Excited oxygen kinetics at electronvolt temperatures via 5-MHz RF-diplexed laser absorption spectroscopy

A multi-MHz laser absorption sensor at 777.2 nm (12,863c m ^-1) is developed for simultaneous sensing of (1) O(⁵ S ⁰) number density, (2) electron number density, and (3) translational temperature at conditions relevant to high-speed entry conditions and molecular dissociation. This sensor leverages a bias tee circuit with a distributed feedback diode laser and an optimization of the laser current modulation waveform to enable temporal resolution of sub-microsecond kinetics at electronvolt temperatures. In shock-heated O ₂, the precision of the temperature measurement is tested at 5 MHz and is found to be within ±5% from 6000 to 12,000 K at pressures from 0.1 to 1 atm. The present sensor is also demonstrated in a CO:Ar mixture, in parallel with a diagnostic for CO rovibrational temperature, providing an additional validation across 7500-9700 K during molecular dissociation. A demonstration of the electron number density measurement near 11,000 K is performed and compared to a simplified model of ionization. Finally, as an illustration of the utility of this high-speed diagnostic, the measurement of the heavy particle excitation rate of O(⁵ S ⁰) is extended beyond the temperatures available in the literature and is found to be well represented by k(³ P→⁵ S ⁰)=2.7×10^-14 T ^0.5 exp⁡(-1.428×10⁴/T)c m ³⋅s ^-1 from 5400 to 12,200 K.