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Molecular beam investigations of surface chemical reactions and dynamics


Mullins, Charles Buddie (1990) Molecular beam investigations of surface chemical reactions and dynamics. Dissertation (Ph.D.), California Institute of Technology.


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Experimental results from molecular beam investigations of trapping and dissociative chemisorption phenomena for several gas-surface systems are presented. The dissociative chemisorption of oxygen on Ir(110)-(1x2) in the limit of zero coverage [...] was studied as a function of incident kinetic energy [...], incident angle [...] and surface temperature [...]. Results from this investigation indicate that two mechanisms account for the initial chemisorption. At low incident kinetic energy (less than 4 kcal/mol) chemisorption mediated by trapping is primarily responsible for the dissociative adsorption while at high energies a direct mechanism can account for the results. In both energy ranges the initial dissociative chemisorption probability is insensitive to incident angle.

The trapping of molecular ethane as well as the dissociative chemisorption of ethane on the clean Ir(110)-(1x2) surface has also been investigated. The initial trapping probability [...] is found to decrease with incident kinetic energy from a value of ~0.98 at 1 kcal/mol to ~0.1 at 16 kcal/mol. These data scale with [...]. The initial dissociative chemisorption of ethane on Ir(110)-(1x2) occurs via a trapping-mediated mechanism at low [...] and a direct mechanism at high kinetic energies. In the trapping-mediated regime [...] decreases rapidly with increasing [...]. These data quantitatively support a kinetic model consistent with a trapping-mediated chemisorption mechanism. The difference in the activation energies for desorption and chemisorption from the physically adsorbed, trapped state [...] is 2.2±0.2 kcal/mol. Chemisorption at high kinetic energies, in the direct regime, is independent of surface temperature.

Additionally, the trapping probability of Ar on Pt(111) has been measured as a function of incident kinetic energy, and angle for [...]=80, 190 and 273 K. The trapping probability decreases with increasing [...] in a manner that depends on both [...] and [...]. The angular scaling law governing the trapping is a function of [...] such that [...] scales with [...] at 80 K, [...] at 190 K and [...] at 273 K. These results suggest that parallel momentum dissipation becomes increasingly more important to the trapping dynamics as the surface temperature is increased.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemical Engineering
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Weinberg, William Henry
Thesis Committee:
  • Unknown, Unknown
Defense Date:29 September 1989
Record Number:CaltechETD:etd-03122007-135158
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:938
Deposited By: Imported from ETD-db
Deposited On:20 Mar 2007
Last Modified:26 Dec 2012 02:33

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