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

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; EL = 50 mJ; L = 10 ns; repetition rate, 10 s−1; bandwidth, 0.05 cm−1) serves as pump for a dye laser ( Ep = 25 mJ; EL = 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.

Self-broadening in singlet spectral lines of helium

The emission profiles of five lines in the singlet spectrum of helium, λλ492.2, 501.5, 504.7, 667.8 and 728.1 nm, have been ’studied under high resolution to test some aspects of line-broadening theory, particularly as it is applied to the resonance interaction. The source was a weak d.c. discharge at 77 or 273 K; the pressure range was 0-10 Torr. Self-absorption was shown to be negligible in separate experiments. Profiles were recorded digitally and analysed by computer into Lorentzian and Gaussian components. The results show that the observed profiles of all the lines are influenced to varying degrees by excitation mechanisms; a discussion of these effects is given. They prevent a quantitative interpretation of λλ492.2 and 667.8 nm, and limit the accuracy of the results for the other lines. Of these, λ728.1nm shows the closest approach to pure resonance broadening; the measured broadening constants are 7.7 ± 0.2 (77K) and 9.3 ± 0.9 (273 K), both in units of 10 -20 cm -1 cm 3 per atom. This line also shows at 77 K a shift to the red of 0.11 ± 0.03 in the same units; this and the temperature dependence are attributed to a non-resonant contribution to the interatomic potential. Preliminary results of recent calculations based on a model potential are in fair agreement with our data. The Lorentzian width of λ728.1nm extrapolated to zero density is found to exceed the radiation width by (1.2 ± 0.5) 10~ 3 cm_ 1 , in agreement with the results of earlier workers, but the possibilities of systematic error (apparatus function, excitation mechanisms) reduce the significance of this finding. Possible experiments to circumvent the difficulties encountered in the present work are discussed.

The Resonant Faraday Effect

The resonant Faraday effect occurs for radiation at frequencies close to the absorption lines of a magneto-active material. In the paper we present a theory for isolated absorption lines with Voigt profiles. The background source is also assumed to have a Voigt profile. The theory shows how the resonant Faraday effect can be used to determine relative transition probabilities, as well as the magnitude of the Lorentzian component of the line profiles.Experimental results for an argon plasma are presented and show that the relative transition probabilities of atomic transitions with a common lower level can be measured to an accuracy of ± 1%. For strong absorption lines, the Voigt "a" parameter can also be determined to an accuracy of 10%, even when the Doppler width is 14 times larger than the Lorentzian width.

Self broadening and f -values in the spectrum of neon

Profiles of lines in the NeI spectrum arising from transitions between the configurations 2 p 5 3 s and 2 p 5 3 p were measured by means of a direct-recording Fabry–Perot interferometer. The d. c. discharge was cooled with liquid nitrogen or ice and currents were typically 0·5 mA. Full analysis of the profiles determines the pressure broadening as a Lorentzian component. For lines with the lower level 2 s 2 (Paschen notation) the broadening can be ascribed to the resonance interaction alone and the f -value of 0·16 8 is obtained for the resonance transition 736 Å. From coupling parameters the f -value of the transition 744 Å is estimated as 0·01 2 which is consistent with the broadening observed for the lines with the lower level 2 s 4 . The nature of the higher order interactions is discussed with reference to the observed temperature dependence and pressure shifts. Extrapolation of the Lorentzian width to zero density and the subtraction of a carefully determined instrumental contribution gives a value in excess of the radiation width for the level 2 s 2 , a result similar to that reported for the level 2 1 P 1 in helium. The explanation of this anomaly as an enhancement effect is discussed.