Demonstration of the extremely high signal-to-noise ratio and advanced O2 B-band line shape analysis in the PDH-locked FS-CRDS experiment

Abstract

We demonstrate an extremely high signal-to-noise ratio of 220000 : 1 obtained by the long-term spectra co-adding and averaging in the Pound-Drever-Hall-locked frequency-stabilized cavity ring-down experiment. The connection of this laser spectroscopy technique with the careful line-shape analysis revealed very subtle line-shape asymmetry of the R7 Q8 16O2 rovibronic B-band transition probed at pressure 933 Pa. This asymmetry could be ascribed to the speed dependence of the collisional shift, as will be shown. Reduction of the minimum detectable absorption coefficient of 2 × 10−10 cm−1 by an order of magnitude, according to the dependence, shifted the bottom limit of measured line intensities toward values as low as 1.3 × 10−30 cm−1/(molecule cm"2). The demonstrated measurement precision enabled the quantification of systematic line-shape deviations, which were approximately 1 part in 80000 of the peak absorption. The influence of slowly drifting etaloning effects on the accuracy of the line-shape analysis is discussed.