Application of Coherent Anti-Stokes Raman Scattering (CARS) Technique to the Detection of NO

Abstract

A coherent anti-Stokes Raman scattering (CARS) setup has been developed to detect contamination of atmospheric nitrogen by nitric oxide (NO). To allow spatially resolved measurements and the possibility of utilizing windows close by the test volume, we chose the folded BOXCARS setup with a CARS lens of focal length 0.5 m and a diameter of 80 mm. A frequency-doubled Nd:YAG laser (= 532 nm; E_L = 50 mJ; _L = 10 ns; repetition rate, 10 s⁻¹; bandwidth, 0.05 cm⁻¹) serves as pump for a dye laser (E_p = 25 mJ; E_L = 2 mJ; bandwidth, 0.03 cm⁻¹), which is tunable between 585 and 615 nm. Nitric oxide CARS spectra including the first hot band have been measured with high spectral resolution in a temperature range from 300 to 800 K. The detection limit of NO is on the order of 0.25% in nitrogen under atmospheric pressure. With suppression of the nonresonant background in the application of polarization CARS, the detection limit could not be scaled down in a desirable manner. The comparison between measured and calculated CARS spectra of NO in an N₂ surrounding confirms the reliability of the energy matrix elements and the Lorentzian width of the type (p, T = 0.06(298 K/T)^0.6 p/p₀ cm⁻¹.