Nitrogen Dioxide Exposure of an Olefin-Terminated Monolayer

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Chapter 2 Instrumentation

Grazing Angle FTIR Spectrometer- Vacuum Chamber

In order to study reactions of surfaces exposed to reactive gases, we have developed an ultrahigh vacuum (UHV) chamber- FTIR system. The system enables in situ time-resolved FTIR studies of the surface reactions during exposure. Figure 2.1 shows a schematic of the Bruker IFS66v/S FTIR and UHV chamber set up.
The IR radiation from the SiC glowbar MIR source (no. 1) is focused onto the aperture wheel by a parabolic mirror (focal length 180 mm). This mirror will move to focus the light from either the glowbar or the Tungsten filament NIR/Vis source (no. 2),as selected by the user. The aperture wheel for these experiments has been set to the 0.7 mm × 4 mm aperture slit in order to maximize the spot size on the surface. Before entering the interferometer, the beam passes through a wire grid polarizer (no. 4), which selects the perpendicular component of the light. Once through the interferometer, the beam’s direction is then determined by the movable mirror (no. 8). This mirror can be positioned so that the beam exits the spectrometer or proceeds into the internal sample compartment. In these experiments, the radiation was reflected into a separate mirror chamber where it was focused by a parabolic mirror (no. 9, focal length 250 mm) onto the sample. The angle of the beam with respect to the surface normal was approximately 83° and was set by moving the parabolic mirror (no. 9). After reflecting off the sample, the beam enters the external detector chamber and is focused by a parabolic mirror (focal length 250 mm), reflected from a flat mirror, and focused into the mercury-cadmiumtelluride (MCT) detector by a second parabolic mirror (focal length 43 mm). The first parabolic mirror in the detector chamber was also moved in parallel with the exit mirror (no. 9) in the spectrometer. The instrument also has the option to collect spectra using the internal sample compartment. The movable mirror (no. 8) is rotated out of the way allowing the beam to be reflected into the internal sample compartment. A second MCT detector as well as a deuterated triglycine sulphate (DTGS) detector (not pictured) are available for analyzes in the internal detector chamber. The internal sample compartment is separated from the interferometer and detector compartment by KBr windows. This enables the sample compartment to be vented for sample introduction without venting the rest of the spectrometer. The internal sample set up was not used in the experiments presented here. The spectrometer and external detector chamber were kept under vacuum, approximately 18 mbar, with a diaphragm pump (Vacuubrand, pumping speed 1.04 Ls-1) to remove background gases such as water and carbon dioxide.The UHV chamber is separated from the spectrometer and external detector chamber by KBr windows (no. 10, 55 mm φ × 5 mm, wedged 0.33°). These windows are housed in differentially pumped window flanges (McAllister) that give the option to pump the window separately. This was not necessary to achieve the desired pressure range for these experiments. The custom window flanges are connected to the UHV chamber via conflat flange flexible bellows (no. 11). An o-ring seals the connection between the spectrometer and the window flange.The UHV sample chamber is pumped by a turbo molecular pump (Varian Model 969-9008, pumping speed 250 Ls-1), which is equipped with a 6” gate valve. For introduction of the sample, the gate valve is closed and the chamber vented. Samples are introduced via a door on the front flange of the chamber. The slide is placed on the sample holder, which is mounted onto a manipulator (PHI model 10-504) that enables the translation of the surface in the x, y and z directions as well as manipulation of the rotation and tilt of the surface. When placing the surface onto the sample holder, it is important that the plane defined by the face of the slide be perpendicular to the plane of the beam. If this is not the case for either the background or sample slide, a sine wave will appear in the baseline of the spectrum. An example of such a spectrum is shown in Figure 2.2. In order to dampen vibrations of the manipulator during collection of spectra,an o-ring is held against the back of the sample holder by a linear translator on the back flange of the chamber.

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Chapter 1 Introduction and Motivation 
1.1 Background
1.2 Reactions of Nitrogen Dioxide with Alkenes
1.2.1 Solution Phase Mechanisms
1.2.2 Gas Phase Mechanisms
1.3 Self-Assembled Monolayers
1.4 Reactions in Monolayers
1.4.1 Steric Effects.
1.4.2 Reactions Starting at Defects
1.5 Reactions with Unsaturated Hydrocarbon Monolayers
1.6 Summary
Chapter 2 Instrumentation 
2.1 Grazing Angle FTIR Spectrometer- Vacuum Chamber 
2.2 Interlock Program
2.2.1 Reading Pressures
2.2.2 Pressure Set Points
2.2.3 Load Lock Chamber
2.2.4 Pressure Bursts
2.2.5 Cryo Pump
2.2.6 Mass Spectrometers and Electronics
2.2.7 Field Point Discrete Input Modules
2.2.8 Field Point Relays
Chapter 3 Nitrogen Dioxide Exposure of an Olefin-Terminated Monolayer
3.1 Introduction
3.2 Experimental
3.2.1 Synthesis of Olefin-Terminated Thiol
3.2.2 Preparation of Self-Assembled Monolayers
3.2.3 Exposure of Monolayers to Nitrogen Dioxide
3.2.4 X-ray Photoelectron Spectroscopy
3.3 Results
3.3.1 Olefin-Terminated Self-Assembled Monolayer
3.3.2 Olefin-Terminated Monolayer Exposed to Nitrogen Dioxide
3.3.3 NO2 Exposure of Mixed Olefin/ Methyl and Methyl-Terminated Monolayers
3.3.4 X-ray Photoelectron Spectroscopy Measurements
3.4 Possible Mechanism for the Reaction of NO2 with an Olefin-Terminated SAM
3.4.1 Via Hydrogen Abstraction
3.4.2 Via Radical Addition
3.5 Summary
References
Appendix

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