Invited Paper VT-MoA1
Low Uncertainty Measurements of Trace Water Vapor Based on Cavity Ring-Down Spectroscopy

Monday, October 31, 2011, 2:00 pm, Room 111

I will discuss how cavity ring-down spectroscopy (CRDS) can be applied to accurately measure the concentration of residual water vapor which is present in a vacuum system or process gas stream. In CRDS, a monochromatic laser beam is injected into an evacuated or sample-gas-containing optical resonator and the transient decay of light exiting the cavity is monitored to quantify the optical losses. For water detection, the laser wavelength is tuned to probe characteristic rotation-vibration absorption features of the water molecule. The sample absorption coefficient is determined from observations of the ring-down cavity decay time and laser frequency, both of which can be precisely measured. Also, because CRDS uses a resonant optical cavity, extremely long effective optical pathlengths (up to tens of km) can readily be achieved in the laboratory. These properties make CRDS a high-spectral resolution, species-selective method, with relatively small combined uncertainty, and high sensitivity. I will show that when CRDS measurements are combined with first-principles spectroscopic models, this technique can yield concentration measurements with sub-percent-level relative uncertainty for absolute concentrations as low as 10¹¹ cm^-3. I will present examples of CRDS-based trace water detection in vacuum and ultra-high purity gas systems, and I will talk about recent CRDS measurements of the vapor pressure of ice over the temperature range 0 deg C to -100 deg C.