Carbon dioxide photolysis from 150 to 210 nm: Singlet and triplet channel dynamics, UV-spectrum, and isotope effects

Abstract

We present a first principles study of the carbon dioxide (CO 2 ) photodissociation process in the 150-to 210-nm wavelength range, with emphasis on photolysis below the carbon monoxide + O( 1 D) singlet channel threshold at ∼167 nm. The calculations reproduce experimental absorption cross-sections at a resolution of ∼0.5 nm without scaling the intensity. The observed structure in the 150-to 210-nm range is caused by excitation of bending motion supported by the deep wells at bent geometries in the 2 1 A′ and 1 1 A″ potential energy surfaces. Predissociation below the singlet channel threshold occurs via spin-orbit coupling to nearby repulsive triplet states. Carbon monoxide vibrational and rotational state distributions in the singlet channel as well as the triplet channel for excitation at 157 nm satisfactorily reproduce experimental data. O) are calculated, demonstrating that strong isotopic fractionation will occur as a function of wavelength. The calculations provide accurate, detailed insight into CO 2 photoabsorption and dissociation dynamics, and greatly extend knowledge of the temperature dependence of the cross-section to cover the range from 0 to 400 K that is useful for calculations of propagation of stellar light in planetary atmospheres. The model is also relevant for the interpretation of laboratory experiments on massindependent isotopic fractionation. Finally, the model shows that the mass-independent fractionation observed in a series of Hg lamp experiments is not a result of hyperfine interactions making predissociation of 17 O containing CO 2 more efficient.mass-independent fractionation | photodissociation dynamics | fine interaction | magnetic isotope effect | Mars C arbon dioxide (CO 2 ) is the main component of the atmospheres of Mars, Venus, and the Hadean Earth (1). Its photoabsorption screens solar UV light, determining altitude-dependent photolysis rates, and its concentrations and infrared absorptions make it a powerful greenhouse gas. CO 2 photodissociation is the basis of these atmospheres' photochemistry and is the primary source of carbon monoxide (CO) and O 2 . Although the initially high concentration of CO 2 during Earth's Hadean era decreased as carbonate rocks accumulated, CO 2 continued to be a prominent atmospheric gas, enhancing surface temperature and attenuating UV light, with a partial pressure >10 mbar in the Archean (2) and >1 mbar in the Proterozoic (3). Its influence on the radiative properties and chemical composition of Earth's atmosphere has continued to the present day; one example is that CO 2 photolysis is the main source of mesospheric CO (4). Variations in the abundances of naturally occurring stable isotopes, including reaction mechanisms exhibiting mass-independent fractionation, are central to efforts to interpret environmental records ranging from sedimentary rocks to oceanic carbonates to glacial ice (5). Thus, CO 2 photolysis is both directly and indirectly linked to isotopic variations found in the environment.The UV absorption band of CO 2 ð120 nm < ...

Wavelength-dependent isotope fractionation in visible light O3 photolysis and atmospheric implications

“…For photolysis of N 2 O, results from ab initio calculations agree better with experimentally determined isotope fractionations than semiempirical calculations [ Schmidt et al , ]. For CO 2 , the ab initio calculations [ Schmidt et al , ] reported 3 times stronger fractionations than predicted by the semiempirical approach applied by Liang et al []. Furthermore, Ndengué et al [] showed that the assumption by Liang et al [] of a proportional relationship between changes in absorption cross sections under isotopic substitution and changes in zero point energy for both central and terminal substitution is not valid.…”

Isotope effects on the formation of the lowest rovibrational level of NaH molecule via pump–dump photoassociation

“…For the bimolecular-collision reactions of Cl( 2 P 3/2 ) + X 2 (υ) → XCl + X(X = H, D) with para-H 2 and ortho-D 2 in the quantum solids, various isotope effects, which may be classified as energetic, translational amplitude, and kinematic, have been predicted based on the quantum reaction dynamics simulations [22]. In the photolysis of carbon dioxide (C 12,13 O 16,17,18 2 ), strong isotopic fractionation can occur as a function of wavelength [23]. In the non-dissociative photoionization spectrum of the N 14,15 2 molecule, significant changes in peak energies and intensities upon isotopic substitution were observed [24].…”

Introduction to Chemistry and Applications in Nature of Mass Independent Isotope Effects Special Feature

“…By convention, the delta notation, is used, which for oxygen is δ 18 For oxygen, the standard is standard mean ocean water (SMOW). Stable isotope ratio measurements are typically reported in this manner, and a similar isotope ratio is reported for δ 17 O. In general, isotope ratio alterations are attributable to conventional thermodynamic, kinetic, translational, and gravitational phenomena (4-6).…”

“…The applications have subsequently expanded, particularly to oxygen isotopes, and in this special issue, many unique uses of both oxygen and sulfur isotopes are presented (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18).…”

“…The work suggests that further measurements in tropical regions would further amplify understanding of the contributing factors. In this issue (17), a study of CO 2 photolysis in the actinic region 150-210 nm has documented the photo physical dynamics occurring in the Earth's mesosphere that influence overall the upper atmospheric (stratosphere and mesosphere) isotopic cycle of CO 2 -O 3 -O 2 , suggested as being important (17,56).…”