Temperature measurements in heated gases and flames using carbon monoxide femtosecond two-photon laser-induced fluorescence

Nonintrusive temperature measurement is crucial in combustion research. Here, we propose a thermometric technique based on femtosecond two-photon laser-induced fluorescence of carbon monoxide (CO-fs-TPLIF). A femtosecond laser with a wavelength of 230 nm was used as an excitation source. Owing to its intrinsic broad bandwidth, dual vibrational bands of the B¹Σ⁺ ← X¹Σ⁺ transition of CO can be simultaneously excited. As a result, the fluorescence from the conventional bands (0,n) and the hot vibrational bands (1,n) of the B¹Σ⁺ → A¹Π_u transition of CO can be simultaneously detected. Hence, the temperature-dependent Boltzmann distribution can be assessed from the relative fluorescence intensity related to different ro-vibrational states, and the temperature can be extracted from the analysis of the recorded fluorescence spectra. Two temperature calibration methods were developed, for the low-temperature range (298–1173 K), the rotational-state-associated bandwidths of the spectra were used; for the flame temperature range, the spectral intensity ratios between the hot vibrational bands (1-n) and the conventional bands (0-n) were used. The CO-fs-TPLIF thermometric technique features the advantages of a simple optical setup and the ability of one-dimensional measurements with high spatial resolution.