Seeded beam technique

If the gas is a mixture of several pure gases of different molecular weights, with one of the gases representing only a minor constituent of the mixture, then the minor constituent takes on the velocity of the major constituent. This is the seeded beam technique as first verified experimentally by Becker et al. (26, 27). Since the macroions constitute only about 10"8 mole % of the final gaseous mixture, intermediate energy beams of macroions can be produced with the macroions having narrow velocity distributions (2). 

The seeded-beam technique has a further advantage in that the final beam is enriched in the heavier constituent (25, 26). This is the so-called Mach focussing factor. ... 

Molecular beam techniques have been used mostly with small molecules, and most of the information about the dynamics of elementary photochemical processes are restricted to such species. It is however also possible to bring relatively large molecules (e.g. aromatics) into molecular beams through seeding in a carrier gas such as He. 

In a seeded beam a condensable material is mixed with a large excess of a rare gas (usually helium) which does not condense under the experimental expansion conditions. This technique is used with metals and other materials which have too low a vapor pressure to be expanded directly. Low boiling metals, such as the alkalis and alkaline earths, can be heated in a simple Knudsen cell. Stronger heating has been used for the more refractory transition metals. Most recently laser evaporation has been used to produce beams of many metals. This technique, which uses a pulsed, focused laser to evaporate metal from a rod, is well suited to the use of a time-of-flight mass spectrometer for cluster analysis. Analysis by laser-induced fluo-resence or laser ionization also allows many spectroscopic studies to be conducted in small clusters. In this way bond lengths and other parameters have been determined for dimers of Cr, Cu, and Sn to name just a few. 

SCBD is a technique consisting in the production of a supersonic beam of inert gas seeded by carbon clusters (covalent aggregates with masses ranging from tens to thousands of atoms) by means of an appropriate... 

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