Master's Thesis Proposal

Prabhav Agrawala

(Professor Jerry Seitzman)

Ab initio Spectroscopy Calculations for Assessing Non-equilibrium Gas Vibrational Effects on Optical Diagnostics

Friday, November 21

12:40 p.m.

Weber Building 200

Abstract:

The accurate prediction of molecular optical properties under thermochemical non-equilibrium (TNE) conditions is essential for advancing optical diagnostics, high-energy laser propagation and ignition, high-temperature plasmas, and hypersonic aerothermodynamics. Among the fundamental optical properties governing such applications, the electric dipole polarizability plays a central role, directly determining the refractive index and Rayleigh/Raman scattering cross-sections. However, these environments often exhibit flowfields 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 such conditions is therefore essential for reliable predictions and diagnostic interpretation. Within the semi-classical framework, the polarizability can be determined entirely from spectroscopic transition data — i.e. line positions and Einstein A coefficients. However, recent efforts to model the non-equilibrium polarizability have been constrained by a lack of transition data for high-lying rovibrational states in standard spectroscopic databases such as HITRAN. Transitions to the continuum are often the dominant contribution to polarizability, yet data here is especially scant.

This work proposes to address these gaps by generating state-resolved transition data using an ab initio approach. Using modern variational quantum mechanical methods implemented in the code, Duo, we generate state-resolved bound–bound and bound–free data for the entire manifold of rovibrational states up to dissociation. For molecular oxygen, using the Schumann-Runge electronic band system as a pilot case, the equilibrium bulk polarizability computed using this approach agrees well with experimental data, demonstrating that the methodology is sound. The resulting framework provides rovibrationally resolved polarizability data suitable for integration with state-to-state kinetics models, and thus evaluation of non-equilibrium impacts on optical diagnostics such as interferometry and Rayleigh scattering.

Committee:

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