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Appl Opt. 2015 Oct 10;54(29):8766-75. doi: 10.1364/AO.54.008766.

Shock-tube measurements of excited oxygen atoms using cavity-enhanced absorption spectroscopy.


We report the use of cavity-enhanced absorption spectroscopy (CEAS) using two distributed feedback diode lasers near 777.2 and 844.6 nm for sensitive, time-resolved, in situ measurements of excited-state populations of atomic oxygen in a shock tube. Here, a 1% O<sub>2</sub>/Ar mixture was shock-heated to 5400-8000 K behind reflected shock waves. The combined use of a low-finesse cavity, fast wavelength scanning of the lasers, and an off-axis alignment enabled measurements with 10 μs time response and low cavity noise. The CEAS absorption gain factors of 104 and 142 for the P<sub>3</sub>5←S520 (777.2 nm) and P<sub>0,1,2</sub>3←S310 (844.6 nm) atomic oxygen transitions, respectively, significantly improved the detection sensitivity over conventional single-pass measurements. This work demonstrates the potential of using CEAS to improve shock-tube studies of nonequilibrium electronic-excitation processes at high temperatures.


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