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As the field of fluid dynamics progresses, the demand for sophisticated diagnostic methods to accurately assess flow conditions rises. In this work, resonantly ionized photoemission thermometry (RIPT) has been used to directly target and ionize diatomic nitrogen (N₂) to measure one-dimensional (1D) temperature profiles in a supersonic jet flow. This technique can be considered non-intrusive as the premise uses resonantly enhanced multiphoton ionization (REMPI) to target molecular nitrogen. This resonance excites N₂into absorption bands of the P, Q, and R rotational branches of N₂(b¹Π_u). The ideal (3 + 1) REMPI scheme excites from the ground state and ionizes N₂(b¹Π_u←X¹Σ_g⁺) where de-excitation results in photoemission from the first negative band of ionizedN₂⁺(B²Σ_u⁺→X²Σ_g⁺) as nitrogen returns to the ground state. The resulting emission can be observed using an intensified camera, thus permitting inference of the rotational temperature of ground-state molecular nitrogen. A linearly regressive Boltzmann distribution is applied based on previous calibration data for this technique to quantify the temperature along the ionized line. This work applies this technique to a pure N₂supersonic jet in cross-flow and counter-flow orientations to demonstrate N₂RIPT’s applications in a supersonic flow. Temperature variations are observed at different locations downstream of the exit in cross-flow, and axisymmetric in counter-flow, to generate profiles characterizing the flow dynamics. Due to the collisional effects resulting from the number density of N₂at higher pressures, a (3 + 2) REMPI scheme is observed throughout this text. 

In this work, a detailed calibration study is performed to establish non-intrusive one-dimensional (1D) rovibrational temperature measurements in unseeded air, based on air resonance enhanced multiphoton ionization thermometry (ART). ART is generated by REMPI (resonance enhanced multi-photon ionization) of molecular oxygen and subsequent avalanche ionization of molecular nitrogen in a single laser pulse. ART signal, the fluorescence from the first negative band of molecular nitrogen, is directly proportional to the 2-photon transition of molecular oxygen C³Π (v = 2) ← X³Σ (v’=0), which is used to determine temperature. Experimentally, hyperfine structures of the O₂rotational branches with high temperature sensitivity are selectively excited through a frequency-doubled dye laser. Electron-avalanche ionization of N₂results in the fluorescence emissions from the first negative bands of N₂⁺near 390, 425, and 430nm, which are captured as a 1D line by a gated intensified camera. Post processing of the N₂⁺fluorescence yields a 1D thermometry line that is representative of the air temperature. It is demonstrated that the technique provides ART fluorescence of ∼5cm in length in the unseeded air, presenting an attractive thermometry solution for high-speed wind tunnels and other ground test facilities. 

Air resonance enhanced multiphoton ionization (REMPI) tagging velocimetry (ART) was demonstrated in quiescent and supersonic flows. The ART velocimetry method utilizes a wavelength tunable laser beam to resonantly ionize molecular oxygen in air and generate additional avalanche-type ionization of molecular nitrogen. The fluorescence emissions from the first negative and first positive bands of molecular nitrogen are, thus, produced and used for flow tagging. Detailed characterization of ART was conducted, including the effects of oxygen resonance to fluoresce nitrogen, nitrogen fluorescence spectrum, laser energy deposition into quiescent flow showing minimal perturbations in flow, fluorescence lifetime study at various pressures, and line tagging without breakdown. Pointwise velocity measurements within a supersonic flow from a nominal Mach 1.5 nozzle have been conducted and characterized.