Cluster ionization threshold, dependence

The adiabatic and vertical ionization potentials for Lis are very similar, both being approximately 3.95 eV. This results because of the similar geometries for the 82 state of LisCCsv) and the Ai state of Li3" (D3h). It must be noted, however, that the vertical ionization process for the removal of an electron from linear Lis to give linear Lis leads to the higher ionization potential, 4.39 eV. If both Csv and forms are present in an experiment, a complicated threshold dependence for the ionization process will be observed (j4). The results for Lis exemplify that the fluxional nature of a small metal cluster may complicate the experimental determination of electron affinities and ionization potentials. 

In the context of this model we thus expect that the addition of ammonia to an iron cluster will lower its ionization threshold. The magnitude of the IP decrease will be dependent on the number of ammonia molecules chemisorbed and on the size of the cluster. We except that if the ionization thresholds and reactivities toward hydrogen were measured for Fe (NH3) or Fe H2 (n variable, m constant), an IP-ln (rate constant) anticorrelation would be found. Expieriments to date have shown that, upon ammoniation, the minimum in reactivity of iron clusters toward hydrogen shifts to smaller cluster sizes and that the rate constant for hydrogen chemisorption for these ammoniated clusters is about a factor of 10 lower than that for the bare clusters. However, the number of chemisorbed ammonias is different for each cluster. Experiments involving metal clusters with a fixed number of chemisorbed ammonias is a needed probe of the detailed interaction between NH3 and a cluster. 

Superimposed on the smooth behavior described by Eq. (I), the experimental data for the ionization potentials of small clusters of. s-cicetron metals show an even odd alternation N-even clusters systematically have slightly larger (0.1-0.2 cV) ionization potentials than their N-odd neighbours [25]. An inverse effect is observed for the electron affinity, with N-odd cluster anions having higher photodetachment thresholds [36]. The odd-even alternation also appears in the mass abundance spectra [25]. This odd-even effect is observed up to some limiting cluster size, which depends on the particular element. 

Hortal et al. [52,53] and Martinez-Haya et al. [54] showed that the LD-MS spectrum of an asphaltene depended on the concentration of species in the sample and on the laser fluence apphed.The thickness of sample applied to the target plate was a factor in the formation of aggregates or clusters of molecules in the reactive gas plume formed by the laser shot thin layers were less likely to form cluster ions than thick layers. High laser fluences were more hkely to lead to cluster ion formation than fluences slightly above ionization threshold. However, these works did not examine fractions of asphaltenes where low- and high-mass molecules were separated from each other. 

Modem supersonic molecular beam techniques provide a direct way to study the stability and valence electron structure of noninteracting, isolated atomic clusters. One of the most widely studied properties is the threshold for photo-ionization and its dependence on cluster size. In Fig. 4.20 we reproduce some experimental results for argon-, krypton-, xenon- (Gantefor et al., 1989), and mercury-clusters (Rademann et al.. 

Asymmetry of Above-Threshold Ionization of Metal Clusters in Two-Color Laser Fields A Time-Dependent Density-Functional Study. 

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