Electron Scattering Cross Sections Experimental Apparatus: : Data collection procedure

2.7 Data collection procedure

The measurement of the total cross section requires accurate measurement of P, L, I_o and I as described by equation 1.8 in chapter 1. The following procedure was adopted for the data collection. First the RGA was turned on and allowed to warm-up for 15 minutes and then checked for purity inside the system. The gas cell and gas delivery lines were purged using the tee purge assembly by filling with the target gas. This was done by first filling the gas lines and gas cell with target gas and keeping the gas inside the system for about 3 seconds. The gas was then pumped from the system. This flushing was carried out using the target gas 5 - 8 times before starting the experiment in order to minimize the atmospheric contaminants within the system. Next, using the RGA, the purity of the target gas inside the system was checked as follows. Prior to admitting the target gas into the gas cell, the background N₂ level in the system was recorded. This is normally about 3 x 10^-7 mTorr. Then the target gas was admitted to the gas cell until the RGA level was 3 x 10^-5 mTorr or more. At the same time the N₂ level in the system was obtained from the RGA. It is normally 2 x 10^-7 mTorr or lower. A reduced N₂ pressure yields a reduction in the out-gassing inside the system and no N₂ is introduce to the system from outside through gas delivery line. From these observed RGA partial pressures, it was verified that 99.5% of target gas was present inside the gas cell during the experiment. Next, the ESA power supply was turned on and the ESA circuit voltage (DV) was set to a predetermined value using a potentiometer. As an example 44.4 Volts were set and checked by using CIRCUITMATE a model DM25L multimeter so that 100 eV electrons were allowed to be transmitted through the ESA. The retarding bias voltage was set to less than 100 V from the electron energy using the computer. For example, to study the 2000 eV electron energy the retarding bias voltage was set to 1900 Volts. The electron gun is turned on next and is first to an electron emission current of about 10 mA. The electron energy is then set close to the energy of interest. The system was allowed to run for another 30 minutes until the electron gun filament was warmed-up. After the filament was fully warmed, the computer control interface bias voltage was matched with the electron gun energy to get the maximum current. The emission current of electrons was set to get 100 pA on the Faraday cup.

The electron beam passed through the gas cell in the absence of the target gas and the current intensity (I_o) was measured on the Faraday cup. The gas cell was then filled to a selected pressure of the target gas and the attenuated electron beam intensity was measured. Gas was present inside the gas cell for not more than 3 seconds. After taking the attenuated current measurement the gas was pumped immediately to avoid secondary scattering and any other possible contamination of the electron gun filament. Before moving into the next measurement, the gas inlet was closed and the gas cell was pumped down to 5 x 10^-7 Torr or better and the electron beam intensity was again measured to make sure it had returned to the original current (I_o). This procedure was followed for every set of I_o and I measurements. The time required to return to the original current depended on the type of gas present and the gas pressure. For higher gas pressures it required about 5 minutes. At each electron energy, I_o and I values were measured for eight to ten different gas pressures. To study the next energy, first the bias voltage was set to less than 100 V from the electron energy of interest. Then the electron energy of the gun was increased to the desired energy. With the increasing electron energy there was a small drop in the emission current in the electron gun. At higher energies the electron emission current had to be increased up to 30 mA to get the maximum current of 100 pA. Using the computer control interface the bias voltage was matched with the electron gun energy to get the maximum current.

During the experiment the temperature near the gas cell was measured frequently using a mercury-glass thermometer. The normal temperature was about 22°C. No sudden changes in the temperature were observed during the experiment but there were some daily variations in the temperature. Daily temperatures were recorded and used in the determination of the number of gas molecules in the gas cell.