EFFECT OF THE THERMAL HISTORY OF ICE CRUSHED AT 77 K ON ITS SURFACE-STRUCTURE AS DETERMINED BY ADSORPTION OF CH4 AT LOW SURFACE COVERAGE

The effect of annealing on the structure of the surface of single crys tals of ice crushed at 77.15 K is investigated using methane adsorptio n at 77.15 and 87.1 K. Methane adsorption isotherms are obtained for s urface coverages, theta, between 0.1 and 1.5 monolayers and between 10 (-6) and 10(-2) monolayer. It is observed that annealing greatly reduc es the number of CH4 molecules adsorbed at low coverage (by a factor o f 10 at P-CH4 = 10(-4) Torr, for annealing at 238 K), but no effect of annealing on adsorption is observed for theta greater than 0.1 monola yer. We propose that these effects are due to the disappearance upon a nnealing of crystal faces with favorable adsorption sites, which could be faces with high Miller-Bravais indices. From our data, and additio nal data obtained by other investigators, we conclude that the mode of formation and the thermal history of the ice surface strongly influen ce its surface structure and adsorption properties. We discuss the pos sibility that the different modes of formation of ice in the atmospher e will result in different surface and adsorption properties.