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

Applied optics

PubMedID: 26479817

Nations M, Wang S, Goldenstein CS, Sun K, Davidson DF, Jeffries JB, Hanson RK. Shock-tube measurements of excited oxygen atoms using cavity-enhanced absorption spectroscopy. Appl Opt. 2015;54(29):8766-75.
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% O2/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 P35?S520 (777. 2 nm) and P0,1,23?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.