AVS 49th International Symposium
    Surface Science Monday Sessions
       Session SS1-MoM

Paper SS1-MoM2
Dynamics of Oligomer Desorption from Surfaces

Monday, November 4, 2002, 8:40 am, Room C-108

Session: Adsorption and Chirality
Presenter: K.R. Paserba, Carnegie Mellon University
Authors: A.J. Gellman, Carnegie Mellon University
K.R. Paserba, Carnegie Mellon University
N. Vaidyanathan, Carnegie Mellon University
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A study of the desorption of long chain oligomers from surfaces has revealed that the measured desorption energies are non-linear in the oligomer chain length. This work has used alkanes, polyetheyleneglycols, and polyethyleneglycol dimethylethers with chain lengths in the range 5 to 60 atoms. These have been adsorbed on the surface of graphite at low temperature and the kinetics of desorption have been measured using temperature programmed desorption. Empirically we find that the desorption energies scale as the square root of the chain length. A model has been proposed for oligomer desorption that accurately accounts for the observed dependence of the desorption energy on chain length. The adsorbed oligomers can be considered to consist of segments that are attaching to and detachng from the surface independently. Within the context of transition state theory these partially detached oligomers are in equilibrium with a transition state to desorption which is fully detached from the surface. The energy of each of the detached states is simply proportional to the number of detached segments. The entropy is given by the number of ways of detaching segments and by the partition function for trans-gauche conformations about each detached bond. These energies and entropies determine the equilibrium constants for each of the partially detached species. These considerations can be formulated into an analytical expression for the measured desorption energy that accurately reproduces the experimental results. One of the interesting insights that our model provides is that the analytical expression for oligomer desorption energies exactly matches the predictions of Tolman's theorem. Because the oligomers on the surface can adopt a huge number of conformations this leads to a substantial non-linearity in the desorption energy as a function of chain length.