Role of Suprathermal Chemistry on the Evolution of Carbon Oxides and Organics within Interstellar and Cometary Ices.

ConspectusLaboratory-based experimental astrochemistry regularly entails simulation of astrophysical environments whereby low-temperature condensed ices are exposed to radiation from ultraviolet (UV) photons or energetic charged particles. Here, excited atoms/radicals are generated that are not in thermal equilibrium with their surroundings (i.e., they are nonthermal, or suprathermal). These species can surpass typical reaction barriers and partake in unusual chemical processes leading to novel molecular species. Often, these are uniquely observable under low-temperature conditions where the surrounding ice matrix can stabilize excited intermediates that would otherwise fall apart. Fourier-transform infrared (FTIR) spectroscopy is traditionally utilized to monitor the evolution of chemical species within ices during radiolysis. Yet, the characterization and quantification of novel species and radicals formed within astrophysical ices is often hindered since many of these cannot be synthesized by traditional synthetic chemistry. Computational approaches can provide fundamental vibrational frequencies and isotopic shifts to help aid in assignments alongside infrared intensities and Raman activities to quantify levels of production. In this Account, we begin with a brief history and background regarding the composition and radiation of interstellar ices. We review details of some of the early work on carbon oxides produced during the radiolysis of pure carbon dioxide ices and contention around the carrier of an absorption feature that could potentially be a product of radiation. We then provide an overview of current and emerging experimental methodologies and some of the chemistries that occur via nonthermal processes during radiolysis of low-temperature ices. Next, we detail computational approaches to reliably predict vibrational frequencies, infrared intensities, and Raman activities based on our recent work. Our focus then turns to studies on the formation of complex organics and carbon oxides, highlighting those aided by computational approaches and their role in astrochemistry. Some recent controversies regarding assignments alongside our recent results on the characterization of novel carbon oxide species are discussed. We present an argument for the potential role of carbon oxides within cometary ices as parent molecular species for small volatiles. We provide an overview of some of the complex organic species that can be formed within interstellar and cometary ices that contain either carbon monoxide or carbon dioxide. We examine how Raman spectroscopy could potentially be leveraged to help determine and characterize carbon oxides in future experiments as well as how computational approaches can aid in these assignments. We conclude with brief remarks on future directions our research group is taking to unravel astrochemically relevant carbon oxides using combined computational and experimental approaches.