Cavity ring-down spectroscopy (CRDS) is an ultrasensitive absorption technique that is capable of measuring absorption changes of 10-10 cm-1. In the simplest form of CRDS, two highly reflective mirrors face one another to form an optical cavity. A laser pulse enters the cavity through the back of one mirror and when sufficient intensity has built up, the laser is turned off, deflected, or blocked. The light in the cavity oscillates back and forth, leaking out a small amount of light. The ring down time, or the rate constant of the exponential decay of the light intensity, depends upon all losses of light within the optical cavity. These losses include mirror transmissions, absorption by the chemical sample, and reflection and scattering caused by the sample.
In most CRDS experiments, the absorbance of the sample is determined to measure a trace amount of a species or to resolve a weak absorption peak that is below the detection limit of traditional absorption techniques. We are interested in using CRDS to look at losses caused by the sample other than absorption, more specifically losses caused by Rayleigh scattering. Much of the theory of Rayleigh scattering was developed over 100 years ago. It has been difficult, however, to make direct measurements in the laboratory owing to the small cross section. The extended path length of CRDS makes it possible to measure the total loss caused by atoms or molecules in the gas phase within the cavity. By operating in regions where there are no absorption peaks, the total loss observed is caused primarily by Rayleigh scattering from which the Rayleigh scattering cross section can be determined.
A three mirror CRDS cavity in the ring configuration has recently been built (Figure 1). This configuration adds more complexity to the setup, but it also provides some advantages. One benefit is it results in a small amount of optical feedback to the laser that can used to affect the properties of the laser, which can lead to a more efficient injection of light into the cavity. By measuring the ring down time when the cavity is filled with a sample gas and seeing how the ring down time changes as the contents are replaced with a different sample gas, the magnitude of the Rayleigh scattering cross section can be determined.