Time-of-flight direct recoil spectrometry : application to liquid surfaces and steps toward quantification Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1r66j3282

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  • Liquid surfaces are very abundant in nature. Despite the importance of the liquid interface in general, experimental molecular-level data was almost completely lacking prior to the last decade and a half. The intent of this work is to provide a means by which experimental data on the composition of liquid surfaces and the average orientation of their constituent molecules can be obtained in order to supplement data from molecular dynamics and related computational techniques. To this end, a unique time-of-flight (TOF) spectrometer, which constitutes the backbone of a new method to study liquid surfaces, was constructed and commissioned. The performance of the spectrometer is demonstrated in a number of exemplary TOF spectra obtained from liquid glycerol. Moving from mere qualitative to quantitative surface analysis necessitates the ability to relate physical quantities such as detection efficiencies, accurate signal intensities, and interaction cross-sections for all elements to one another. As a first step, the absolute detection efficiency of a channel electron multiplier, used as particle detector in the spectrometer, was measured for the noble gas ions He⁺, A⁺, and Xe⁺ The data obtained led to an empirically derived, general expression of the detection efficiency that is applicable to particles of any atomic number. The results also show that the threshold velocity, below which a multiplier does not respond to impinging ions, cannot be regarded as independent of the ion's atomic number as previously reported in the literature. The second step involved a comprehensive investigation of ion-atom interactions and spectral features that are crucial for the processing of experimental signal intensities for quantitative analysis. For this purpose, the binary collision code Marlowe was used in extensive trajectory calculations simulating TOF spectra. The simulation results confirm the high surface sensitivity of the technique and reveal the strong dependence of the sampling depth on the primary ion type and energy. Finally, theoretically calculated interaction cross-sections for hydrogen, which had often been reported as being abnormally high, where investigated and a correction factor to the screening function of the atomic interaction potential was empirically derived. This constituted a crucial step toward a more accurate determination of surface concentrations involving hydrogen.
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