15min:

HAIFENG HUANG, Department of Chemistry, University of Virginia, Charlottesville VA, 22904-4319; KEVIN K. LEHMANN, .

Allan variance can be used to characterize the slow drift of CW cavity ring-down spectroscopy (CW-CRDS) system, caused by the instability of experimental environments. By including the differential measurement of on- and off-peak decay rates in the system, the drifting of both decay rates can cancel out greatly. A sensitivity of 5.6 × 10^-12 cm^-1 during an optimum integration time of 15.4 minutes has been obtained with our CW-CRDS system. This sensitivity corresponds a 3 methane detection limit of 0.3 parts per billion by volume (ppbv) in N₂ at 20 torr or 37 parts per trillion by volume (pptv) in N₂ at 760 torr measurement pressure, near 1.65 µm laser wavelength.