Cavity enhanced absorption spectroscopy (CEAS) allows the accurate study of non-equilibrium, high temperature gas processes (typical, for example, of the environment near hypersonic vehicles). The combination between a resonant cavity with a special off-axis alignment and a fast wavelength-scanning laser scheme, as the one devised by M. Nations and co-authors, enables direct measurements of atomic oxygen transitions from excited electronic states generated in shock tubes with fast response and high sensitivity. Shock-tube experiments can be used to simulate aerodynamic flow and combustion reactions of gases under highly-transient conditions and a wide range of temperatures and pressures that are difficult to obtain with other tools. Using the CEAS approach, the authors provide quantitative information on the chemical kinetics of oxygen atoms that form after dissociation of the molecular dimer in a very-high temperature environment (at temperatures larger than 5000 K): population levels, translational temperature, collision-broadening coefficients are retrieved from in-situ absorption spectra. Their spectroscopic scheme proves non-intrusive, highly-selective and extremely efficient thanks to the sensitivity gain of CEAS as well as the low measurement noise associated with the use of a moderate-finesse cavity. Its capability can readily be extended to non-equilibrium excited-state kinetic studies of many other neutral and ionized species in hypersonic flowfields.
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