Multiphoton ionization, metal clusters

Herrmann A, Leutwyler S, Schumacher E and Woste L 1978 On metal-atom clusters IV. Photoionization thresholds and multiphoton ionization spectra of alkali-metal molecules Hel. Chim. Acta 61 453... 

We have also carried out preliminary experiments in which we have detected the laser desorption of ethylene, cyanogen, methanol, and benzene from the Pt(s)[7(111) x (100)] surface. These spectra are shown in Figure 9. In the experiments involving ethylene, cyanogen, and methanol only neutral species are desorbed. In the case of benzene we observe the molecular parent ion in the absence of the electron beam. We believe that this is due to resonance multiphoton ionization of the benzene by the laser after desorption (resonance multiphoton ionization of benzene is very efficient with 249 nm radiation). These spectra are in marked contrast to the results of SIMS experiments which produce a wide variety of complex metal-adsorbate cluster ions. In the case of ethylene, our experiments were performed at 140 K, and under these conditions ethylene is known to be a molecular x-bonded species on the surface. In SIMS under these conditions the predominant species is CH (15)t but in the laser desorption FTMS experiments neutral ethylene is the principal species detected at low laser power. 

Other studies of CO loss from small molecules include the photolysis of OCS on Ag clusters, in which an odd-even dependence on the number of Ag atoms in the cluster was discovered. OCS desorbs non-dissociatively from even numbered clusters, but dissociates to CO and AgnS on odd numbered clusters. This alternation correlates with the ionization potentials of the naked metal clusters, and the photoreactivity pattern can be explained in terms of a charge-transfer mechanism. A numerically efficient algorithm has been developed in the theory of ladder-climbing and IR multiphoton dissociation, and has been applied to HCO. ... 

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]... 

FELIX is also shown schematically in Fig. 1.19. This method of infrared resonance enhanced multiphoton ionization (IR-REMPI) has been successfully applied to study fullerenes, metal carbide, metal oxide, and metal nitride clusters [126,128-130] as well as metal-adsorbate complexes [131]. 

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