Interference effects of C2‐radicals in nitrogen vibrational CARS thermometry using a frequency‐doubled Nd:YAG laser

Generally, in coherent anti‐Stokes Raman spectroscopy a frequency‐doubled Nd:YAG laser emitting at 532 nm along with a broadband dye laser is used for nitrogen vibrational CARS thermometry, resulting in a signal wavelength of 473 nm. Unfortunately, emission and absorption bands of C2 radicals occur in the same spectral region, hence the CARS signal can be disturbed by interferences. C2 radicals are an intermediate species especially in hydrocarbon flames and, moreover, they can be produced by laser‐induced vaporization of soot particles present in the flame. The described interferences can be a severe problem resulting in erroneous temperature data. The influence of the interferences on the determined temperatures was analyzed in this work. For this purpose, an experimental set‐up was chosen that simultaneously generates the vibrational nitrogen signal at 473 and 487 nm, i.e. in a undisturbed spectral region. This was realized by using the dual‐pump CARS technique with an annular phase‐matching geometry (USED CARS). Hence a direct comparison of temperature results from disturbed and undisturbed spectra can be performed. Measurements conducted in one‐dimensional ethylene–air flames established on a McKenna‐type burner are presented.