Total Internal Reflection Absorption Spectroscopy (TIRAS) for the Detection of Solvated Electrons at a Plasma-liquid Interface

The overall goal of this experiment is to directly detect solvated electrons in the plasma-liquid interface. This method can help answer important questions regarding the free radical chemistry created by a plasma in contact with an aqueous solution. The main advantage of this technique is that it provides direct measurement of a short lived free radical, the solvated electron, though its absorption of light.

The implications of this technique extend toward medicine, as low temperature plasma is now being tested as a form of cancer treatment. This method can also provide insight into chemical synthesis techniques, as low temperature plasmas are used in many indocile processes. For this method, use a custom plasma electrochemical cell, two inches in diameter, with two optical windows, at angles of approximately 20 degrees down from the normal plane.

Use a well fitting PTFE cell lid, with four ports for the electrodes and gas lines. Use an anode made of a piece of platinum foil attached to a stainless steel rod and a cathode made of a sharpened stainless steel capillary. Use as the absorption spectroscopy light source a 670 nanometer diode laser in line with an adjustable iris and a 50 millimeter lens, all mounted in a 30 millimeter optical cage system mounted on goniometer to allow adjustment of the angle of incidence.

Use as a photodetector a large area photodiode wired in reverse bias leakage circuit with a 670 nanometer bandpass filter in front of the detector. Mount the photo detector on a second goniometer. Ensure that the goniometers are positioned so that the laser will be directed into one of the optical windows of the electrochemical cell and the light will be reflected though the other optical window to the photo detector.

Use a solid state relay circuit to modulate the plasma current at a 20 kilohertz carrier frequency, including a lock in amplifier to obtain a sufficiently high signal to noise ratio to detect the extremely small optical absorbent signals of solvated electrons. Lastly, design an automated data collection program to control the instrument, and record signals from the lock in amplifier. To begin the set up, place in the electrochemical cell 60 milliliters of a 0.163 molar solution of sodium perchlorate and deionized water.

Insert the platinum foil anode into the electrochemical cell lid, and place the lid on the cell. Insert the platinum foil anode into the electrochemical cell lid. Submerge the anode about one centimeter below the solution surface.

Ensure that only the platinum foil contacts the solution. Then, connect the blunt end of the stainless steel plasma electrode capillary to an argon gas line via a mass flow meter. Insert the sharp end of the plasma capillary through the lid so that the point is about one to two millimeters above the surface of the solution.

Confirm the distance between the solution and the capillary tip with a camera. Then connect a narrow hose to an argon gas line via another mass flow meter. Insert the hose through one of the ports in the electrochemical cell lid to serve as the purge line.

Leave the fourth port unobstructed as a vent. Ensure that the lid will not move during the experiment. Then complete the electrochemical circuit by connecting the capillary to the voltage source and the anode to the ground.

Purge the cell with argon at 250 cubic centimeters per minute for at at least five minutes before preceding. Then set the argon gas flow through the plasma capillary to about ten cubic centimeters per minute. Turn on the diode laser, and align the laser so that it will strike the plasma liquid interface as indicated by light being scattered off the dimple by the argon flow.

Stop the flow of argon through the capillary and wait for the laser spot to return to its normal size. Leave argon flowing through the main purge line at 250 cubic centimeters per minute. Align the photo detector so that the laser reflection hits the center of the detector.

Connect the photo detector output to a volt meter. And measure the voltage produced by the absorbent signal by the laser. Record the voltage to be used later to normalize the absorbance data from the lock in amplifier.

The lock in amplifier essentially measures an AC voltage from the photo detector. This voltage must be normalized by the absolute intensity of the laser, measured as the DC offset of the detector. Then, connect the photo detector output to the lock in amplifier input.

Connect the transistor transistor logic output of the function generator to the frequency input of the lock in amplifier. Turn on the function generator, lock in amplifier, and high voltage power supply. Ensure that the instruments are communicating with the data collection program and that the lock in amplifier is locked with a signal at the 20 kilohertz carrier frequency.

Initiate the experiment and monitor the system as the laser is automatically turned on and the plasma is ignited. Observe the recorded signals, as steady state data is recorded both with and without the laser. We only expect to see the optical absorbency from solvate electrons when the laser and the plasma are both on.

Therefore, the signal with only the laser and no plasma, or vice versa, should be relatively small by comparison. Once automated data collection has finished, average the data, subtract noise, and calculate the concentration of solvated electrons in the solution over time. Information about amplitude and phase relative to the switching circuit was obtained from the cosine and sine components of the absorbent signal, or x and y.

The signal amplitude could then be used to estimate the concentration of solvated electrons and reaction rate constants of various chemical species. The amplitude of the absorbance was about 10 parts per million, emphasizing the need for a sufficiently large signal to noise ratio for a successful experiment. If unknown noise significantly affects the signal, it is best to discard the data and repeat the experiment.

Once mastered, a measurement with this procedure should take no more than 30 minutes. It is critical to provide an inert atmosphere such as argon, and to properly align the laser. Trace amounts of oxygen can quickly quench any free electrons in the plasma and prevent them from entering the solution.

It should also be noted that this method does not directly measure concentration of solvated electrons. Rather, it measures the AC amplitude of optical absorbance, which is proportional to the AC amplitude of the 20 kilohertz plasma current. After its development, this technique has yielded important insights into the fast reaction kinetics of solvate electrons at a plasma liquid interface.

We would eventually like to extend this to other free radicals produced by plasmas in contact with liquids. Lastly, don't forget that working with high voltages can be hazardous. Precautions such as grounding the optical table and other equipment should be taken before performing this procedure.