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

Computational Infrared Spectroscopy of 958 Phosphorus-Bearing Molecules

Phosphine is now well-established as a biosignature, which has risen to prominence with its recent tentative detection on Venus. To follow up this discovery and related future exoplanet biosignature detections, it is important to spectroscopically detect the presence of phosphorus-bearing atmospheric molecules that could be involved in the chemical networks producing, destroying or reacting with phosphine. We start by enumerating phosphorus-bearing molecules (P-molecules) that could potentially be detected spectroscopically in planetary atmospheres and collecting all available spectral data. Gaseous P-molecules are rare, with speciation information scarce. Very few molecules have high accuracy spectral data from experiment or theory; instead, the best current spectral data was obtained using a high-throughput computational algorithm, RASCALL, relying on functional group theory to efficiently produce approximate spectral data for arbitrary molecules based on their component functional groups. Here, we present a high-throughput approach utilizing established computational quantum chemistry methods (CQC) to produce a database of approximate infrared spectra for 958 P-molecules. These data are of interest for astronomy and astrochemistry (importantly identifying potential ambiguities in molecular assignments), improving RASCALL's underlying data, big data spectral analysis and future machine learning applications. However, this data will probably not be sufficiently accurate for secure experimental detections of specific molecules within complex gaseous mixtures in laboratory or astronomy settings. We chose the strongly performing harmonic ωB97X-D/def2-SVPD model chemistry for all molecules and test the more sophisticated and time-consuming GVPT2 anharmonic model chemistry for 250 smaller molecules. Limitations to our automated approach, particularly for the less robust GVPT2 method, are considered along with pathways to future improvements. Our CQC calculations significantly improve on existing RASCALL data by providing quantitative intensities, new data in the fingerprint region (crucial for molecular identification) and higher frequency regions (overtones, combination bands), and improved data for fundamental transitions based on the specific chemical environment. As the spectroscopy of most P-molecules have never been studied outside RASCALL and this approach, the new data in this paper is the most accurate spectral data available for most P-molecules and represent a significant advance in the understanding of the spectroscopic behavior of these molecules.</jats:p