Spectroscopy ionized cluster beam

Figure 1. Experimental set-up for performing transient two-photon ionization spectroscopy on metal clusters. The particles were produced in a seeded beam expansion, their flux detected with a Langmuir-Taylor detector (LTD). The pump and probe laser pulses excited and ionized the beam particles. The photoions were size selectively recorded in a quadrupole mass spectrometer (QMS) and detected with a secondary electron multiplier (SEM). The signals were then recorded as a function of delay between pump and probe pulse.

As long as one deals with small clusters, beam analysis is possible by combining spectroscopy with expansion modeling. It is possible to use, for example, the soft ionization methods to obtain a better idea of the relative concentration of different clusters in the beam. Soft ionization can be achieved either by direct photoionization or by applying the multiphoton ionization methods (see Cheshnovsky and Leutwyler as a recent example). This technique does not solve completely the fragmentation problem, since if the positively charged species formed is not stable, it will fall apart. However, combining it with sp>ectroscopy, satisfactory results can be obtained. 

Figure 24.1 Top schematic view of the pick-up technique a molecular beam apparatus to investigate the spectroscopy and dynamics of sodium-containing clusters is shown. The metallic cluster is produced by the pick-up technique under crossed-beam conditions. Adapted with permission from Polanyi et al, J. Phys. Chem. 99 13691. Copyright 1995 American Chemical Society. Bottom schematic view of a pick-up technique based on a beam-gas arrangement. After nozzle expansion, a skimmer extracts the beam that subsequently collides with the particles in the gas cell. The cluster beam is ionized by a pulsed laser and mass analysed in a TOE mass spectrometer. Reproduced from Nahler et al, J. Chem. Phys., 2003, 119 224, with permission of the American Institute of Physics...

While the use of direct absorption methods has grown, indirect action spectroscopic methods continue to be widely and successfully used in the study of neutral molecular clusters. As mentioned earlier, there are two commonly used detection methods, mass spectrometers and bolometers. Because of the variety of mass-spectroscopic methods, there is an equally wide range of techniques used in neutral cluster spectroscopy. One of the oldest among these involves electron-impact mass spectrometry of a cw neutral beam combined with vibrational predissociation spectroscopy using a tunable cw or pulsed laser. The advent of continuously tunable infrared sources (such as color center lasers and LiNbOa optical parametric oscillators) allowed for detailed studies of size and composition variation in neutral clusters. However, fragmentation of the clusters within the ionizer of the mass spectrometer, severely limited the identification of particular clusters with specific masses. Isotopic methods were able to mitigate some of the limitations, but only in a few cases. 

Cluster synthesis by supersonic expansion normally yields broad distribution of cluster sizes. The measurement of any size-dependent property depends on the availability of a mass-specific detection scheme [20]. Relatively nondestructive and selective methods for assaying neutral cluster size distributions have been provided by combinations of mass spectrometry with (a) laser techniques (resonant two-photon ionization (R2PI) [21-26], depletion spectroscopy [27]) and (b) helium cross-beam scattering techniques [28]. 

This is not only important to understand the chemistry of loosely bound complexes but also contributes to elucidating the molecular structure in the transition region between free molecules and solids. One example for this is the formation of clusters in supersonic alkali beams where molecules Na have been observed from x = 2 to x = 12 [10.16]. The spectroscopy of such multimers (clusters) yields dissociation energies, ionization energies, and vibrational structure as a function of the number x of atoms in the cluster. The comparison of these figures with the values in the solid allows the proof of theoretical models, which explain the transition from molecular orbitals to the band structure of solids. 

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