Student Name: Prabhav Agrawala

Advisor: Dr. Jerry Seitzman

Milestone: MS Thesis Final Examination (Defense)

Degree Program: Aerospace Engineering

Title: AB INITIO SPECTROSCOPY CALCULATIONS FOR ASSESSING NON-EQUILIBRIUM GAS VIBRATIONAL EFFECTS ON OPTICAL DIAGNOSTICS

Abstract: Accurate prediction of molecular optical properties under thermochemical non-equilibrium is essential for advancing optical diagnostics, high-energy laser propagation, plasmas kinetics, and hypersonic aerothermodynamics. Among the optical properties that govern such applications, the electric dipole polarizability plays a central role, directly determining the refractive index and elastic scattering cross-sections. However, these environments often exhibit flow fields where translational, rotational, and vibrational energy modes are decoupled, producing non-Boltzmann populations that persist over significant spatial and temporal scales. Vibrational non-equilibrium, in particular, has been shown to significantly alter the polarizability, especially for homonuclear diatomics where vibrational relaxation is slow. Accurately modeling the polarizability under these conditions is therefore essential for reliable interpretation of experimental diagnostic data. Semi-classical light-matter interaction theory determines the polarizability entirely from spectroscopic data: line positions, Einstein A coefficients, and continuum absorption cross-sections. However, efforts to model the polarizability under non-equilibrium have been constrained by inadequacies in these data for high lying energy levels, particularly state-resolved photodissociation spectra, which often end up dominating the total polarizability. This work addresses these gaps by generating state-specific transition data via ab initio spectroscopic calculations. Using electronic data as input and employing variational solutions to the nuclear Schrödinger equation implemented in the Duo code, we generate state-resolved, bound-bound and bound-free transition data for the complete rovibrational manifold up to dissociation. We validate this approach on molecular oxygen, using the Schumann-Runge electronic band system as a pilot case, achieving agreement within 10 % for bound-bound spectra compared to HITRAN, and within 30 % for photodissociation cross-sections. The equilibrium polarizability at 300 K computed using the same data, however, matches experiment to within 1 %, validating the approach. Using this validated framework, we quantify diagnostic errors for two canonical non-equilibrium cases: normal shocks and supersonic expansions. For the range of conditions examined, equilibrium assumptions introduce systematic errors of up to 8 % for interferometric measurements and 16 % for Rayleigh scattering experiments. These results provide a rigorous, state-resolved assessment of non-equilibrium polarizability effects and establish a transferable methodology for other molecular species and optical diagnostics. More generally, this work shows that ab initio spectroscopy is a viable alternative to move beyond spectroscopic data "blindspots" that constrain advanced aerospace research. 

Date and time: 2026-04-06, 13:15

Location: MK-317 Conference Room

Committee:
Dr. Jerry Seitzman (advisor), School of Aerospace Engineering
Dr. Joshua Kretchmer, School of Chemistry & Biochemistry
Dr. Adam Steinberg, School of Aerospace Engineering