AVS 47th International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuM

Paper SS1-TuM11
Isothermal Desorption Kinetics of H@sub 2@O from @super 1@H@sub 2@@super 16@O, @super 1@H@sub 2@@super 18@O and @super 2@H@sub 2@@super 16@O Ice Multilayers

Tuesday, October 3, 2000, 11:40 am, Room 208

Session: Reactions on Oxides and Environmental Chemistry
Presenter: J.A. Smith, University of Colorado at Boulder
Authors: J.A. Smith, University of Colorado at Boulder
F.E. Livingston, University of Colorado at Boulder
S.M. George, University of Colorado at Boulder
Correspondent: Click to Email
The mechanism of H@sub 2@O desorption from ice can be explored by examining the H@sub 2@O desorption kinetics from ice composed of the various H@sub 2@O isotopomers. The isothermal desorption kinetics of H@sub 2@O from @super 1@H@sub 2@@super 16@O, @super 1@H@sub 2@@super 18@O and @super 2@H@sub 2@@super 16@O ice multilayers were measured using optical interferometry. These experiments were performed at temperatures between 175-195 K using ice multilayers grown epitaxially on a Ru(001) surface. The desorption rates of @super 1@H@sub 2@@super 16@O and @super 1@H@sub 2@@super 18@O were very similar. The desorption kinetics for @super 1@H@sub 2@@super 16@O were E@sub d@ = 13.9 +/- 0.2 kcal/mol and @nu@@sub d@ = 10@super 32.6 +/- 0.3@ molecules/cm@super 2@ s. In contrast to the expectation that the H@sub 2@O desorption rate should scale with the square root of molecular mass, the desorption rate of @super 2@H@sub 2@@super 16@O was approximately a factor of two slower over the measured temperature range. The desorption kinetics for @super 2@H@sub 2@@super 16@O were E@sub d@ = 14.8 +/- 0.4 and @nu@@sub d@ = 10@super 33.4 +/- 0.5@ molecules/cm@super 2@ s. The desorption kinetics for the three H@sub 2@O isotopomers are explained using transition state theory. Th e rotational degrees of freedom yield the main differences. The differences in the activation energies are related to the zero-point energies of frustrated rotations on the ice surface. The differences in pre-exponentials are associated with the moments of inertia of the desorbing molecules.