The importance of pores in the electron stimulated production of D2 and O2 in low temperature ice

The effects of porosity and morphology of amorphous and crystalline D2O ices on the electron stimulated generation and trapping of D2 and O2 have been studied by post-irradiation thermal desorption. The trapped product yields increase from crystalline ice to amorphous ice to highly porous amorphous ice, similar to observed ESD yields. This is attributed to the increased number of defects, traps and pores in amorphous ice. Molecular deuterium is released in the temperature range from 55 to 105 K for each of the samples, with two notable bursts at 115 and 132 K for porous amorphous ice. Low temperature release is associated with diffusion of D2 through micropores in the ice matrix. Highly porous amorphous ice shows a striking spike in the release of D2 at 80 K which has not previously been observed and must be a result of trapping of D2 within enclosed macropores. The trapped D2 in macropores may result from transport of excitation energy to the pore interface or from transport of D2 through micropores at lower temperatures during TPD, or both. Molecular oxygen is retained within ice until much higher temperatures (>140 K). The release at this temperature is attributed to sintering and diffusion along grain boundaries in crystalline ice. The majority of trapped O2 evolves with the desorption of the ice matrix, suggesting that clathrate hydrates may be important trapping sites. Amorphous ice releases a surge of trapped O2 at the 160 K amorphous to crystalline phase transition, which is consistent with codeposition experiments. Highly porous ice exhibits much higher total yields of oxygen and shows a second strong spike in trapped O2 release at 175 K.