Development of a trans-rotational temperature diagnostic for vibrationally-excited carbon monoxide using single-photon laser-induced fluorescence
A new trans-rotational temperature diagnostic with ±50K accuracy has been developed for use in nonequilibrium, low temperature, monatomic gases seeded with carbon monoxide (CO). The scheme utilizes single-photon laser induced fluorescence (LIF) of CO under vibrationally-excited conditions in which single-photon transitions from the CO X1Sig+ ground electronic state to upper electronic A1Pi or D'1Sig+ states become accessible to a tunable, narrowband ArF excimer laser at 193 nm. Two vibrationally excited environments in which the chemistry is well understood were used as a testbed; an optically-pumped 3% CO/Ar plasma at 100 torr and a 4% CO/He d.c. glow discharge at 8 torr. For the optically-pumped CO/Ar plasma, a spatially-averaged temperature of 536±103 K (2sig) was obtained from rotationally resolved X1Sig+(v"=20)-D'1Sig+(v'=2) LIF excitation spectra. Temperature measurements pumping the X1Sig+(v"=7)-A1Pi(v'=1) 4th Positive (528±51 K) were also found to compare well with line-of-sight Fourier Transform-InfraRed (FT-IR) emission measurements (536±10 K). Spatially averaged FT-IR spectroscopy of the CO 1st overtone was used to verify a vibrational population of ~0.1% within the positive column of the CO/He d.c. glow discharge. The A-X (7,1) transition was pumped and subsequent (8,1) emission at 200.8 nm collected. Spectral peaks were assigned and used to determine a spatially averaged rotational temperature of 432±44 K on the discharge centerline. This was found to be in good agreement with FT-IR spectroscopy measurements (395±10 K). As a prelude to Planar-LIF (PLIF) temperature measurements, vibrationally-resolved emission from laser excitation of various rotational lines within the A-X and D'-X bands were used to investigate spectral interferences. This information was used to determine that a simple aqueous organic filter (urea) in the A-X case, or commercial glass filter (UG-11) in the D'-X case, are adequate for rejecting elastically-scattered radiation and extraneous ro-vibrational bands during PLIF imaging. Single-shot and accumulated PLIF images were obtained for the 3% CO/Ar optically pumped plasma. Using a Two-Line ratio method, A-X band Q-branch images were used to spatially resolve the trans-rotational temperature. The single-shot precision was ~±200 K (2s), the accuracy ~±100 K.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:laser induced fluorescence temperature planar argon fluoride excimer spectroscopic diagnostic carbon monoxide fourth positive bands vibrationally excited
Date of Publication:01/01/2004