APPLICATION OF COHERENT ANTI-STOKES-RAMAN SCATTERING (CARS) TECHNIQUETO THE DETECTION OF NO

A coherent, anti-Stokes Raman scattering (CARS) setup has been develop ed to detect contamination of atmospheric nitrogen by nitric oxide (NO ). To allow spatially resolved measurements and the possibility of uti lizing windows close by the test volume, we chose the folded BOXCARS s etup with a CARS lens of focal length 0.5 m and a diameter of 80 mm. A frequency-doubled Nd:YAG laser (lambda = 532 nm; E-L = 50 mJ; tau(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 i ncluding the first hot band have been measured with high spectral reso lution in a temperature range from 300 to 800 K. The detection limit o f NO is on the order of 0.25% in nitrogen under atmospheric pressure. With suppression of the nonresonant background in the application of p olarization CARS, the detection limit could not be scaled down in a de sirable manner, The comparison between measured and calculated CARS sp ectra of NO in an N-2 surrounding confirms the reliability of the ener gy matrix elements and the Lorentzian width of the type Gamma(p,T = 0. 06(298 K/T)(0.6)p/p(0) cm(-1).