Degenerate four wave mixing of C-2 in forward geometry: Spectral lineshapeanalysis and temperature determination

Forward phase-matched degenerate four-wave mixing (DFWM) spectrum of C-2 mo lecules is studied to determine the temperature of a premixed oxyacetylene flame. Both theoretical calculation and experimental measurement of the d(3 )pi (g) <----> d(3)pi (g) swan band of C? are performed at weak field inten sity. The measured spectra excited with a narrow band optical parametric os cillator (OPO) showed good matches with the results of calculations. When a DFWM spectrum corresponding to the molecular transitions of both high (J = 36-32) and low (J = 8-15) rotational levels is used for a Boltzmann plot, we can determine the flame temperature of 3143 K with a very small temperat ure uncertainty of 1.6%. In addition, the uncertainty in temperature measur ement caused by the line interference of the DFWM spectrum in the forward g eometry is estimated to be only 0.5%, which is negligible for practical app lications.