Knowledge Gaps in the Cometary Spectra of Oxygen-bearing Molecular Cations

Abstract

Molecular cations are present in various astronomical environments, most notably in cometary atmospheres and tails where sunlight produces exceptionally bright near-UV to visible transitions. Such cations typically have longer-wavelength and brighter electronic emission than their corresponding neutrals. A robust understanding of their near-UV to visible properties would allow these cations to be used as tools for probing the local plasma environments or as tracers of neutral gas in cometary environments. However, full spectral models are not possible for characterization of small, oxygen-containing molecular cations given the body of molecular data currently available. The five simplest such species (H2O+, CO2 +, CO+, OH+, and O 2 + ) are well characterized in some spectral regions but are lacking robust reference data in others. Such knowledge gaps hinder fully quantitative models of cometary spectra, specifically hindering accurate estimates of physical-chemical processes originating with the most common molecules in comets. Herein, the existing spectral data are collected for these molecules, and the places where future work is needed are highlighted, specifically where the lack of such data would greatly enhance the understanding of cometary evolution.

Complete, Theoretical Rovibronic Spectral Characterization of the Carbon Monoxide, Water, and Formaldehyde Cations

“…The full set of rotational constants, distortion constants, and geometrical parameters predicted by these and other levels of theory herein are given in Table A2 for the A state. As these are not widely available at high levels of theory [ 2 ], these data could prove useful in modeling complete UV/Vis spectra of H O .…”

“…Small molecular cations would be potent markers for comet evolution in the Solar System, as these species are believed to be abundant in cometary comae [ 1 ]. Understanding the rovibronic transitions of these small molecules would aid in their potential inclusion as markers of photochemical processes in these environments [ 2 ]. Previously unidentified lines in the spectra of the Hyakutake and Ikeya–Zhang comets [ 3 ] have been attributed to transitions of higher-excited levels of H O [ 4 ], exemplifying the importance of these rovibronic transitions.…”

“…Similarly, H CO has also been explored with both experiment and theory [ 28 , 29 , 30 , 31 , 32 ]. However, significant gaps still remain for their spectral classification [ 2 ].…”

DFT + F12 QFFs for Cost-Effective Rovibrational Spectral Data Predictions of Ground and Excited Electronic States

“…The heavy atom stretching frequencies are significantly reduced due to an increase in the antibonding character in formaldehyde and a decrease in the bonding character from protonated carbon dioxide. Both of these molecules are likely present in the coma of comets as they approach perihelion, and the novel rovibrational data for these three states of these two molecules will assist in the spectral characterization of such solar system bodies, providing insights into the behavior and evolution of oxygen, , from the dawn of the solar system until now.…”

F12+EOM Quartic Force Fields for Rovibrational Predictions of Electronically Excited States

“…Ab initio spectroscopic modeling of electronically excited states would aid in clarifying astronomical spectra potentially leading to the discovery of new molecules in space. − Other applications include providing complete partition functions to molecules in highly energetic environments, enhancing a complete understanding of photophysics needed for solar energy harvesting, and characterizing species involved in charge-transfer reactions to name a few. An immediate planetary science application would be in the classification of cometary spectra.…”