Metal cluster source laser vaporization

Figure Cl. 1.1. Schematic of a typical laser vaporization supersonic metal cluster source using a pulsed laser and a pulsed helium carrier gas.

Figure 1. Schematic illustration of the laser-vaporization supersonic cluster source. Just before the peak of an intense He pulse from the nozzle (at left), a weakly focused laser pulse strikes from the rotating metal rod. The hot metal vapor sputtered from the surface is swept down the condensation channel in dense He, where cluster formation occurs through nucleation. The gas pulse expands into vacuum, with a skinned portion to serve as a collimated cluster bean. The deflection magnet is used to measure magnetic properties, while the final chaiber at right is for measurement of the cluster distribution by laser photoionization time-of-flight mass spectroscopy.

Previously, intense beams of metal clusters could only be produced for the most volatile metals. The limitation arose from significant materials problems involved in the construction of high temperature ovens. The development of a source that utilizes laser vaporization and subsequent condensation in a rapidly flowing gas eliminated the materials problem and has enabled just about any material to be studi ed(la,8). 

The field of gas-phase transition metal cluster chemistry has expanded rapidly due to the development of the laser vaporization source and the fast flow chemical reactor. The work from the three major laboratories have been reviewed. Many additional laboratories are developing cluster chemistry programs and will soon certainly make significant contributions. 

There are several preparative methods for the production of bare metal clusters including the fast flow reactor (PER), the fast flow tube reactor (FTR), the SIDT (24), the GIB (23), and a supersonic cluster beam source (SCBS) (198). Essentially, all of these methods are similar. The first process is to vaporize the metal sample producing atoms, clusters, and ions. Laser vaporization is generally favored although FAB or FIB may be used. The sample is located in a chamber or a tube and so vaporization generally takes place in a confined environment. An inert gas such as helium may be present in the vaporization source or may be pulsed in after the ionization process. 

Many of the metal cluster anions have been produced by cathode discharge in He doped with Ar and studied in a flow tube reactor. A few clusters have been prepared by laser vaporization using a supersonic cluster beam source and the reac-... 

Figure 9.61 ToF mass spectrum of metal-carbon cluster ions (TiC2+ and ZrnCm+ cluster ions) using a titanium-zirconium (50 50) mixed alloy rod produced in a laser vaporization source (Nd YAG, = 532 nmj and ionization by a XeCI excimer laser (308 ). ( . M. Davis, S. J. Peppernick and A. W Castleman, J. Chem. Phys., 124, 164304(2006). Reproduced by permission of American Institute of Physics.)...

However, it is rare when all the atoms in an aromatic species are metals. One such system was synthesized in 2001 by A. I. Boldyrev andL.-S. Wang and their colleagues. Using a laser vaporization supersonic cluster source and a Cu/Al... 

The ionization potentials (IPs) of ammonia clusters containing alkali metal atoms, such as Li [10], Na [8] and Cs [9], have been reported by Hertel s and Fuke s groups. These clusters have been prepared by pickup sources coupled with a heated oven (Na and Cs) or a laser-vaporization source (Li). The IP(n) values decrease almost linearly with (n-f 1) , which is approximately proportional to the inverse of the cluster radius. Although the IPs of free atoms are different (5.392, 5.139 and 3.894 eV for Li, Na and Cs, respectively), those of the clusters (n > 5) are almost the same irrespective to the metal atoms. The intercept at (n + 1) 0... 

The laser vaporization approach allows the use of even the most refractory target materials. The source configuration used in Fig. 1 involves a target rod that is rotated and translated in a continuous screw motion to expose fresh metal to the laser beam. This has been found necessary to provide acceptable pulse-to-pulse reproducibility. Target rods of refractory metals, semiconductors, carbon, polyethylene, alumina, and alloys have all been vaporized successfully to make clusters in many laboratories. For some materials a disk target is preferred due to the ease in sample preparation. Molecular solids, liquids, and solutions could also be used, though care must be taken to consider the additional complex plasma chemistry one is likely to encounter. 

In a seeded supersonic expansion source [35-37[, the material is heated in an oven and a mixture of gaseous material and a seed gas expands through a nozzle into the vacuum. This source produces a highly intense beam of small pure and mixed metal clusters, but it is limited to metals with a low boiling point (Li, Na, K, etc.). Laser vaporization sources [13,38[ are more widely used as they produce pure and mixed clusters of most elements and when operated at low frequency (1-10 Hz) they have been successfully used... 

Fig. 1.19. Scheme of the experimental setup for infrared multiphoton ionization or dissociation of clusters or of metal clusters-rare gas complexes. The charged and neutral clusters are directly emitted from the laser vaporization/supersonic expansion source. The beam passes a skimmer and is subsequently crossed by the tightly focused beam of the FELIX. At some time after the FELIX pulse is over, the time-of-flight mass spectrometer acceleration plates are pulsed to high voltage, and a mass spectrum is recorded in a standard reflectron setup. Also schematically depicted is the particular pulse structure of the FELIX light [126,127]... 

Fig. 1.29. Schematic sketch of the collision cell method for the study of metal cluster reactivity. The supersonic laser vaporization source is depicted on the right hand side. The clusters subsequently pass two collision cells in which reactions can take place. Finally, laser ionization mass spectrometry serves to detect the neutral reaction products [3]...

Platinum and palladium were among the first metals that were investigated in the molecular surface chemistry approach employing free mass-selected metal clusters [159]. The clusters were generated with a laser vaporization source and reacted in a pulsed fast flow reactor [18] or were prepared by a cold cathode discharge and reacted in the flowing afterglow reactor [404] under low-pressure multicollision reaction conditions. These early measurements include the detection of reaction products and the determination of reaction rates for CO adsorption and oxidation reactions. Later, anion photoelectron spectroscopic data of cluster carbonyls became available [405, 406] and vibrational spectroscopy of metal carbonyls in matrices was extensively performed [407]. Finally, only recently, the full catalytic cycles for the CO oxidation reaction with N2O and O2 on free clusters of Pt and Pd were discovered and analyzed [7,408]. 

Several techniques have been used to investigate the reactivity of the metal carbide cluster ions formed in a laser vaporization source. The earliest investigations performed by Castleman s group relied on a preliminary mass selection of the desired cluster. The ion beam was then injected into a drift tube where the selected cluster encounters the reactant mixed with helium as a buffer gas. The FTICR (Fourier-transform ion cyclotron resonance) mass spectrometer studies reported by Byun, Freiser and co-workers basically rely on the same principle even though the total pressure of the reaction chamber is 10 torr, compared with 0.7 torr in Castleman s experiments. A new method of forming met-car ligand complexes was then reported by Castleman et al. this involved the direct interaction of the vaporized metal with mixtures of methane and selected reactant gases. ... 

The aromaticity concept has been introduced to the chemistry literature through benzene and other cyclic molecules and their extension to inorganic molecules like borazine and silicazine. The seminal work of Boldyrev and coworkers has paved the way to have a recent upsurge of interest in the field of aromaticity and antiaromaticity of metal clusters (Fig. 13-17). Negative ion photoelectron spectroscopy using a laser vaporization source augmented by ab initio calculations have authenticated the aromatic nature... 

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