Transport and guest -host interactions in amorphous and crystalline ice.

摘要

Interactions of 13CO2 guest molecules with vapor-deposited porous H2O ices have been examined using temperature programmed desorption (TPD) and Fourier transform infrared (FTIR) techniques. Specifically, the trapping and release of 13CO2 by amorphous solid water (ASW) has been studied.;Samples were prepared by: (i) depositing 13CO2 on top of ASW; depositing 13CO2 underneath ASW; and (iii) co-depositing 13CO2 and H2O during ASW formation. The use of 13CO2 eliminates problems with background 12CO2. Some of the deposited 13CO2 becomes trapped when the ice film is annealed. The amount of 13CO2 trapped in the film depends on the deposition method (i.e., on top of the ASW, underneath the ASW, and co-deposition).;The release of trapped molecules occurs in two stages. The majority of the trapped 13CO2 escapes during the ASW-to-cubic ice phase transition at 165 K and the rest desorbs together with the cubic ice film at 185 K. We speculate that the presence of 13CO 2 at temperatures up to at 185 K is due to 13CO 2 that is trapped in cavities within the ASW film. These cavities are similar to ones that trap the 13CO2 that is released during crystallization. The difference is that 13CO2 that remains at temperatures up to 185 K does not access escape pathways to the surface during crystallization.;The UHV system was modified to incorporate a novel laser induced desorption (LID) technique in addition to TPD and FTIR. The source of the IR laser radiation at 2.92 mum based on the deuterium gas Raman shifter was developed as a part of the LID setup. Preliminary results of the H2O LID from the ASW films are discussed. Future experiments to investigate the phase transformations of ASW and participation of boundaries in it, dopant transport, and lateral flow of amorphous materials and supercooled liquids are outlined.