Ultraviolet spectroscopy of pressurized and supercritical carbon dioxide

Timothy W Marin,Ireneusz Janik

doi:10.1038/s42004-021-00516-z

Abstract

Carbon dioxide (CO2) is prevalent in planetary atmospheres and sees use in a variety of industrial applications. Despite its ubiquitous nature, its photochemistry remains poorly understood. In this work we explore the density dependence of pressurized and supercritical CO2 electronic absorption spectra by vacuum ultraviolet spectroscopy over the wavelength range 1455-2000 Å. We show that the lowest absorption band transition energy is unaffected by a density increase up to and beyond the thermodynamic critical point (137 bar, 308 K). However, the diffuse vibrational structure inherent to the spectrum gradually decreases in magnitude. This effect cannot be explained solely by collisional broadening and/or dimerization. We suggest that at high densities close proximity of neighboring CO2 molecules with a variety of orientations perturbs the multiple monomer electronic state potential energy surfaces, facilitating coupling between binding and dissociative states. We estimate a critical radius of ~4.1 Å necessary to cause such perturbations.

Highlights

Carbon dioxide (CO2) is prevalent in planetary atmospheres and sees use in a variety of industrial applications
We focused on examining spectral changes with increasing pressure/ density at 308 K, just above the critical temperature of 304 K, for the lowest-lying electronic absorption band
We observed no pressure effect on the band peak energy position, the known diffuse vibrational structure inherent to the gas-phase VUV absorption spectrum gradually decreases in magnitude with increasing pressure/density

Summary

Introduction

Carbon dioxide (CO2) is prevalent in planetary atmospheres and sees use in a variety of industrial applications. CO2 photochemistry must be understood over a wide range of excitation energies, as well as temperatures and pressures, in order to fully interpret the atmospheric chemistry of such worlds and to help assess its applicability as a potential nuclear power plant coolant. An increase of pressure/density of a subcritical or supercritical fluid increases the frequency of molecular collisions and simultaneously decreases the average distance between neighboring molecules This alters the physicochemical properties of the fluid. The electronic perturbation of water monomers in supercritical water is evident upon increase of pressure/density, indicated by a gradual disappearance of diffuse vibrational structure and blueshift of its lowest-energy electronic absorption band[9]. As interaction between neighboring CO2 molecules upon condensation is arguably limited only to weak Van der Waals character, one may reason that with increased pressure/density a perturbation of the CO2 upper electronic states should dominate and be easier to distinguish from effects occurring in the ground state

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Conclusion