powerful methods presently applied to elucidate metal cluster geometric structure will be presented in the following. These are mass-selected negative ion photoelectron spectroscopy, infrared vibrational spectroscopy made possible by very recent advances in free electron laser (FEL) technology, gas-phase ion chromatography (ion mobility measurements), and rf-ion trap electron diffraction of stored mass-selected cluster ions. All methods include mass-selection techniques as discussed in the previous section and efficient ion detection schemes which are customary in current gas-phase ion chemistry and physics [71]. 

FELIX is also shown schematically in Fig. 1.19. This method of infrared resonance enhanced multiphoton ionization (IR-REMPI) has been successfully applied to study fullerenes, metal carbide, metal oxide, and metal nitride clusters [126,128-130] as well as metal-adsorbate complexes [131]. 

In a conventional mass spectrometer, an ion M+ is indistinguishable from an ion because both possess the same mass to charge ratio. How- 

The most direct approach to the geometric structures of molecules and also of clusters in the gas phase are diffraction methods, in particular the diffraction of an electron beam. Since an adequate cluster flux for such electron diffraction experiments has been so far only possible, if the full source output beam was sampled, the uncertainties in cluster size (no mass-selection) and internal energy prevent an unambiguous interpretation of electron diffraction patterns [138-145]. In a new development, a technique has been recently reported that relies upon an rf-Paul trap [146] to take advantage of the current 

As an example, an electron diffraction investigation of (CsI) Cs cluster structures (n = 30-39) are presented in Fig. 1.24. From the mass-resolved diffraction pattern contributions of both, rock salt (NaCl) and bulk cesium chloride lattice (CsCl) derived isomeric structures are observed at size n = 32. This particular size can form a closed shell rhombic dodecahedron corresponding to the Csl bulk structure. Interestingly, all other investigated sizes (n 32) are dominated by the rock salt structure [147]. In Fig. 1.24a the molecular diffraction data and the best fits for (CsI) Cs cluster sizes n = 31, 32, and 33 obtained at 300 K are displayed. The plot in Fig. 1.24b shows the diffraction 

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