Trace gas uptake on ice surface using Rutherford Backscattering

Collaboration: W. A. Lanford, SUNY at Albany, Albany, N.Y., U.S.A.

Surfaces play an especially important role in the chemistry of high vapor pressure material. Measurement of elemental or isotopic concentration profiles on such materials would provide key data for understanding the microscopic mechanism underlying their physical chemistry. However, such measurements present a significant analytical challenge. Most modern profiling methods rely on either sputtering or ion energy loss to derive depth information. Ion beam analysis - e.g. Rutherford backscattering (RBS) - might seem incompatible with the high vapor pressures associated with these materials. However, we have demonstrated that a simple modification of a conventional ion beam analysis chamber allows these materials to be studied. This approach provides for the first time in situ quantitative concentration profiles for volatile liquids or solids held at controlled ambient pressures up to a few hPa and opens up whole new avenues to the study of this important class of materials.

Two fields are currently pursued: (a) Measurements of solubility and diffusion constants of different trace gases (HCl, SO₂, ...) in ice and in acid hydrates under atmospheric conditions, (b) Investigations for the existence and characterization of the quasi-liquid layer (QLL) on ice surfaces. The samples (polycrystalline ice samples, ice single crystals and hydrates of sulfuric acid and nitric acid are examined in the helium or proton beam of a linear accelerator by Rutherford backscattering (RBS). This technique permits the non-destructive measurement of absolute concentration profiles with a depth resolution of 30Å over a total depth of about 10.000Å. From such concentration profiles the diffusion constant (D) and the solubility (Henry’ s law constant, H) can be determined independently, if the gas phase is known or measured (mass spectroscopy). These are quantities needed as a function of temperature, trace gas concentration and morphology of the ice for modeling atmospheric chemistry e.g. in polar regions or cirrus clouds. Apart from the measurement of the quantities D and H the quasi liquid layer on ice will be examined. Concentration profiles of a trace gas in the solid phase will be measured in the proximity of the coexistence line between ice and the appropriate aqueous solution of the trace gas. By the differences in the diffusion constants these concentration profiles should differ depending on how the surface of the ice is represented: by an expanded, relatively homogeneous liquid film with sharp transitions both to the solid and the gaseous phase, or by a gradual reduction of the crystal order to the gaseous phase.

Publications

M. Hess, U. K. Krieger, C. Marcolli, T. Huthwelker, M. Ammann, W. A. Lanford, and Th. Peter
Journal of Physical Chemistry A, 111, (2007) 4312-4321
Bromine Enrichment in the Near-Surface Region of Br-Doped NaCl Single Crystals Diagnosed by Rutherford Backscattering Spectrometry

U. K. Krieger, T. Huthwelker, C. Daniel, U. Weers, Th. Peter, W. A. Lanford
Science, 295 (2002) 1048-1050
"Rutherford backscattering to study the near-surface region of volatile liquids and solids"
Abstract full text

T. Huthwelker, U. K. Krieger, U. Weers, Th. Peter, W.A. Lanford
Nuclear Instruments and Methods in Physics Research B, 190 (2002) 47-53
"RBS analysis of trace gas uptake on ice"

learn something about RBS:

- RBS Theory Tutorial by Dr. Ron Fleming
- RBS Instrumentation Tutorial by Dr. Ron Fleming