Nitrogen CARS spectrum

Temperature information from CARS spectra derives from spectral shapes either of the 2-branches or of the pure rotational CARS spectra of the molecular constituents. In combustion research it is most common to perform thermometry from nitrogen since it is the dominant constituent and present everywhere in large concentration despite the extent of chemical reaction. The 2-branch of nitrogen changes its shape due to the increased contribution of higher rotational levels which become more populated when the temperature increases. Figure 6.1-21 displays a calculated temperature dependence of the N2 CARS spectrum for experimental parameters typically used in CARS thermometry (Hall and Eckbreth, 1984). Note that the wavenumber scale corresponds to the absolute wavenumber value for the 2320 cm 2-branch of N2 when excited with the frequency doubled Nd.YAG laser at 532 nm ( 18796 cm ), i. e. = 18796 -1- 2320 = 21116 cm. The bands lower than about 21100 cm are due to the rotational structure of the first vibrational hot band. 

Fig. 0.4. Experimental nitrogen Q-branch of coherent anti-Stokes Raman scattering spectrum (CARS) measured at 700 K and different pressures [14].

Figure 5 A CARS vibrational spectrum produced by monitoring the output beam intensity (at C04) while wavelength scanning an OPO (see Figure 4(B)). This spectrum shows Raman-active peaks from benzene (b), oxygen (0), nitrogen (n), and cyclohexane (c) covering a range from 681 cm- (Aqpo = 552 nm) to 3098 cm- (Iqpo = 637 nm). Zero frequency shift corresponds tolopo = 532 nm.

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