Potentials photoionization, clusters

Yang S and Knickelbein M B 1990 Photoionization studies of transition metal clusters ionization potentials for Fe... 

Knickelhein MB, Yang S, Riley SJ. 1990. Near-threshold photoionization of nickel clusters Ionization potentials for Nis to Nipo. J Chem Phys 93 94-104. 

Yang S, KnickelbeinMB (1990) Photoionization studies oftransitionmetal clusters ionization potentials for Fe and Co . J Chem Phys 93 1533-1539... 

For larger clusters much spectroscopic detail is lost, but measurements of the photoionization thresholds provide information concerning cluster ionization potentials. Sodium clusters show a relatively smooth decrease in ionization potential from 5.1 eV for the atom to 3.5 eV for the 14-atom cluster. This is still significantly above the 1.6-eV work function of bulk sodium. For the smaller clusters the odd sizes have a lower ionization potential than the neighboring even-sized clusters because of effects due to open versus closed shell configurations. Recently measurements have been made of the ionization potential of iron clusters up to 25 atoms. In this experiment the ionization potentials were bracketed by the use of various ionizing lasers. There is a decrease in the ionization potential from 7.870 eV for an iron atom to the 4.4-eV work function of the bulk metal, but the trend is by no means linear. Thus, the ionization potential of Fea is about 5.9 eV, while those of Fes and Fe4 are above 6.42 eV. Clusters in the range of 9-12 atoms have ionization potentials below... 

Photoionization ti me-of-fli ght mass spectrometry is almost exclusively the method used in chemical reaction studies. The mass spectrometers, detectors and electronics are almost identical. A major distinction is the choice of ionizing frequency and intensity. For many stable molecules multi photon ionization allowed for almost unit detection efficiency with controllable fragmentation(20). For cluster systems this has been more difficult because high laser intensities generally cause extensive dissociation of neutrals and ions(21). This has forced the use of single photon ionization. This works very well for low i oni zati on potential metals ( < 7.87 eV) if the intensity is kept fairly low. In fact for most systems the ionizing laser must be attenuated. A few very small... 

It was also observed, in 1973, that the fast reduction of Cu ions by solvated electrons in liquid ammonia did not yield the metal and that, instead, molecular hydrogen was evolved [11]. These results were explained by assigning to the quasi-atomic state of the nascent metal, specific thermodynamical properties distinct from those of the bulk metal, which is stable under the same conditions. This concept implied that, as soon as formed, atoms and small clusters of a metal, even a noble metal, may exhibit much stronger reducing properties than the bulk metal, and may be spontaneously corroded by the solvent with simultaneous hydrogen evolution. It also implied that for a given metal the thermodynamics depended on the particle nuclearity (number of atoms reduced per particle), and it therefore provided a rationalized interpretation of other previous data [7,9,10]. Furthermore, experiments on the photoionization of silver atoms in solution demonstrated that their ionization potential was much lower than that of the bulk metal [12]. Moreover, it was shown that the redox potential of isolated silver atoms in water must... 

This is a severe drawback in the case of equilibrium studies of metal molecules since, as a rule, such molecules are minor vapor components and maximum sensitivity is required for their thermodynamic evaluation. However, very precise ionization potentials can be measured using photoionization spectroscopy (5,28). Berkowltz (28) reviewed early work concerning alkali metal dimers. Herrmann et al. ( ) have measured the ionization potentials of numerous sodium, potassium and mixed sodium-potassium clusters. For most of these clusters the atomization energies of the neutral molecules are not known. Therefore, the dissociation energies of the corresponding positive ions cannot be calculated. 

The ultraviolet photoelectron spectra of diatomic alkali halide molecules are reviewed and interpreted. Data for lithium halide dimers, 112X2> are presented and it is shown that the dimers have significantly larger ionization thresholds than the corresponding monomers. Some historical controversies regarding the presence of dimers and their ionization energies are clarified. Photoionization mass spectrometry is used to determine the adiabatic ionization potential of lithium chloride trimer, in order to probe the trend of I.P. with cluster size. The predictions of Hartree-Fock, Xa and ionic model calculations on this point are presented. 

The reaction AB —) I hv AB e is the basis of photoelectron spectroscopy and photodetachment methods. Many precise and accurate ionization potentials of molecules have been obtained by studying the photoionization of neutral molecules. The same principles apply to the photon methods for determining electron affinities, except that negative ions are studied. The electron affinities of over 1,000 atoms, radicals, clusters, and small molecules have been determined using... 

The techniques that have been most employed for investigating the electronic properties of small particles are photoemission (UPS, XPS), soft X-ray spectroscopy, EXAFS, photoionization mass spectrometry, and AES (23, 111, 240, 257d,e). While there is some controversy from theoretical work about the minimum particle size required to give bulk properties—from 10 (258) to several hundred atoms (259)—there seems to be a consensus that a cluster of about ISO atoms or more is required to observe a photoemission spectrum similar to that of the corresponding bulk metal (23, 260). When other properties are considered (ionization potential, density of states, valence bandwidth, etc.), the agreement is less satisfactory between the results obtained with different techniques (23). 

Experimentally we now have a great deal of information about electrons attached to water clusters. From the work of Haberland et. al. (1984) and Knapp et al. (1986) we know that an electron can be attached to a cluster of about 11 water molecules or more. It can also be attached very loosely to clusters of two or three water molecules but that special case will not be discussed here. [See Wallqvist et al. (1986) or Landman et al. (1987).] We believe it has a rather simple explanation. Bowen and colleagues (Coe, 1986) had also measured the photoionization spectra of those larger clusters. Their ionization potentials are of the order of one volt and increase monotonically with cluster size for the cases studied. These experimental data, however, yield very little information about the structure of the complexes and it... 

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