| AVS 54th International Symposium | |
| Surface Science | Thursday Sessions |
| Session SS1-ThA |
| Session: | Environmental Surfaces |
| Presenter: | S.C. Langford, Washington State University |
| Authors: | S.C. Langford, Washington State University K. Kimura, Washington State University J.T. Dickinson, Washington State University |
| Correspondent: | Click to Email |
On single crystal NaCl, 248-nm laser radiation can roughen steps and produce monolayer islands and etch pits at fluences well below the macroscopic damage threshold.2 These effects are dramatically altered by the presence of 10-5 Pa water vapor. In this work, we extend these observations to single crystal KCl and KBr. Atomic force microscopy (AFM) images of NaCl, KCl, and KBr cleavage surfaces exposed to pulsed 248-nm laser radiation in ultrahigh vacuum show monolayer islands; the island density increases with increasing laser fluence. We attribute island formation to the aggregation of alkali halide monomers desorbed from steps onto terrace sites as a consequence of UV exposure; this desorption accounts for the observed step roughening. In the presence of water vapor at pressures of 10-5 to 10-3 Pa, fifty 248-nm pulses at 100 mJ/cm2 produce monolayer etch pits on KCl and KBr. Higher fluences are required for NaCl. Quadrupole mass spectrometry on the products emitted during laser irradiation show much more intense alkali and halogen emissions in the presence of water vapor, consistent with the observed etching. Although the mechanism for enhanced etching in the presence of water vapor is not clear, the erosion of alkali halides during electron irradiation at room temperature is hindered by the accumulation of alkali metal.3 Water vapor may hinder alkali aggregation, perhaps by dissociative adsorption at halogen vacancies. We show that water vapor at similar partial pressures has comparable effects on alkali halide surfaces exposed to 2 keV electrons.
1This work was supported by the U.S. Department of Energy-under Grant DE-FG02-04ER-15618.
2K.H. Nwe, S.C. Langford, and J.T. Dickinson, J. Appl. Phys. 97, 043502 (2005).
3B.M. Szymonski, J. Ruthowski, A. Poradzisz, Z. Postawa, and B. Jørgensen, in Desorption Induced by Electronic Transitions—DIET II (Springer, Berlin, 1985), p. 160-168.