# A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics
## 2 Femtosecond Two-Photon Laser-Induced Fluorescence (fs-TPLIF)
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Species**} & \multicolumn{2}{c}{**Excitation**} & \multicolumn{2}{c}{**Detection**} & \multirow{2}{*}{**Ref.**} \\ \cline{2-2} \cline{4-5}  & **Wavelength** & **Transition** & & **Wavelength** & **Transition** \\ \hline OH & 620 nm*2 & \(A^{2}\Sigma^{+}\gets X^{2}\Pi^{1}\) & \(\sim\)310 nm & \(A^{2}\Sigma^{+}\to X^{2}\Pi^{1}\) & [31,32] \\  & 230 nm*2 & \(B^{1}\Sigma^{+}\gets X^{1}\Sigma^{+}\) & 400–600 nm & \(B^{1}\Sigma^{+}\to A^{1}\Pi^{+}\) & [27, 33, 34, 35, 36, 37, 38] \\  & 230 nm*2 & \(B^{1}\Sigma^{+}\gets X^{1}\Sigma^{+}\) & 280–380 nm & \(b^{2}\Sigma^{+}\to a^{2}\Pi^{1}\) & [34] \\  & 205 nm*2 & \(n=3\to n=1\) & 656 nm & \(n=3\to n=2\) & [30, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50] \\ H & 307 nm*3 & \(n=3\gets n=1\) & 656 nm & \(n=3\to n=2\) & [40, 47] \\  & 243 nm*2 + 486 nm & \(n=4\gets n=1\) & 656 nm & \(n=3\to n=2\) & [51] \\ O & 226 nm*2 & \(3p^{3}P\gets 2p^{3}P\) & 845 nm & \(3p^{3}P\to 3s^{3}S\) & [49, 52, 53] \\  & 204.1 nm*2 & \(5p^{\prime}[3/2]_{2}\gets 4p^{6}[^{1}s_{0}]\) & 826 nm & \(5p^{\prime}[3/2]_{2}\to 5s^{\prime}[1/2]_{1}\) & [49, 50, 54] \\ Kr & 204.1 nm*2 & \(5p^{\prime}[3/2]_{2}\gets 4p^{6}[^{1}s_{0}]\) & 750–840 nm & & [55, 56] \\  & 212.6 nm*2 & \(4s^{2}4p^{5}5p/4s^{2}4p^{5}5p^{\prime}\gets 4s^{2}4p^{6}\) & 759 nm & & [57] \\ Xe & 225 nm*2 & \(6p^{\prime}[3/2]_{2}\gets 5p^{61}S_{0}\) & 835 nm & \(6p^{\prime}[3/2]_{2}\gets 6s^{\prime}[1/2]_{1}\) & [49, 53] \\ \hline \hline \end{tabular} Note: Refs. [50, 52, 55, 55] are measured in plasma.  
\end{table} 
Table 1: Excitation and detection strategies of various species by multi-photo femtosecond laser induced fluorescence. Figure 1: Spectra of CO femtosecond two-photon laser-induced fluorescence (fs-TPLIF): (**a**) Spatially resolved imaging spectrum recorded in a laminar premixed CH\({}_{4}\)/air jet flame (\(\Phi=1.5\)); (**b**) integrated spectral curve of the flame; (**c**) spectral curve of a gas mixture of CO/N\({}_{2}\) at room temperature [27]. (©) _2017 Optical Society of America_). Figure 2: Excitation and detection strategy of atomic hydrogen using the multiphoton femtosecond laser-induced fluorescence (MP-fs-LIF). [PERSON] et al. [52] also performed an fs-TPLIF diagnosis for O in premixed laminar flame. An fs laser at 226 nm was used for two-photon excitation of O to its excited state (\(3p^{3}P\gets 2p^{3}P\)), and the subsequent O fluorescence at 845 nm from the transition \(3p^{3}P\to 3s^{3}S\) was detected. The advantage of fs lasers excitation over traditional ns lasers excitation in O measurements is that the photolysis of vibrationally excited CO\({}_{2}\), known as the main interference that produces additional O atoms [59], can be eliminated. Combustion radicals can also be measured by fs-TPLIF. Hydroxyl (OH) is one of the essential combustion radicals, and OH-PLIF is widely used to visualize the reaction zone and product zone of a flame [60; 61; 62; 63]. Different from CO, H, and O, OH can be excited with single-photon strategy. A typical single-photon OH-LIF uses an ns laser at \(\sim\)283 nm to excite OH through a transition line in \(A^{2}\Sigma^{+}\gets X^{2}\Pi\) (0, 1) band, which allows observation of the fluorescence emission from the (1, 1) and (0, 0) bands at \(\sim\)310 nm [64]. There are many researches of OH-LIF, including ns-laser excitation [62; 63] and ps-laser excitation [65]. [PERSON] et al. [31; 32] performed the fs-TPLIF of OH measurements with two-photon excitation at 620 nm. However, they did not directly observe any obvious advantages of two-photon fs-laser excitation relative to single-photon ns- or ps-laser excitation. Femtosecond-TPLIF has also been used to measure inert gases such as Kr [54; 55; 56; 57] and Xe [49]. Although these studies do not directly measure the combustion species, they are still relevant to combustion research. For example, fs-TPLIF imaging of Kr was achieved to measure the mixture fraction in gaseous flows [57], which is a crucial parameter in the combustion flow field.