Browsing by Author "Laaksonen, Reima Tapani"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Hydrogen chemisorption on transition metal clusters
    (1991) Laaksonen, Reima Tapani; Smalley, Richard E.
    Positively charged clusters of V, Nb, Ta, Co, Rh, Ir, and Ni in the size range of 3-26 atoms were generated by laser vaporization in a supersonic nozzle and injected into the ion trap of a Fourier transform ion cyclotron resonance apparatus, where they were studied by exposing them to hydrogen gas. Some clusters experience structural changes during the chemisorption process, and some clusters have multiple stable isomers. The reactivity of positive V, Nb, and Ta clusters toward H$\sb2$ increases with cluster size, but the reactivity oscillates so that the even clusters can be several orders of magnitude less reactive than their odd neighbors. The reactivity of positive Co, Rh, Ir, and Ni clusters toward H$\sb2$ varies with the cluster size, but the variation is smooth. In general clusters made of atoms with the same number of valence electrons behave quite similarly. The high reactivity of most positive clusters is attributed to unoccupied d-like states lying near the fermi level--these states are needed to avoid the Pauli repulsion, which creates the activation barrier for the H$\sb2$ chemisorption.
  • Thumbnail Image
    Item
    Short range order in lattice dynamics and the feasibility of exafs measurements
    (1985) Laaksonen, Reima Tapani; Wang, C. C.; Trammell, George T.; Miettinen, Hannu E.
    We studied theoretically the kinetics of a bcc lattice near the critical temperature. Our motivation is to examine the feasibility of using the recently developed time-resolved EXAFS technique to study the lattice dynamics. We derived the kinetic equations using the quasi-chemical approximation and keeping the number of particles constant and allowing only atom-vacancy exchange process. We solved these equations for a relaxation process following a sudden temperature change. Our calculations show that the long-range order exhibits critical slowing-down, as expected. Also the non-linear coupling between short- and long-range order was observed. Because of this non-linear coupling the relaxation of the short-range also slows down near the critical temperature. Our results put stringent conditions on possible experiments.