Development of a frequency-stabilized cavity ring-down spectrometer for direct measurement of HO2 radicals

The hydroperoxyl radicals (HO₂) play a crucial role in atmospheric chemistry. Direct in-situ measurement of HO₂ concentration using laser spectroscopy has always been a challenge, requiring very high detection sensitivity. In this presentation, we report the development of a frequency-stabilized cavity ring-down spectrometer (FS-CRDS) for the direct measurement of HO₂ concentration. The optical cavity of the spectrometer was made of perfluoroalkoxy (PFA) tube with an inner diameter of 9 mm. The distance between the two high reflectivity mirrors (double coated, with reflectivity R = 95% at λ = 632 nm and R = 99.998% at λ = 1506 nm) was about 51.4 cm, with one of the cavity mirrors mounted on a piezo-electric transducer (PZT). A stable red He-Ne laser with a frequency stability of ±2 MHz was used as the reference laser for the cavity length stabilization servo. A 1506 nm fiber laser was used as the probe laser. The probe laser beam was split into two beams: one beam was used to lock the probe laser to the stable cavity using Pound-Drever-Hall (PDH) locking method; the other beam passed sequentially through an acousto-optic modulator (AOM) and a fiber electro-optic modulator (EOM) for cavity ring down spectroscopy (CRDS) measurement. By tuning the frequency of the microwave source drive of the EOM, and using a frequency-agile, rapid scanning spectroscopy method, the laser sidebands were sequentially switched to different optical cavity models, thereby achieving rapid full-spectrum scanning. With a 1 s integration time, the spectrometer achieved a detection sensitivity of about 2.6×10^-11 cm^-1, which was about 12 times improved compared with normal CRDS system without electronic locking. The corresponding detection limit for HO₂ radicals was about 1.2×10⁸ molecule/cm³ (the absorption line for HO₂ detection was located at 6638.207 cm^-1, with a line strength of 7.09 × 10^-21 cm^-1/(molecule cm^-2)). This work demonstrates that FS-CRDS is a feasible technique for high sensitivity direct measurement of HO₂ radicals. Further improvements will be made in the future to enhance detection sensitivity.