The relevant research work was published in Optics Express.
Hydroxyl (OH) radicals are important active oxidants in the atmosphere and are important reaction chain initiators in the oxidation of most compounds. In order to study their reaction processes and mechanisms in depth, they must be measured directly in "real time", that is, to study them on the time scale of their actual existence. Therefore, detection techniques must be able to respond to rapid changes in the concentration and lifetime of OH radicals.
To address this problem, the research team developed a frequency-modulated Faraday rotational spectroscopy technique for highly sensitive and rapid measurement of OH radicals, which is characterized by a high measurement bandwidth, high sensitivity, and good selectivity, and is particularly suitable for kinetic studies of short-lived intermediate radicals.
CHENG Feihu, member of the team further explained how the spectrometer worked.
"We used a special laser beam that modulated at a very high frequency, up to 150MHZ," he said.
In this study, they used the spectroscopic technique to measure the time-resolved spectral signal of OH radical near 2.8 μm. The detection limit for OH reached 6.8×108 molecules/cm3 (1σ, 0.2 ms) after three pulse averaging, and decreased to 8.0×107 molecules/cm3 after 100 event integrations, which was in general agreement with the trend of the system performance in the white noise limit.
This work will provide a new and alternative tool for the study of radical dynamics, not only for OH radicals but also for other paramagnetic transient molecules, according to the team.
Schematic of the frequency-modulated Faraday rotational spectrometer setup. (Image by CHENG Feihu)
Concentration-time profile of OH radical obtained after averaging 3 pulses at a 200 μs sampling interval. (Image by CHENG Feihu)