Bulk diffusion of HDO and H{sub 2}{sup 18}O into single-crystal H{sub 2}{sup 16}O ICE multilayers
- Univ. of Colorado, Boulder, CO (United States)
Heterogeneous reactions on polar stratospheric cloud (PSC) particles, composed of ice (Type II PSC) and HNO{sub 3} hydrates (Type I PSC), play a critical role in the depletion of ozone in Antarctic regions. The dynamical nature of ice may affect these reactions and our understanding of chemistry on PSCs. The bulk diffusion of HDO and H{sub 2}{sup 18}O into single-crystal H{sub 2}{sup 16}O ice multilayers grown epitaxially on Ru(001) were measured utilizing a novel technique of isothermal desorption/depth profiling combined with laser induced thermal desorption (LITD) probing. The measured bulk diffusion constants for HDO into single-crystal H{sub 2}{sup 16}O ice multilayers ranged from D(HDO)=4.2x10{sup -14}{plus_minus}0.3 cm{sup 2}s{sup -1} at T=153 K to D(HDO)=9.0x10{sup -14{plus_minus}0.6} cm{sup 2}s{sup -1} at T=170 K. The bulk diffusion coefficients for H{sub 2}{sup 18}O in crystalline ice varied from D(H{sub 2}{sup 18}O)= 4.4x10{sup -16{plus_minus}0.2} cm{sup 2}s{sup -1} to 5.0x10{sup -15{plus_minus}0.2} cm{sup 2}s{sup -1} over the temperature range T=155-165 K. An Arrhenius analysis of the HDO and H{sub 2}{sup 18}O bulk diffusion coefficients yielded the bulk diffusion activation energies (E{sub A}) and preexponential factors (D{sub o}):E{sub A} (HDO)=2.0{plus_minus}1.0 kcalmol{sup -1}, D{sub o}(HDO)=8.7x10{sup -11{plus_minus}0.6} cm{sup 2}, E{sub A}(H{sub 2}{sup 18}O)= 13.1{plus_minus}0.5 kcalmol{sup -1}, D{sub o}(H{sub 2}{sup 18}O)=9.0x10{sup 2{plus_minus}0.3} cm{sup 2}s{sup -1}. The activation barrier for molecular diffusion is consistent with the breakage of two hydrogen bonds. The smaller barrier for HDO diffusion suggests H/D exchange with possible tunneling. The bulk diffusion coefficients predicted at stratospheric temperatures indicate that the ice surfaces are dynamic with diffusion into the bulk competitive with thermal desorption.
- OSTI ID:
- 370000
- Report Number(s):
- CONF-960376--
- Country of Publication:
- United States
- Language:
- English
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