Computational Molecular Spectroscopy Towards New Physics

Several theories of modern physics go beyond the standard model of particle physics to describe as of yet unexplained phenomena of the universe. A common method of testing new theories of physics is using spectroscopy to compare transition positions at different times. Non-trivial calculations are required to determine the sensitivity coefficients of transitions to a variation of fundamental constants. These calculations can be done using nuclear motion programs with adequate spectroscopic models. In this work, 27 small molecules with spectroscopic models are evaluated as molecular probes to constrain the variation of the proton-to-electron mass ratio. The diatomic radical CN is used as a case study to develop and explain the construction of spectroscopic models. Over 40,000 experimental transitions from 22 unique sources were validated to generate a network of 8083 interconnected spin-rovibronic energy levels. These empirical energy levels, along with ab initio dipole moment curves have been used to construct and fit a spectroscopic model for the three lowest coupled electronic states of CN in the nuclear motion program Duo. The resultant line list is further refined in a novel hybrid style with the replacement of energy levels from empirical and perturbative sources to produce over 2.2 million transitions up to 60,000 cm-1. A comprehensive high-throughput methodology is developed to calculate the sensitivity coefficients for transitions in CN, 21 other diatomic and 5 small polyatomic molecules of astrophysical relevance. In diatomics, near degenerate vibronic levels and parity transitions within non-singlet-sigma ground states can cause enhanced transition sensitivity. Unfortunately, many of the enhanced transitions, especially those showing anomalously large sensitivities, have extremely low intensities at 100 K. Expanding to polyatomic molecules, tunnelling transitions (a natural progression from parity transitions) show enhanced sensitivity, especially combination rotation-tunnelling transitions. Enhanced transitions are compared against previous calculations, and some previously identified enhanced transitions are excluded from astrophysical consideration based on their very low intensity at 100 K. Selection criteria that consider factors both sensitivity and observability of transitions to be used as molecular probes for a variation in the proton-to-electron mass ratio are considered for both diatomic and polyatomic molecules.