# A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics
## 2 Femtosecond Two-Photon Laser-Induced Fluorescence (fs-TPLIF)
Considering that the photolytic interferences are often linearly proportional to the laser pulse energy, the photodissociation problem will be mitigated to a large extent for fs-TPLIF. Besides the advantage of photolytic interference suppression, the effective utilization of the whole broadband profile of the fs laser pulse due to the existence of multi photon-pairs in matching the two-photon resonant excitation has also been mentioned by [PERSON] et al. [30]. So far, fs-TPLIF has been applied to measure various combustion intermediates and products, including molecules (e.g., CO), atoms (e.g., H and O), and radicals (e.g., OH). Inert gases (e.g., Kr and Xe) are also of interest because crucial parameters, like mixing fraction, could be extracted by measuring them. The excitation and detection strategies of the abovementioned species are listed in Table 1.  
The measurement of CO by fs-TPLIF was first performed by [PERSON] et al. [37] in a steady flame. In their research, excitation wavelength at 230.1 nm was used to induce two-photon transition (\(B^{1}\Sigma^{+}\gets X^{1}\Sigma^{+}\)) of CO, and fluorescence in the range of 362-516 nm was captured. Later on, this excitation scheme has been extended into high-pressure flames [34], sooty flames [34], and piloted liquid-spray flames [33]. Our group also employed this scheme in a premixed laminar jet flame [27]. A hot band (1,n) together with the conventional band (0,n) of \(B^{1}\Sigma^{+}\to A^{1}\Pi^{+}\) transitions were observed in the burned zone of the flame. As shown in Figure 1b, the \(B^{1}\Sigma^{+}\gets X^{1}\Sigma^{+}(1,1)\) band was excited due to the broadband nature of the fs laser pulse. We have seen that the two vibrational bands are sensitive to temperature and can potentially be used for accurate flame temperature measurements. In addition, the CO fs-TPLIF signal recorded across the focal point of the excitation beam shows a relatively flat intensity distribution despite the steep laser intensity variation, which is beneficial for CO imaging in contrast to ns and ps TPLIF. For atomic intermediates in combustion, fs-TPLIF has been applied to H and O measurements. Atomic hydrogen is a critical species in the combustion of hydrogen or hydrocarbon fuels [58], which gets involved in flame ignition, propagation, and extinction. The H fs-TPLIF technique was first demonstrated in combustion by [PERSON] et al. [30,46]. In their scheme, H was excited by a 205 nm fs laser, and the subsequent \(H_{\alpha}\) fluorescence at 656 nm from the transition \(n=3\to n=2\) was detected, as shown by the red line in Figure 2. This excitation scheme has also been adopted for quantitative measurements [44] and 2D imaging [45]. Another extinction scheme, shown as the blue line in Figure 2, was proposed by our group [51]. In the scheme, H is electronically excited through a two-photon process by a 243 nm laser from the ground state to its first excited state (\(n=3\gets n=1\)), where the excited H is instantaneously re-excited through another one-photon process by a relay laser (486 nm) to excited state of \(n=3\). In order to avoid the stray light interference from the 486 nm laser, the secondary fluorescence at 656 nm (\(n=3\to n=2\)) was collected. Consequently, interference-free H imaging was achieved in a laminar jet flame. Furthermore, a three-photon (307.7 nm) excitation scheme (\(n=3\gets n=1\)) with detection fluorescence at 656 nm was demonstrated by [PERSON] et al. [40], as shown by the green line in Figure 2.