AVS 51st International Symposium
    Surface Science Friday Sessions
       Session SS1-FrM

Paper SS1-FrM3
Diffusion of He in Amorphous Solid Water: Observation of an Inverse H/d Lattice Isotope Effect

Friday, November 19, 2004, 9:00 am, Room 210B

Session: Hydrated Surface Phenomena
Presenter: J.L. Daschbach, Pacific Northwest National Laboratory
Authors: J.L. Daschbach, Pacific Northwest National Laboratory
G.K. Schenter, Pacific Northwest National Laboratory
P. Ayotte, University of Sherbrooke, Canada
R.S. Smith, Pacific Northwest National Laboratory
B.D. Kay, Pacific Northwest National Laboratory
Correspondent: Click to Email
The diffusion of He through amorphous solid water (ASW) has been measured in H@sub 2@O and D@sub 2@O between 50 K and 125 K. He is embedded in a thin layer of ASW grown on Pt(111) at 25 K by molecular beam techniques and subsequently capped with an ASW cap of precise thickness. Linear temperature programmed desorption of the embedded He diffusing through the ASW cap is measured by mass spectroscopy. We find that He diffuses more rapidly in D@sub 2@O than in H@sub 2@O ASW. To our knowledge, this is the first observation of a lattice isotope effect in diffusion. A microscopic kinetic model for He hopping between cage sites in the ASW, reflecting at the Pt(111) interface, and desorbing at the vacuum interface closely fits the data. The diffusion kinetic parameters are found to be D@sub 0@ = 1.3 ± 0.3 x 10@super -3@ cm@super 2@/s, E@sub a@ = 11.9 ± 0.17 kJ/mole and D@sub 0@ = 0.8 ± 0.2 x 10@super -3@ cm@super 2@/s, E@sub a@ = 11.2 ± 0.17 kJ/mole, for H@sub 2@O and D@sub 2@O respectively. The origin of this inverse isotope effect in the host lattice has been studied using transition state theory. These results indicate the inverse isotope effect arises predominantly from the frustrated rotations of the lattice water molecules.