Neutral Cluster Beams

FIGURE 3.5 Gas streamlines (top) and trajectories (bottom) simulations of 20-nm-diameter particles in a gas expansion from left to right in aerodynamic lenses. Reprinted with permission from Ref. [91]. American Institute of Physics. 

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Mass spectrometry involves the detection of charged particles, and, in the present case, a portion of the neutral cluster beam is ionized. Ionization essentially involves electronic excitation and occurs on the time scale of the order of 10 16 s (Haberland 1985 Mark 1987). The mass spectrometric detection of the ions is usually achieved on a microsecond time scale after the ionization event. As a result, the ionization process is taken to be time zero in the discussion of the processes which occur following the actual ionization of the neutral clusters, yet before the mass selection of the cluster ions. That is, the resulting cluster ion will incubate in the ionizer for microseconds before being accelerated into the mass filter. On that time scale, the cluster ion may lose monomer units, and the cation within the cluster may fragment or react chemically with the adjacent molecules. 

Neutral clusters from supersonic beam 157 Spectroscopy of neutral clusters 158 Sources of ionic clusters 167 Mass spectrometry of ionic clusters 170... 

In subsection 7.2.1, it was discussed that the neutral clusters generated from the beam source have a wide range of cluster sizes. Although there is no single, easily available technique which allows the size specific detection of neutral clusters, it is now well accepted that the cluster size distribution can be manipulated, to a certain extent, by careful control of the various experimental parameters. Mass spectrometric detection of clusters in general and tandem mass spectrometry in particular offer the inherent advantage of enabling size specific studies of the cluster ions. 

With free jet expansion techniques, we have produced clusters of aqueous nitric acid (3 ), hydrochloric acid, sulfuric acid (4, pure acetic acid ( 5), and sulfur dioxide (6). For analogy to buffering, the formation of clusters containing ammonia have also been examined. These have included ammonia with aqueous nitric acid (7 ), hydrogen sulfide (7J), and sulfur dioxide (8). The basic experiment involves expansion of vapor through a nozzle, collima-tion of the jet with a skimmer to form a well-directed molecular beam, and detection of clusters via electron impact ionization and quadrupole mass spectrometry. Some variations include the introduction of a reactive gas into vacuum near the expansion as described elsewhere (4, 8) and the implementation of an electrostatic quadrupolar field to examine the polarity of the neutral clusters. The electric deflection technique is described by Klemperer and coworkers (9). 

What is the relation between the magic numbers observed in different distributions For example, how are the magic numbers observed in the Boltzmann mass distribution related to the magic numbers observed in a cluster beam study In attempting to answer this question we ignore possible fragmentation processes that may occur when neutral clusters are ionized. 

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]... 

For the more volatile (alkali) metals it has been possible since the late 1970s to create a beam of cold, naked clusters of 1 < n < 100, where n is the number of atoms per cluster 348, 349). Metal vapor is produced in an oven and then cooled by expansion. The clusters are separated in a time-of-flight mass spectrometer (TOFMS). Later it became possible to handle less volatile (transition) metals through vaporization by a laser beam 347, 350-352). For the mass analysis the neutral clusters are photoionized by a suitable UV laser. 

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