Browsing by Author "Jin, Changming"
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Item Clusters as models of bulk surfaces and novel materials(1992) Jin, Changming; Smalley, Richard E.Studies on gas phase clusters show that common ground exists between clusters and bulk surfaces such that clusters can serve as models of bulk surfaces. Several cluster systems are studied to probe this cluster-surface analogy with both ultraviolet photoelectron spectroscopy (UPS) and Fourier transform ion cyclotron resonance (FT-ICR). Ultraviolet photoelectron spectra of GaAs cluster anions are measured with a photon energy of 7.9 eV. The electron affinity displays a strong even/odd oscillation suggesting the presence of a substantial gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the corresponding neutral clusters. The absence of unpaired electrons in even numbered neutral clusters is reminiscent of the healed dangling bonds caused by the reconstruction and relaxation at GaAs bulk surfaces. Chemical reactions of noble and transition metal clusters with CO and H$\sb2$ are studied with FT-ICR. The chemistry and physics of these metal cluster plus adsorbates systems are also modeled well by using the cluster shell model originally developed for metal bulk solids and surfaces. Excellent interplay between theory and experiment on the Cu$\sb{\rm x}\sp+$CO systems reveals the major advantage for the cluster model of bulk surfaces. New materials are prepared in the course of studying gas phase carbon clusters. Laser vaporization of a graphite/BN composite disk produces fullerenes in which one or more atoms of the hollow carbon cage is replaced by a boron atom. These boron-doped fullerenes are found to act as Lewis acid when they chemisorb ammonia molecules. Fullerenes with one or more metal atoms inside are also generated both in gas phase and in macroscopic quantities. The formation mechanism of the multi-metal doped fullerenes is proposed as due to coalescence. These new materials are expected to have interesting, novel, and useful properties.Item Magnetic and critical behavior of thin terbium(0001)/tungsten(110) films studied by electron capture spectroscopy (ECS)(1988) Jin, Changming; Rau, CarlEpitaxial films of hcp Tb(0001) on bcc W(110) substrates are prepared and their geometric, electronic and magnetic properties are studied. The atomic flatness and cleanness and the single-crystalline state of the W(110) substrates and the Tb films are checked using scanning tunneling microscopy (STM), Auger electron spectroscopy (AES) and low-energy and high-energy electron diffraction (LEED and RHEED). Using electron capture spectroscopy (ECS), the surface electron spin polarization of the Tb(0001) films is studied as a function of temperature. The topmost layer is found to order ferromagnetically below a surface Curie temperature T$\sb{\rm C\sb s}$ = 249.96 K, which lies above both the bulk Curie and Neel temperatures T$\sb{\rm C\sb b}$ = 220 K and T$\sb{\rm N\sb b}$ = 228 K, respectively, and exhibits strongly non-monotonic behavior close to 245 K. Novel critical behavior is observed near T$\sb{\rm C\sb s}$, suggesting strong surface anisotropies, in accord with a recently predicted and previously unobserved surface phase transition.