Optimal spectral region for real-time monitoring of sub-ppm levels of water in phosphine by cavity ring-down spectroscopy

Abstract

We have utilized cavity ring-down spectroscopy (CRDS) to characterize the pressure and carrier gas effects on the shape of a water absorption line. The half-width at half-maximum pressure-broadening coefficient for the water line at 10687.36 cm −1 was measured to be 58±6 MHz kPa −1 (0.20±0.02 cm −1 atm −1) for water in phosphine (PH 3). This value is twice as large as the pressure-broadening coefficient for the same absorption transition in the case of water in air. Strong interference from neighboring PH 3 lines limits the sensitivity of the system in the region of this absorption line. We have also characterized the spectral neighborhood of several other water absorption transitions for the water–PH 3 system; these additional water–PH 3 CRDS spectra are presented. We have identified the spectral region around the water line at 10667.76 cm −1 as optimal for CRDS measurements of the water–PH 3 system. Minimal interference from adjacent PH 3 absorption transitions in this region enables high-sensitivity, real-time measurements of trace water in bulk PH 3. Utilizing this water line, our CRDS apparatus has an estimated detection limit of 50 nmol mol −1 H 2O in PH 3.