Interactions of CCl 4 with Thin D 2 O Amorphous Ice Films, Part I: A Nanoscale Probe of Ice Morphology

High sensitivity temperature-programmed desorption mass spectrometry (TPDMS) was used in order to investigate the adsorption of carbon tetrachloride on thin (∼500 ML) amorphous D 2 O ice films at 95 K. The TPDMS experiments demonstrate that the desorption kinetics of CCl 4 are extremely sensitive to the ice morphology. At submonolayer coverages, the adsorption of CCl 4 on ice films prepared by vapor deposition at 130 K results in TPD spectra which consist of a single desorption feature α-CCl 4 at 133−140 K. The TPD spectrum of CCl 4 from ice films vapor-deposited at 95 K, however, consists of three desorption peaks labeled σ-, δ-, and ε-CCl 4 at 145, 180, and 190 K, respectively. We attribute the observed differences in the TPD spectra of CCl 4 from the two types of ice to differences in the microstructure of the ice surface. D 2 O vapor deposition at 95 K apparently results in the formation of microporous amorphous ice, while deposition at 130 K results in the formation of pore-free, solid ice. Analysis of the TPD spectra demonstrates that CCl 4 adsorption on ice prepared at 130 K results in the formation of metastable, two-dimensional islands. CCl 4 adsorption on ice prepared at 95 K, however, proceeds through formation of three-dimensional clusters in the pores of microscopically rough ice. At very low coverages, small CCl 4 clusters are trapped at the ice surface during TPD. Trapped CCl 4 evolves into the gas phase at two different temperatures giving rise to δ- and ε-CCl 4 desorption states. The dependence of δ- and ε-CCl 4 yields on the temperature of ice film deposition indicates that two structurally different forms of ice coexist at temperatures between 95 and 130 K.