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This paper presents an extensive parameter study of a non-intrusive and non-seeded laser diagnostic method for measuring one dimensional (1D) rotational temperature of molecular nitrogen (N₂) at 165 - 450 K. Compared to previous efforts using molecular oxygen, here resonantly ionized and photoelectron induced fluorescence of molecular nitrogen for thermometry (N₂RIPT) was demonstrated. The RIPT signal is generated by directly probing various rotational levels within the rovibrational absorption band of N₂, corresponding to the 3-photon transition of N₂(X¹Σ_g⁺,v=0→b¹Π_u,v^′=6) near 285 nm, without involving collisional effects of molecular oxygen and nitrogen. The photoionized N₂produces strong first negative band of N₂⁺(B²Σ_u⁺−X²Σ_g⁺) near 390 nm, 420 nm, and 425 nm. Boltzmann analyses of various discrete fluorescence emission lines yield rotational temperatures of molecular nitrogen. By empirically choosing multiple rotational levels within the absorption band, non-scanning thermometry can be accurately achieved for molecular nitrogen. It is demonstrated that the N₂RIPT technique can measure 1D temperature profile up to ∼5 cm in length within a pure N₂environment. Multiple wavelengths are thoroughly analyzed and listed that are accurate for RIPT for various temperature ranges. 

The frame rate of conventional high-speed imaging devices is limited by exposure time and signal read-out time. Structured imaging is able to push the frame rate beyond the limit of the imaging device. This work demonstrates the feasibility of potential TeraHz rate structured imaging with a multiplexed structured imaging setup. In this work, a multiplexed structured image capture system was employed to image UV laser induced krypton-II emission lines at 10 GHz. The measured emission intensity of the 764 nm line over time suggests a temporal resolution of 97 ps. The temporal resolution is adjustable, and the frame rate can be further increased to the TeraHz level. 

This paper demonstrates a simultaneous Thomson scattering and rotational Raman scattering spectroscopy in a weakly ionized plasma in air. Thomson scattering was collected in the forward scattering direction, in order to compress the relative spectra width of Thomson scattering from the plasma. Simultaneous measurements of rotational Raman scattering were obtained in the same direction, which was not affected by the collection angles. The measurements thus yielded electron temperature (T_e) and electron number density (n_e) as well as gas temperature in a weakly ionized atmospheric pressure plasma. The separation of rotational Raman scattering and Thomson scattering occurred when the scattering angle decreased to 20 degrees in the plasma, where the air temperature was found to be 150 ± 25 °C, and electron temperature of the plasma was 0.587 ± 0.087 eV, and electron number density was (1.608 ± 0.416) × 1021 m-3. The technique could be used for various plasma and combustion diagnostics in realistic engineering environments. 

Characterization of the thermal gradients within supersonic and hypersonic flows is essential for understanding transition, turbulence, and aerodynamic heating. Developments in novel, impactful non-intrusive techniques are key for enabling flow characterizations of sufficient detail that provide experimental validation datasets for computational simulations. In this work, Resonantly Ionized Photoemission Thermometry (RIPT) signals are directly imaged using an ICCD camera to realize the techniques 1D measurement capability for the first time. The direct imaging scheme presented for oxygen-based RIPT (O₂RIPT) uses the previously established calibration data to direct excite various resonant rotational peaks within the S-branch of theC³Π, (v = 2) ← X³Σ(v^′ = 0) absorption band of O₂. The efficient ionization of O₂liberates electrons that induce electron avalanche ionization of local N₂molecules generating N₂⁺, which primarily deexcites via photoemissions of the first negative band of $N_2^{+}\left(B^2{\mathrm{\Sigma <\#comment/>}}_u^{+}-<\#comment/>X^2{\mathrm{\Sigma <\#comment/>}}_g^{+}\right)$. When sufficient lasing energy is used, the ionization region and subsequent photoemission signal is achieved along a 1D line thus, if directly imaged can allow for gas temperature assignments along said line; demonstrated here of up to five centimeters in length. The temperature gradients present within the ensuing shock train of a supersonic under expanded free jet serves as a basis of characterization for this new RIPT imaging scheme. The O₂RIPT results are extensively compared and validated against well-known and established techniques (i.e., CARS and CFD). The direct imaging capability fully realizes the technique’s fundamental potential and is expected to be the standard of implementation going forward. The direct imaging capability can play instrumental roles in future scientific studies that rely upon acute characterization of thermal gradients within a medium that cannot be easily resolved by a point. Furthermore, the removal of the spectrometer greatly reduces the cost, complexity, and optical alignment associated with prior RIPT measurements. 

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.