Femtosecond ultraviolet laser absorption spectroscopy for simultaneous measurements of temperature and OH concentration

This Letter reports a femtosecond ultraviolet laser absorption spectroscopy (fs-UV-LAS) for simultaneous in situ measurements of temperature and species. This fs-UV-LAS technique was demonstrated based on X-2 pi-A(2)sigma(+) transitions of OH radicals near 308 nm generated in low temperature plasmas and flames. The fs-UV-LAS technique has revealed three major diagnostic benefits. First, a series of absorption features within a spectral bandwidth of ~& nbsp;3.2 nm near 308 nm were simultaneously measured and then enabled simultaneous multi-parameter measurements with enhanced accuracy. The results show that the temperature and OH concentration could be measured with accuracy enhanced by 29-88% and 58-91%, respectively, compared to those obtained with past two-narrow-line absorption methods. Second, an ultrafast time resolution of ~& nbsp;120 picoseconds was accomplished for the measurements. Third, due to the large OH X-2 pi-A(2)sigma(+) transitions in the UV range, a simple single-pass absorption with a 3-cm path length was allowed for measurements in plasmas with low OH number density down to ~& nbsp;2 x 10(13) cm(-3). Also due to the large OH UV transitions, single-shot fs absorption measurements were accomplished in flames, which was expected to offer more insights into chemically reactive flow dynamics. Published under an exclusive license by AIP Publishing.