Anal. Chem. 2000, 72, 5590-5599
Depth-Profiling and Diffusion Measurements in Ice Films Using Infrared Laser Resonant Desorption Frank E. Livingston, Jamison A. Smith, and Steven M. George*
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215
A new infrared laser resonant desorption (LRD) technique has been developed that permits depth-profiling and diffusion measurements in ice. This LRD technique utilizes an Er:YAG rotary Q-switched laser with an output wavelength of λ ) 2.94 µm and a pulse duration of ∼100 ns. The Er:YAG laser light resonantly excites O-H stretching vibrations in the H2O molecules that form the ice. This laser resonant heating induces H2O desorption at the ice surface. Control experiments were conducted on pure and isotopically mixed laminated ice films to determine the optimum experimental parameters for the LRD depthprofiling and diffusion measurements. Depending on laser energy, the measured desorption depth was either less than, comparable to, or larger than the optical penetration depth of ∼0.8 µm at λ ) 2.94 µm. LRD studies were used to analyze H218O/H216O stacked multilayers and laminate sandwich structures. These measurements revealed that the LRD technique can depth-profile into ice films with submicrometer spatial resolution and high sensitivity. Two types of experiments employing LRD depth-profiling were demonstrated to monitor diffusion in ice. HCl hydrate diffusion in ice was measured versus time after depositing ice/HCl/ice sandwich structures. Na diffusion into ice was studied after adsorbing Na using a continuous Na source for a given exposure time at the diffusion temperature. Ice represents the most important and prominent molecular solid on Earth. The importance of ice in chemical and physical processes on Earth and in the atmosphere is well documented.1-3 Glacial and polar ice cores provide critical information concerning chronological records of climate variations, geologic events, and atmospheric composition dating back over 420 000 years.2,4-7 Ice particles in the form of polar stratospheric clouds have also been * Corresponding author: (fax) 303-492-5894; (e-mail) georges@ spot.Colorado.edu. (1) Hobbs, P. V. Ice Physics; Clarendon Press: Oxford, U.K., 1974. (2) Wolff, E. W.; Bales, R. C. Chemical Exchange Between the Atmosphere and Polar Snow; Springer-Verlag: Berlin, 1996. (3) Pruppacher, H. R.; Klett, J. D. Microphysics of Clouds and Precipitation; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1997. (4) Mayewski, P. A.; Lyons, W. B.; Spencer, M. J.; Twickler, M. S.; Buck, C. F.; Whitlow, S. Nature 1990, 346, 554-556. (5) Taylor, K. C.; Lamorey, G. W.; Doyle, G. A.; Alley, R. B.; Grootes, P. M.; Mayewski, P. A.; White, J. W.; Barlow, L. K. Nature 1993, 361, 432-436. (6) Delmas, R. J.; Legrand, M. In The Environmental Record in Glaciers and Ice Sheets Dahlem Workshop Report; Oeschger, H., Langway, C. C., Eds.; Wiley and Sons: Chichester, U.K., 1989.
5590 Analytical Chemistry, Vol. 72, No. 22, November 15, 2000
directly linked to ozone depletion in the Antarctic region.8-10 Despite the significance of ice, a detailed understanding does not exist for many microphysical processes in ice such as adsorption, desorption, diffusion, and reaction kinetics. This incomplete picture of the elementary kinetic processes limits ice core analysis and a full understanding of heterogeneous atmospheric chemistry. Quantitative measurements of diffusion in ice are almost nonexistent for species other than H2O isotopes.1,11 Microtome sectioning and scintillation or mass spectrometric detection techniques have been used to measure H218O, D2O, and T2O diffusion in macroscopic crystalline and polycrystalline ice.1,11 X-ray topography experiments have also examined the growth of interstitial-type dislocation loops and dipoles on the surface of crystalline H2O ice to determine H2O diffusion coefficients.12,13 In addition, nuclear magnetic resonance studies have measured H2O self-diffusion in the “quasi-liquid layer” on micrometer-sized (25 µm precludes measurements at low temperatures representative of the polar stratosphere and upper troposphere. The microtome technique may also be susceptible to serial contamination effects during ice sectioning using a lathe tool or microtome knife.20 Laser-induced thermal desorption (LITD) and isothermal desorption depth-profiling techniques have recently been developed to measure the diffusion of H2O isotopes into single-crystal ice multilayers.24-29 The LITD isothermal desorption depthprofiling method allows diffusing species to be monitored in real time with high sensitivity in ultrathin (
