Pulsed cluster source

The final probe of molecular clusters is that of selected chemical reactions. The use of probe reactions to study supported cluster catalysts is well established, and we are attempting the development of similar probes of unsupported clusters. The first steps in this direction are the design of a pulsed chemical reactor to go with the pulsed cluster source and the development of criteria for reactions. It is important to recall that at present... 

The cluster reactor is attached to the pulsed cluster source s condensation channel, as shown in Figure 6. (16) To it is attached a high-pressure nozzle from which a helium/hydrocarbon mixture is pulsed into the reactor at a time selected with respect to the production and arrival of the clusters. The effect of turbulent mixing with the reactant pulse perturbs the beam, but clusters and reaction products which survive the travel from the source to the photoionization regime ( 600y sec) and the photoionization process are easily detected. 

Figure 6. Scale-drawn schematic of the cluster reactor in relation to the pulsed cluster source. The letters A-F indicate the various stages of cluster preparation or synthesis, cooling, mixing and reacting, and finally flowing into vacuum toward detection.

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

The size distribution of the clusters produced in the cluster source is quite smooth, containing no information about the clusters except their composition. To obtain information about, for example, the relative stability of clusters, it is often useful to heat the clusters. Hot clusters will evaporate atoms and molecules, preferably until a more stable cluster composition is reached that resists further evaporation. This causes an increase in abundance of the particularly stable species (i.e., enhancing the corresponding peak in the mass spectrum, then commonly termed fragmentation spectrum ). Using sufficiently high laser fluences (=50 /iJ/mm ), the clusters can be heated and ionized simultaneously with one laser pulse. 

The pulsed molecular beam cluster source has produced clusters of virtually every material—we have made clusters of even the most refractory transition metals, of group IIIB and IVB elements, and numerous oxides, carbides, and intermetallic alloys of these elements. 

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.

Nanostructured carbon thin films containing carbynoid species were grown by SCBD of carbon clusters produced by a pulsed microplasma cluster source (PMCS, schematically shown in Figure 2.1) [27]. 

The first chamber hosts the cluster source, evacuated by two turbo-molecular pumps (3001/s). They are horizontally mounted facing each other, in order to assure the correct operating conditions for the pulsed supersonic... 

Figure 21.1. Photoionization time-of-flight mass spectrum of neutral silicon clusters. The clusters have been ionized with an excimer laser hv = 7.89 eV). In the inset a schematic drawing of the pulsed laser vaporization cluster source is shown.

Photoelectron spectroscopy can also be carried out by measuring the distribution of flight times of photoemitted electrons. This method is useful in systems in which the absorbing species are produced by a pulsed source and the photolysis radiation is also pulsed. These electron time of flight methods have been used to elucidate structures of transient species such as free radicals and clusters produced in pulsed photolysis sources and in assessing the vibrational-state purity of ions produced in multiphoton ionization processes, particularly those in which the final photon absorption process is from a Rydberg state whose geometry is similar to that of the ion. 

Figure 2.36 Schematic arrangement of pulsed laser equipment used to produce carbon clusters. Source Reprinted from Curl RF, Smalley RE, Scientific American, 265 54-63, October 1991.

Fig. 6.1. PumpVcontrol scheme for Na2(H2 0)n. (a) Potential-energy surfaces of Na2 with double minimum state (2) 17 and shelf state (4) Eg. (b) A pickup cluster source produces Na2(H2 0)n clusters, which are excited by two pump pulses to the shelf state. After a certain delay time the control pulse transfers the wave packet to the outer minimum of the double minimum state. Here the reaction of Na2 with the water molecules takes place...

The combination of high resolution tunable IR lasers, slit jet cooled supersonic expansions, quantum shot noise limited absorbance detection and pulsed discharge sources has provided a general and remarkably powerful tool for spectroscopic study of cold clusters as well as highly reactive radicals and molecular ions. Yet for all that has been done, this seems just the beginning, particularly for the more recent studies of radicals and molecular ions. 

The size distribution of the cluster beam can be tuned by the delay of the He pulse, its gas background pressure and is dependent on the geometry and distance between nozzle and thermalization chamber [2, 8, 18]. For smaller cluster sizes the He pressure (in the cluster source chamber) needs to be higher than for bigger ones [19]. The kinetic energy distribution upon impact is measured by... 

W.R. Gentry Low-energy pulsed beam sources, in Atomic and Molecular Beam Methods /, ed. by G. Scoles (Oxford Univ. Press, New York 1988) p.54 S.B. Ryali, J.B. Fenn Clustering in free jets. Ber. Bunsenges. Phys. Chem. 88, 245 (1984)... 

Figure 3.9. Transient C02 formation rates on Pd30 (a) and Pd8 (b) mass-selected clusters deposited on a MgO(lOO) film at different reaction temperatures [74]. In these experiments CO was dosed from the gas background while NO was dosed via a pulsed nozzle molecular beam source. The turnover frequencies (TOFs) calculated from the experiments displayed in (a) and (b) are displayed in the last panel (c). C02 formation starts at lower temperatures but reaches lower maximum rates on the larger cluster. (Figure provided by Professor Heiz and reproduced with permission from Elsevier, Copyright 2005).

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