Browsing by Author "Craycraft, Mary Jo"

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    The ultraviolet photoelectron spectroscopy of Group IV 2-15 atom cluster anions
    (1989) Craycraft, Mary Jo; Smalley, Richard E.
    The ability to map valence electronic structure is the result of a recent advance in photoelectron spectroscopy; its union with cluster molecular beam technology. The task of interpreting the spectra is hampered by a serious lack of understanding of cluster electronic structure in general. Recently progress has been made in finding models for single s valence electron systems. Alkali and noble metal clusters can be treated as free electron systems and simple interatomic potentials can be used with rare gas clusters. Neither a smeared jellium background nor a simple interatomic potential is adequate to describe covalent bonding, however. The isoelectronic Group IV members have a valence configuration of ns$\sp2$np$\sp2$. All readily form clusters, and the elements differ in both their atomic and bulk properties; thus the series provides an ideal system for studying electronic structure. The mass selected cluster ion beam is crossed with a beam (6.42 or 7.9eV) and the resulting photodetached electrons collected with the aid of judiciously arranged magnetic fields. The spectra are found to be unique for each size cluster. Some spectra show a significant gap between the two lowest binding energy features, indicating that the neutral cluster is a closed shell species. The clusters with such gaps are minima in a plot of EA as a function of cluster size. The UPS also vary with the cluster composition. Carbon is unique; an even -odd alternation in electron affinities switches from odd minima for clusters containing less than ten atoms to odd maxima for larger clusters. This corresponds with an alternation in singlet and triplet ground states and a switch from chain to ring structures previously predicted by theory (K. S. Pitzer, E. Clementi, J. Amer. Chem. Soc. 81 4477 (1958) and R. Hoffmann, Tetrahedron 22 521 (1965)). The spectra of the remaining group IV members are remarkably similar to each other for clusters of up to ten atoms, as is the trend in the electron affinities as a function of cluster size. This similarity in electronic structure may imply similarity in geometries. At ten, Si and Ge behavior diverges from that of Sn and Pb.
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    Ultraviolet photoelectron spectroscopy of clusters with a magnetic time of flight photoelectron spectrometer
    (1988) Craycraft, Mary Jo
    The nature of metal and semi-conductor bonding has been investigated via UV photoelectron spectroscopy of cluster anions. Electrons from negative cluster ions of the desired mass are photodetached and then collected with the aid of judiciously arranged magnetic fields. Pulsed operation allows the use of time of flight techniques on both the parent ions and the photoelectrons. Photodetaching with UV grants access to the details of valence electronic structure. Our experiments show marked variations in electronic structure as a function of cluster size and composition. Cu cluster UPS show an even-odd alternation in electron affinities, predicted by cluster shell models. The UPS of sixty atom C Clusters corroborates the proposed soccerball structure for some C$\sb{60}$. Semi-conductor cluster UPS indicate shell closings for certain cluster sizes.