The Interaction of HCl on Water Ice Nanoscale Films
EMSL Project ID
24821
Abstract
Our description of the role played by ice particles in altering the chemical composition of the Earth's atmosphere is currently undermined by our poor understanding of the interaction of molecules with ice surfaces. Tackling the complex coupled kinetics responsible for interfacial reactivity (adsorption and desorption, surface and bulk diffusion, reaction, phase separation and crystallization) has proven a daunting task even for idealized systems. However, it has become increasingly clear that any further progress in our understanding of heterogeneous atmospheric chemistry will require a quantitative description of the still controversial adsorption state (molecularly adsorbed or ionically dissociated) of atmospherically relevant molecules on ice surfaces. This issue has come into the spotlight recently because of the postulated reactivity of the resulting partially hydrated, surface segregated halide anions towards gas phase species. It has been proposed that this mechanism could initiate the well-known ozone consuming catalytic cycles in the polar stratosphere [1-3]. These issues are also intimately coupled to other outstanding problems in the chemistry and physics of ice such as impurity uptake and diffusion, as well as the phase separation and crystallization kinetics for various hydrates from icy material with complex compositions. Our approach will use thin films as models for atmospherically relevant systems. We will perform quantitative studies of the complex coupled kinetics in water-acid binary ices with a particular emphasis on uptake, diffusion, phase separation and crystallization in the HCl-water system. The best approach to undertake this ambitious endeavor requires a combination of molecular beam and thermal desorption techniques, complemented with simultaneous in situ characterization by vibrational spectroscopy. An important question is whether HCl dissociates on an ice surface and if so at what temperature. Numerous theoretically calculations and some experimental results have been published but a clear answer remains elusive.[4-6] In preliminary work, we have grown ice layers at 20 K and then deposited using a molecular beam precise amounts of HCl. A plot of the integrated HCl stretching band versus coverage for these films. The results show that the molecular HCl stretch band intensity is missing for coverages less than a monolayer of HCl, but is observed for coverages above a monolayer. One interpretation of this observation is that, even at 20 K, HCl in contact with the ice substrate has dissociated. Another possibility is that HCl remains molecular but lies flat and is therefore infrared inactive. Ongoing and future research activities are aimed at unraveling the mechanistic details of the interaction of HCl with both crystalline and amorphous ice.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2007-05-31
End Date
2009-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Ayotte P, P Marchand, JL Daschbach, RS Smith, and BD Kay. 2011. "HCI Adsorption and Ionization on Amorphous and Crystalline H2O Films below 50 K." Journal of Physical Chemistry A 115(23):6002-6014.
F. Cholette, T. Zubkov, R. S. Smith, Z. Dohnalek, B. D. Kay, and P. Ayotte, Journal of Physical Chemistry B 113 (13), 4131 (2009).
J.L. Daschbach, J. Kim, P. Ayotte, R.S. Smith, and B.D. Kay, Journal of Physical Chemistry B 109, 15506 (2005).