title: Measurements of OH radicals in a low-power atmospheric inductively coupled plasma by cavity ringdown spectroscopy.
abstract: Cavity ringdown spectroscopy is applied to line-of-sight measurements of OH radicals in an atmospheric-pressure argon inductively coupled plasma, operating at low power (200 W) and low gas flows (approximately 18 liters/min). Density populations of the single S21(1) rotational line in the OH A2sigma(+)-X2Pi (0-0) band are extracted from the measured line-of-sight absorbance. Plasma gas kinetic temperatures, derived from the recorded line shapes of the S21(1) line, ranged from 1858 to 2000 K with an average uncertainty of 10%. Assuming local thermodynamic equilibrium, an assumption supported by the comparison of the experimental and simulated spectra, the spatially averaged total OH number density at different observation heights was determined to be in the range of 1.7 x 10(20)-8.5 x 10(20) (m(-3)) with the highest OH density in the plasma tail. This work demonstrates that ringdown spectra of the OH radical may be used both as a thermometer for high-temperature environments and as a diagnostic tool to probe the thermodynamic properties of plasmas.
authors.affiliation: Diagnostic Instrumentation & Analysis Laboratory, Mississippi State University, 205 Research Blvd., Starkville, Mississippi 39759, USA. wang@dial.msstate.edu
authors.forename: Chuji
authors.initials: C
authors.lastname: Wang
authors.forename: Fabio J
authors.initials: FJ
authors.lastname: Mazzotti
authors.forename: Sudip P
authors.initials: SP
authors.lastname: Koirala
authors.forename: Christopher B
authors.initials: CB
authors.lastname: Winstead
authors.forename: George P
authors.initials: GP
authors.lastname: Miller
mesh_terms: D001128:Argon / Q000737:chemistry; D006358:Hot Temperature; D017665:Hydroxyl Radical / Q000032:analysis*; D013057:Spectrum Analysis / Q000379:methods*
chemical_list: D017665:Hydroxyl Radical; D001128:Argon