Abstract:
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.
