We use several different techniques to characterize gas phase species in our plasmas. One of these techniques is called optical emission spectroscopy (OES) and makes use of the light emitted by excited state species in the plasma as they relax back to the ground state. The wavelength of the emitted light is characteristic of species present in the plasma, and the intensity of the emitted light can be related to their number density. This can all be done in a barrel-type glass reactor similar to the ones we use for sample processing, but in this case, a quartz window is incorporated into the reactor to allow better transmission of the light at low wavelengths.

This picture shows another system we can use to characterize gas-phase species in the plasma. Part of this system consists of a barrel-type glass reactor (oriented vertically in the upper right part of the picture), but the particular arrangement used on this system allows the incorporation of a mass spectrometer. Thus, we’re able to use mass spectrometry to analyze ionized and neutral species that are generated in the plasma. In addition, this system can accommodate the Langmuir probe that we have in our lab. This probe provides information about plasma properties including the electron temperature, ion density, and plasma potential.

One of the more unique experiments we are able to do in our lab is the imaging of radicals interacting with surfaces (IRIS) technique. This apparatus is shown in the picture below and is housed on the basement level of the Chemistry building. The experiment has several different parts and is easier to visualize in the schematic shown in the lower left. As the feedgas becomes fragmented, the species that are generated move through a set of metal slits to form a molecular beam as it enters the main chamber (we also call this the interaction region). The plasma molecular beam is intersected by a laser beam entering the chamber at a 45° angle. This laser beam, which is focused by a lens outside the vacuum chamber, is tuned to a specific wavelength so that it will excite fluorescence in a certain molecule in the molecular beam. By collecting spatially-resolved images of this fluorescence intensity with and without a sample substrate in the path of the molecular beam, we’re able to determine a molecule’s surface reactivity, which describes the probability that a molecule will react at a surface. This non-intrusive technique provides valuable insight into some of the subtle processes that take place during the plasma treatment of a sample. With some adjustments, we can also use this apparatus to collect fluorescence excitation spectra and measure the gas phase properties of specific molecules in the molecular beam. These properties include relative number densities, translational temperatures, and rotational temperatures.