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Carbon Dioxide Photolysis from 150 to 210 Nm: Singlet SPECIAL FEATURE and Triplet Channel Dynamics, UV-Spectrum, and Isotope Effects

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Carbon Dioxide Photolysis from 150 to 210 Nm: Singlet SPECIAL FEATURE and Triplet Channel Dynamics, UV-Spectrum, and Isotope Effects Carbon dioxide photolysis from 150 to 210 nm: Singlet SPECIAL FEATURE and triplet channel dynamics, UV-spectrum, and isotope effects Johan A. Schmidta,1, Matthew S. Johnsona,1, and Reinhard Schinkeb aDepartment of Chemistry, University of Copenhagen, DK-2100 Copenhagen Ø, Denmark; and bMax-Planck-Institut für Dynamik und Selbstorganisation, D-37077 Göttingen, Germany Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved May 23, 2013 (received for review September 21, 2012) We present a first principles study of the carbon dioxide (CO2) vibrational structures; those overlaid on the first band are com- photodissociation process in the 150- to 210-nm wavelength range, plex, whereas those on the second are more regular. The triplet, with emphasis on photolysis below the carbon monoxide + O(1D) COð1Σ+Þ + Oð3PÞ, and singlet, COð1Σ+Þ + Oð1DÞ, dissociation singlet channel threshold at ∼167 nm. The calculations reproduce channels open at 227.5 nm and 167.2 nm, respectively. Because of ∼ experimental absorption cross-sections at a resolution of 0.5 nm the very small actinic flux below λ < 167 nm, atmospheric pho- without scaling the intensity. The observed structure in the 150- to tolysis of CO2 occurs almost exclusively via the triplet channel 210-nm range is caused by excitation of bending motion supported except at very high altitudes. by the deep wells at bent geometries in the 21A′ and 11A″ potential The electronic structure of CO has been the subject of several energy surfaces. Predissociation below the singlet channel threshold 2 theoretical studies (7–9). The first quantum dynamics analysis of occurs via spin-orbit coupling to nearby repulsive triplet states. Car- bon monoxide vibrational and rotational state distributions in the the UV photoabsorption of CO2 was provided only recently by singlet channel as well as the triplet channel for excitation at 157 nm Grebenshchikov (10). This work determined 3D potential en- satisfactorily reproduce experimental data. The cross-sections of ergy surfaces (PESs) of six singlet states and calculated the ab- 12 16 12 17 16 12 18 16 individual CO2 isotopologues ( C O2, C O O, C O O, sorption cross-section starting near the singlet channel threshold. 13 16 13 18 16 C O2, and C O O) are calculated, demonstrating that strong In contrast, in the present study, we focus on the wavelengths EARTH, ATMOSPHERIC, isotopic fractionation will occur as a function of wavelength. The longer than 167 nm, which are important for most atmospheric AND PLANETARY SCIENCES calculations provide accurate, detailed insight into CO2 photoab- applications. sorption and dissociation dynamics, and greatly extend knowl- CO2 is a linear molecule with 16 valence electrons, properties edge of the temperature dependence of the cross-section to cover common to two other key terrestrial trace gases: nitrous oxide the range from 0 to 400 K that is useful for calculations of prop- (N2O) and carbonyl sulfide (OCS)(6). We have recently com- agation of stellar light in planetary atmospheres. The model is also pleted first principle computations of the UV absorption spectra relevant for the interpretation of laboratory experiments on mass- of these two molecules using accurate PESs, the transition dipole independent isotopic fractionation. Finally, the model shows that moment (TDM) functions coupling them with the ground state, the mass-independent fractionation observed in a series of Hg lamp and the quantum mechanical wave packet methodology (11– experiments is not a result of hyperfine interactions making pre- 17 16). Our calculations do an excellent job of reproducing available dissociation of O containing CO2 more efficient. experimental data, including the OCS isotopologue absorption mass-independent fractionation | photodissociation dynamics | spectra (17, 18) and the temperature- and isotopologue-dependent fine interaction | magnetic isotope effect | Mars N2O cross-sections (14, 19). The wave packet methodology extracts a wealth of information from the potential energy and transition dipole surfaces, including arbon dioxide (CO2) is the main component of the atmos- Cpheres of Mars, Venus, and the Hadean Earth (1). Its photo- vibrational frequencies; temperature- and isotopologue-dependent absorption screens solar UV light, determining altitude-dependent absorption spectra; and detailed descriptions of the photodissoci- photolysis rates, and its concentrations and infrared absorptions ation dynamics, including product quantum state, and velocity and make it a powerful greenhouse gas. CO2 photodissociation is the angular distributions (20). In practice, the method is limited mainly basis of these atmospheres’ photochemistry and is the primary by the quality of the surfaces. Using the method we have built and source of carbon monoxide (CO) and O2. Although the initially tested on N2O and OCS, we are able to present a similar analysis of ’ high concentration of CO2 during Earth s Hadean era decreased as the CO2 UV absorption and dissociation from the onset of the UV carbonate rocks accumulated, CO2 continued to be a prominent spectrum (>210 nm) to ca. 150 nm. Our calculations reproduce atmospheric gas, enhancing surface temperature and attenuating a variety of existing experimental data; therefore, we can report > UV light, with a partial pressure 10 mbar in the Archean (2) reliable absorption cross-sections for a set of isotopologues and for and >1 mbar in the Proterozoic (3). Its influence on the radiative ’ a wide range of temperatures by changing the isotopic masses and/ properties and chemical composition of Earth s atmosphere has or the populations of vibrational states. continued to the present day; one example is that CO2 photolysis is the main source of mesospheric CO (4). Variations in the abun- dances of naturally occurring stable isotopes, including reaction Author contributions: J.A.S., M.S.J., and R.S. designed research; J.A.S. performed research; mechanisms exhibiting mass-independent fractionation, are central J.A.S. and R.S. contributed new reagents/analytic tools; J.A.S., M.S.J., and R.S. analyzed to efforts to interpret environmental records ranging from sedi- data; and J.A.S., M.S.J., and R.S. wrote the paper. mentary rocks to oceanic carbonates to glacial ice (5). Thus, CO2 The authors declare no conflict of interest. photolysis is both directly and indirectly linked to isotopic varia- This article is a PNAS Direct Submission. tions found in the environment. 1To whom correspondence may be addressed. E-mail: [email protected] The UV absorption band of CO2 ð120 nm < λ < 210 nmÞ con- or [email protected]. sists of two broad, strongly overlapping bands peaking around This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. 145 nm and 133 nm, respectively (6). Both exhibit pronounced 1073/pnas.1213083110/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1213083110 PNAS Early Edition | 1of6 Downloaded by guest on September 30, 2021 A In our previous studies of photodissociation in N2OandOCS, which, near linearity, have electronic structures identical to that of CO2, we used the same model and were able to reproduce the absorption cross-sections very well both at low energies and throughout the first UV band (12, 13). We have calculated PESs for the ground state X, the three excited singlet states A–C, and the three triplet states a–c using the MRCI+Q method on large 3D grids (SI Text, Figs. S1–S3, and Table S1). The calculated energies for the three fundamental vibrations in X½ð0; 11; 0Þ; ð1; 0; 0Þ; and ð0; 0; 1Þ deviate from the experimental data by 1% (Table S2), which gives an indication of the quality of the potentials. Our earlier studies of N2O (13) and OCS (11, 12) found equally good agreement between experi- mental and theoretical vibrational excitation energies. As an ex- B ample of an excited state PES, Fig. 2 shows 2D representations of the A state PES, VA, which is the most important one; the cor- responding contour plots for the B state are very similar. VA has a deep well at bent geometries, whose equilibrium energy is 5.532 eV [i.e., 1.961 eV below the (classical) singlet dissociation chan- nel]. The well leads to substantial trapping of the O and CO products during dissociation, even for energies above the singlet channel threshold, and therefore has a strong effect on the ab- sorption cross-section and on the product state distributions. Although the A and B PESs of OCS and N2O have similar po- tential wells, they are not as deep and produce only minor structures in the respective absorption spectra (12, 13). The TDMs of A, B, and C with the ground electronic state μ (e.g., A) have been calculated at the MRCI level on small Fig. 1. 1D potential energy curves for the lowest four singlet (full curves) and lowest three triplet (broken curves) electronic states as a function of r1, one of the two C-O bond lengths (A), and as a function of the O-C-O bend- A ing angle α (B). The A′ states (X, A, and a) are shown in black, and the A″ states (B, b, C, and c) are shown in red. In this figure and throughout this article, energy is measured relative to the global minimum of the X state. deg., degree. Low-Energy Absorption and Dissociation Mechanisms Six excited electronic states are energetically accessible below ≈ 8:5 eV (λ ≈ 150 nm) in the Franck–Condon (FC) region (Fig. 1): the three singlet states 21A′ (A), 11A″ (B), and 21A″ (C) and the three triplet states 13A′ (a), 13A″ (b), and 23A″ (c) (letters in parentheses indicate abbreviations used below). Fig. 1A, which shows 1D cuts along one of the two C-O bonds, r1, provides an overview of these states and how they connect with the dissociation channels B (Table S1); the potential of the ground electronic state 11A′ (X) is also included.
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