Outline of the fast thermal desorption spectroscopy apparatus. The filament assembly consists of a tungsten filament spot welded to the supports. The supports are in thermal contact, but electrically isolated from a liquid nitrogen cooled heat sink. The vaporization flux from the filament is monitored with a quadrupole mass spectrometer (QMS) positioned in a differentially pumped vacuum chamber from the filament. Vaporization products enter the QMS chamber through a skimmer. During experiments, the voltage drop across the filament and the current through the filament are measured every .
flux from the filament and the corresponding temperature of the filament. The flux was measured with the fast ionization gauge (FIG), positioned away from the filament. After the temperature of the filament is stable within . Achieving a steady state temperature coincides with the onset of desorption, which follows zero-order kinetics for the next .
Typical results of ultrafast microcalorimetry measurements. Dashed line: the effective heat capacity of the ice-free filament. Solid line: the effective heat capacity of the filament with an film vapor deposited at . Dotted line: the effective heat capacity of the filament with an film vapor deposited at . In the case of deposition at , the lack of exothermic transitions demonstrates that the ice sample is crystalline. In the case of films deposited at , the effective heat capacity of the film shows partially overlapping irreversible exotherms due to the crystallization of the initially amorphous ice samples.
FTDS spectrum of from a thick “sandwich-like” // film (upper panel). The tracer layer initially positioned approximately under the film surface, as illustrated in the diagram on top the graph. The vaporization temperature was . Isothermal desorption kinetics of the entire ice film monitored by the figure are shown for comparison (lower panel).
Isothermal desorption spectra of from thick polycrystalline ice films at . The diagram on top of the graph shows the initial positions of the isotopic tracer layers.
Isothermal desorption spectra of from thick polycrystalline ice films measured at various temperatures. The diagram on top of the graph shows the initial positions of the isotopic tracer layers.
Isothermal desorption spectrum of from thick pure polycrystalline ice films and from films uniformly doped with acetic acid. The mole fraction of was 0.01. The overall vaporization rate of doped ice was indistinguishable from that of pure ice. The diagram on top of the graph shows the initial positions of the isotopic tracer layers. The addition of acetic acid results in significant changes in the feature; however, the position and width of the feature remains unaffected.
Two possible mechanisms of vaporization. (a) The case where the changes in the ice film’s surface morphology are assumed to be negligible; whereas, the diffusion of along the grain boundaries is rapid. Rapid diffusive transport of isotopic tracer molecules to the film’s surface results in the peak. The subsequent peak arises from desorption of the molecules trapped in ice crystallites. (b) The case where the diffusion of the tracer species within the film’s bulk is negligible but the surface undergoes thermal etching during vaporization. Isotopic tracer species located at or near the grain boundaries vaporize, when grooves and pits reach the tracer layer, thus resulting in the peak in the desorption spectra of . The peak signifies desorption of the molecules trapped in ice crystallites.
Isothermal desorption spectra of from thick polycrystalline ice films at and plot of the initial tracer layer position as a function of vaporization time of . According to the interpretation of the spectra described in the text, the slope of the line gives the vaporization rate of the single crystal part of the polycrystalline film.
Absolute vaporization rate of single crystal ice (dark circles), absolute vaporization rate of polycrystalline ice (grey circles), and the maximum vaporization rate (line) of ice as function of temperature.
Upper shaded region: The range of values of the vaporization coefficient derived from experimental measurements of the absolute vaporization rate of single crystal ice. The shaded region in the middle of the plot shows the range of values for vaporization coefficient predicted by the classical mobile precursor mechanism.
The average depth of the surface grooves as a function of vaporization time, during vaporization of ice films at various temperatures. The slopes of the lines give the rate of thermal etching of the polycrystalline surface.
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