The measurement of Rayleigh scattering and stable carbon isotope ratios using cavity ring-down spectroscopy.

摘要

This thesis describes two different projects using the technique cavity ring-down spectroscopy. In the first project, a cavity ring-down spectrometer has been built in the three mirror ring configuration. This configuration results in optical feedback to the laser from a backward traveling wave in the cavity that affects the properties of the laser in a beneficial way. The optical feedback causes the narrowing of the spectral width of the laser leading to a more efficient injection of light into the cavity. The intensity of light transmitted from the cavity with the optical feedback effect is approximately thirty times greater than without the optical feedback effect. The initial instrument was designed with a Fabry-Perot diode laser operating at 635 nanometers as the light source, which it was hoped would lead to a less expensive experimental setup than is traditionally used. The use of the Fabry-Perot laser led to complications that prevented accurate measurements from being made with the instrument. An external cavity diode laser operating around 655 nanometers was used with the instrument and it produced improved results due to the better stability of the laser. The baseline noise of the instrument using the external cavity diode laser is 2 x 10-10 cm-1.;An investigation of the suitability for using this instrument in the measurement of Rayleigh scattering of small molecules was undertaken. The measurement of the Rayleigh scattering cross sections for hydrogen, argon, nitrogen, oxygen, and carbon dioxide at 651 and 659 nanometers, which are on the order of 10-27 cm2, were on average within 7 x 10-29 cm2 of the cross sections calculated using the index of refraction. The accuracy of the measured cross sections is within a few percent of the amounts calculated using the index of refraction, but it is approximately ten times worse than predicted by the baseline noise in the cavity which is due mainly to the relatively long time required to make the measurements. Potential improvements for the measurement of Rayleigh scattering with the instrument are discussed.;In the second project, the measurement of stable carbon isotope ratios using cavity ring-down spectroscopy is described. An instrument consisting of a gas chromatograph, a combustion chamber, and a cavity ring-down spectrometer has been developed for the measurement of 13C/12C ratios in organic compounds, which can provide useful information on the formation of the compounds. The instrument has a precision of 0.95‰ and 0.67‰ for ethane and propane, respectively. The accuracy of the instrument is within 3‰ of the amounts measured with an isotope ratio mass spectrometer. With future improvements, this instrument could develop into an attractive alternative to mass spectrometry based instruments.