AVS 47th International Symposium
    Surface Science Tuesday Sessions
       Session SS3-TuA

Paper SS3-TuA8
Diffusion of Organic Molecules in Ice Measured Using Laser Resonant Desorption Depth-Profiling

Tuesday, October 3, 2000, 4:20 pm, Room 210

Session: Water/Surface Interactions
Presenter: F.E. Livingston, University of Colorado
Authors: F.E. Livingston, University of Colorado
S.M. George, University of Colorado
Correspondent: Click to Email
Knowledge of the diffusion of organic molecules in ice is important for understanding the history of the Earth's atmosphere preserved in ice cores. Molecular diffusion in ice is affected by chemical and physical properties such as hydrophilicity, size and geometry. To explore these properties, we have measured the diffusion rates of different organic molecules such as carboxylic acids and alcohols. Diffusion was measured using an infrared laser resonant desorption (LRD) technique to depth-profile into the ice. LRD was accomplished using a Q-switched Er:YAG laser that emits light at @lambda@=2.94 µm. The laser radiation is resonantly absorbed by the O-H stretching vibration of H@sub2@O molecules and thermalized to facilitate desorption in the surface region. We have employed LRD depth-profiling to study the diffusion kinetics of formic (HCOOH) and acetic (CH@sub3@COOH) acid. LRD analysis revealed that acetic acid diffuses ~3 times faster than formic acid. The diffusion coefficients for formic acid vary from D~2 x10@super-13@ cm@super2@/s at T=175 K to D~2x10@super-10@ cm@super2@/s at T=195 K. Arrhenius analysis yielded a diffusion activation energy of E@subA@~24 kcal/mol and a diffusion preexponential D@subo@~4x10@super16@ cm@super2@/s. For acetic acid, the measured diffusion coefficients range from D~7x10@super-13@ cm@super2@/s at T=170 K to D~7x10@super-10@ cm@super2@/s at T=195 K. Arrhenius analysis yielded diffusion kinetic parameters of E@subA@~18 kcal/mol and D@subo@~1x10@super11@ cm@super2@/s. The faster diffusion for acetic acid argues that the lower hydrophilicity and larger size of acetic acid does not reduce its diffusion relative to formic acid.