Two-color cavity ringdown spectroscopy: Optical detection of room-temperature $^{14}$CO$_2$ samples at parts-per-quadrillion concentration level

Abstract

Detection sensitivity is often one of the most important attributes to consider during selection of spectroscopic techniques. However, high sensitivity alone is insufficient for spectroscopic measurements and peak assignments in spectrally congested regions. Two-color cavity ringdown spectroscopy (2C-CRDS), based on intra-cavity pump-probe detection, simultaneously achieves high detection sensitivity, selectivity, and accuracy. Thanks to its built-in compensation for instrumental baseline fluctuations and one-photon background molecular signals, 2C-CRDS enables mid-IR detection of radiocarbon dioxide ($^{14}$CO$_2$) in $room$-$temperature$ CO$_2$ samples, with better than 10 parts-per-quadrillion (ppq) measurement $sensitivity$ and $accuracy$ (4 ppq on average from four separate trial measurements). This detection capability is prohibitively challenging for other existing spectroscopic techniques based on one-photon absorption, because of severe spectral overlap between the very weak $^{14}$CO$_2$ signals and the orders-of-magnitude stronger hot-band signals from other CO$_2$ isotopologues at room temperature. 2C-CRDS measurements of room-temperature $^{14}$CO$_2$ samples at the record sub-10-ppq concentration level could directly benefit a wide range of scientific fields that utilize $^{14}$C for dating and isotope tracing, especially during field deployment. The 2C-CRDS technique significantly enhances the sensitivity and general utility of high-resolution mid-IR detection for analytical trace measurements and fundamental chemical dynamics studies.Comment: 14 pages, 5 figures, 1 tabl