Flow cell

The Pyrex cell measured a total length of 175 cm with an inner diameter of∼5 cm and was supported by two aluminum boxes, which were connected to either ends of the cell via O-ring seals. The entire box-cell-box volume was enclosed by wedged CaF2windows attached to the boxes and the total window-to-window length of the cell was 208 cm [2]. The cell was jacketed to allow for temperature control using liquid nitrogen-chilled methanol as the coolant, and an insulated cover encased the

Figure 2.1: Schematic of the IRKS apparatus. A pre-cooled gas mixture entered the temperature-controlled Pyrex flow tube, where radicals were generated by pulsed excimer laser photolysis (beam path shown in blue). Three independent absorption probe beams were used to temporally monitor the reactant and product species: NIR and MIR light (solid and dashed red lines) from two diode lasers were coupled to multi-pass Herriott optics and broadband UV light made a single-pass through the reaction cell (purple). Flip mirrors were used (red rectangles) to switch between the two IR lasers.

entire length of the jacketed region. The temperature inside the reaction cell was measured by a calibrated type T thermocouple (Omega) that was inserted into the jacket of the cell and in contact with the coolant. The pressure inside the cell was measured by a calibrated absolute capacitance manometer (MKS-220CA 1000 Torr, accuracy: 0.15% of reading).

Known concentrations of precursor gases were introduced into the cell using calibrated flow controllers (MKS 1149, 1147, GE50, and GM50 series). Vapors from condensed-phase precursors (e.g., CH3OH, H2O, CH3CHO, CH3C(O)CH3) were introduced into the cell by flowing N2gas through glass bubblers containing the liquid compounds, held inside temperature-controlled baths. The bubbler pressures and temperatures were measured with calibrated absolute capacitance manometers (MKS-220CA 1000 Torr) and type T thermocouples (Omega), respectively, and the concentrations were calculated from the vapor pressures using the Antoine equation.

The reagent gases were pre-mixed and pre-cooled in a meter-long jacketed tube prior to entering the main reaction cell through an off-center inlet port and were

pumped out via two exit ports on either sides of the cell. The distance between the centers of the exit ports was 138 nm. Room temperature dry nitrogen purge gas flowed through both aluminum chambers to confine the reagent gases to the temperature-controlled region. The purge gas also served to protect the Herriott mirrors housed inside the aluminum boxes from corrosion from caustic gases such as Cl2and from condensation at lower temperatures. The optimum gas flow conditions for confining the reactant gas between the two exit ports were determined by previous measurements performed by Dr. Aaron Noell [3]. Experiments were typically conducted using purge gas flows that were 100% of the total flow. During non- operation, the flow cell was held under vacuum using a scroll pump (Edwards nXDS15i) in order to protect the Herriott mirrors from deterioration by outgassing caustic chemicals adsorbed on the walls of the cell.

2.2.1 Path length and temperature profile

At low temperatures, the room temperature purge gas flowing through the aluminum chambers prevented condensation of water on the Herriott mirrors; however, this protective measure also introduced temperature gradients in the flow cell. Characterization of the path length was explored at the beginning of my thesis work in collaboration with Dr. Aaron Noell and Professor Fred Grieman. The reactant path length as well as the length and locations of the mixing regions between the purge and reactant gas flow were determined by measuring the temperature profile of the gas along the length of the cell using a T-type thermocouple probe attached to the end of a long stainless steel rod. The temperature probe was inserted into the cell through a flange mounted at one end of the cell to allow controlled movement of the probe along the entire length of the cell, while simultaneously providing sufficient seal to maintain constant pressure inside the cell. The temperature at various points along the length of the cell was recorded at a total pressure of 100 Torr and 300 Torr (10 s residence time) with the main gas at 213 K and the purge gas at room temperature (Figure 2.2). The purge gas was flowed at 100% of the total flow of the reagent gas. All thermocouples were calibrated with a precision platinum resistance thermometer (Omega RTD HH376 PT).

As shown in Figure 2.2, while the cell temperature continued to decrease beyond

∼10 cm past the first pump-out port (TCprobe position = 0 cm), the temperature remained low until past the second pump-out port (TCprobe position = 138 cm). One possible explanation is that because the reagent gas inlet port was not centered with respect to the pump-out ports, the gas near the left side of the cell was slightly

Figure 2.2:Temperature profiles of the cell at P=100 Torr (top panel) and at P=300 Torr (bottom panel) measured by the movable thermocouple probe (TCprobe: blue

markers). The temperature readings by the stationary thermocouple (TCport: red

markers), located at∼13 cm from the edge of the port (marked by red dashed line),

are plotted as a function of the probe position indicating that the temperature of the cell remained constant during the experiment. Dashed lines are shown with the schematic of the flow cell to visually map the location of TCprobeinside the flow cell where the temperature measurements were taken.

warmer than the right side. The gas was pre-cooled prior to entering the cell except in the small section where the pre-cooling glassware joins to the cell. This was likely causing some heat transfer, which is demonstrated by the small rise in temperature (∼3-5◦C) located at TCprobeposition = 46 cm. The deviation in temperature at this position was not considered to be a significant source of error.

The total path length, including the entire mixing region, was∼160 cm at 100 Torr and ∼170 cm at 300 Torr. While the path length between the midpoints of the mixing region was∼150 cm for both 100 Torr and 300 Torr, the mixing region extended over a wider volume at 300 Torr: comparing the temperature gradients on the right side of the cell, the length of the mixing region was∼10 cm at 100 Torr and

suspicions that the rate constants of the HO2 and RO2 self-reactions measured at higher pressures by the IRKS apparatus were unreliable due to the presence of mixing regions in the cell that caused large concentration gradients. The flow conditions that most reliably contained the reagent gas were concluded to be 100 Torr, 10 s residence time, and 100 % purge. Under these conditions, the absorbance of Cl2at 320 nm (σ =2.37×10−19 cm2) gave a path length of 143±4 cm. For the kinetics experiments conducted for this thesis, a path length of 147±10 cm was used (an average of the path lengths determined from Cl2absorption and from the temperature profile experiments).

The temperature profile measurements also revealed that temperature reading by TCprobe depended on how well the thermocouple was positioned in the cell. Due to the lack of coolant flowing in the port through which the thermocouple entered the cell, the thermocouple had to be threaded sufficiently far from the port to get an accurate measurement of the temperature of the gas inside the cell. For example, the temperature readings by TCportwhen it was positioned 1 cm away from the edge of the port were 10◦C warmer than the readings by TCprobe. At the same time, care had to be taken to ensure that the thermocouple tip was not touching the wall of the cell or blocking any of the optical paths of the IR, UV, and excimer beams. Due to the physical challenges of getting TCportplaced in the correct position, a separate thermocouple was placed inside the jacket of the cell for measuring the cell for all kinetics experiments conducted in this thesis. This thermocouple was in contact with the coolant and was calibrated against the temperature measured by TCport. During calibration, TCportwas positioned∼13 cm away from the edge of the port (as in Figure 2.2), where it was clear that TCportwas correctly measuring the cell temperature since its temperature readings matched the TCprobereadings.