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Abstract: Observations of CO emissions in the visible and near-infrared (NIR) have been rare for comets, and no measurements from orbiters are currently available in the visible for the dayglows of Mars or Venus. Analysis of the ultraviolet CO(a–X) Cameron bands from Mars Express dayglow observations supports the conclusion that these bands have very high rotational temperatures, some thousands of kelvins. The most plausible source for the CO rotational excitation is its generation by CO2 photodissociation. Recent laboratory measurements investigating the photodissociation of CO2 in the extreme ultraviolet (EUV) reveal strong emissions in the visible and NIR region by the triplet CO(a′, d, e) states, which we take to be a primary source for the UV CO(a–X) Cameron bands. Thus, detection of visible emissions from the triplet CO states in planetary dayglows and comets provides an upper limit to the CO2 density. The presence of CO high rotational excitation along with the intense visible and NIR band emissions should be considered as a practical way by which planetary dayglow and cometary spectra provide information on the presence of CO2. Finally, we report on existing observations of cometary atmospheres and estimate the altitude for the emitting layer of the CO triplet states in Mars and Venus. [Copyright &y& Elsevier]

Emission from the OH Meinel bands is routinely used to determine rotational temperatures that are considered proxies for the kinetic temperature near the mesopause region. Previous observations determined OH rotational temperatures that show a dependence on the vibrational level, with the temperature rising overall as the OH vibrational quantum number v increases. The source of this trend is not well understood and has generally been attributed to deviations from thermodynamic equilibrium.This report shows the existence of bimodal OH rotational population distributions is an inherent feature of rotational relaxation in gases and can provide an explanation for the previously reported temperature trend. The use of only a few lines from rotational transitions involving low rotational quantum numbers to determine rotational temperatures does not account for the bimodality of the OH rotational population distributions and leads to systematic errors overestimating the OH rotational temperature. We will report selected examples, discuss the relevant implications, and consider strategies that could lead to more reliable OH rotational temperature determination [1].[1] K. S. Kalogerakis, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1047 (2018). [ABSTRACT FROM AUTHOR]

Abstract: The rate coefficient for collisional removal of by O(3P), kO, is an essential parameter in modeling the emissions from in the Earth’s thermosphere. We report a laboratory measurement of this rate coefficient at room temperature. A fluorine laser is used to photodissociate O2 at 157.6 nm, followed by detection of the time-resolved fluorescence from subsequently generated . The determined value is . The rate coefficient for collisional removal of by O2 is also determined, with a value of (1.5±0.1)×10−11 cm3s−1. [Copyright &y& Elsevier]

Branching ratios for N( 2 D 0 3/2 ) and N( 2 D 0 5/2 ) produced by predissociation of state selected excited nitrogen molecules in the vacuum ultraviolet region have been measured for the first time. The quantum numbers of the excited nitrogen molecule are defined by selective excitation of the nitrogen molecule in the Franck-Condon region from the ground electronic, 1 Σ g + , vibrational, v″, and rotational, J″ state to an excited E u ', v', J' state with a tunable vacuum ultraviolet, VUV 1 , laser. The neutral atoms produced by predissociation from this excited state are then selectively ionized with a second tunable VUV 2 laser. Measurement of the relative populations of these two atoms formed in their spin-orbit states defines the quantum states for the atomic products. This means that the wave functions of the initial state and knowledge of the relative yields define all the experimental parameters for this series of unimolecular reactions. The ions formed by VUV 2 are mass analyzed with a time-of-flight mass spectrometer and detected with a time slice velocity ion imaging mass spectrometer. In this manner, we can determine the recoil velocity associated with the predissociation process. Two different techniques are used to determine the spin-orbit ratios, namely, resonant VUV photoionization (RVUV-PI) spectroscopy and total kinetic energy release (TKER) spectroscopy determined from the image produced when the atoms are selectively ionized by VUV 2 in the interaction region. The TKER spectra obtained from the lines at 110 296.25 and 110 304.96 cm -1 that couple to a newly discovered autoionization line at 129 529.4255 ± 0.0015 cm -1 prove that the lines observed in this region originate from the N( 2 D 0 3/2 S 2 D 0 5/2 ) atoms. Two other lines in this region at 110 286.20 and 110 299.89 cm -1 originate from the nitrogen N( 4 S 0 3/2 ) that is photoionized in a 1+ 1 VUV-UV resonant multiphoton ionization process. The spin-orbit branching ratios have been evaluated for valence and Rydberg electronic excited states from 104 129.4 to 118 772.1 cm -1 , and it shows that they are independent of the rotational and vibrational quantum numbers. They are not appreciably affected by the symmetry properties of the wave function in the Franck-Condon region of the excited states. In the energy region below 117 153.8 cm -1 the pathways at long internuclear distances appear to determine [N( 2 D 0 3/2 )]/[N( 2 D 0 5/2 )] branching ratios of ∼0.38, ∼0.62, and ∼1.04. At higher energies, TKER and RVUV-PI spectroscopy have been used to show that the average fraction of the N( 2 D 0 3/2 ) and N( 2 D 0 5/2 ) atoms produced in the spin-allowed channels that produce two N( 2 D 0 J ) is 0.85 versus 0.15 for spin-forbidden channels. The importance and need for this information for comparison with theory and applications in astrochemistry are briefly discussed.

Collisional removal of electronic energy from O 2 in the low-lying a 1 Δ g state is typically an extremely slow process for the v = 0 level. In this study, we report results on the deactivation of O 2 ( a 1 Δ g , v = 1-3) in collisions with O 2 and CO 2 . Ozone photodissociation in the 200-310 nm Hartley band is the source of O 2 ( a, v), and resonance-enhanced multiphoton ionization is used to probe the vibrational-level populations. Deactivation of the a( v = 1-3) levels in collisions with O 2 at 300 K is fast, with rate coefficients of (5.6 ± 1.1) × 10 -11 , (3.6 ± 0.4) × 10 -11 , and (1.9 ± 0.4) × 10 -11 cm 3 s -1 (2σ) for v = 1, 2, and 3, respectively. The relaxation process appears to involve a near-resonant electronic energy transfer pathway analogous to that observed in vibrationally excited O 2 ( b 1 Σ g + ). With CO 2 collider gas, the removal rate coefficient at 300 K is (1.8 ± 0.4) × 10 -14 and (4.4 ± 0.6) × 10 -14 s 3 s -1 (2σ) for v = 1 and 2, respectively. Despite the small mole fraction of O 2 in the atmospheres of Mars and Venus, O 2 is at least as important as CO 2 in the final stages of collisional relaxation within the O 2 vibrational-level manifold.

We report a laboratory measurement of the rate coefficient for the collisional removal of O(2)(X(3)Sigma(g) (-),upsilon=1) by O((3)P) atoms. In the experiments, 266-nm laser light photodissociates ozone in a mixture of molecular oxygen and ozone. The photolysis step produces vibrationally excited O(2)(a(1)Delta(g)) that is rapidly converted to O(2)(X(3)Sigma(g) (-),upsilon=1-3) in a near-resonant electronic energy-transfer process with ground-state O(2). In parallel, a large amount of O((1)D) atoms is generated that promptly relaxes to O((3)P). Under the conditions of the experiments, only collisions with the photolytically produced O((3)P) atoms control the lifetime of O(2)(X(3)Sigma(g) (-),upsilon=1), because its removal by molecular oxygen at room temperature is extremely slow. Tunable 193-nm laser light monitors the temporal evolution of the O(2)(X(3)Sigma(g) (-),upsilon=1) population by detection of laser-induced fluorescence near 360 nm. The removal rate coefficient for O(2)(X(3)Sigma(g) (-),upsilon=1) by O((3)P) atoms is (3.2+/-1.0)x10(-12) cm(3) s(-1) (2sigma) at a temperature of 315+/-15 K (2sigma). This result is essential for the analysis and correct interpretation of the 6.3-mum H(2)O(nu(2)) band emission in the Earth's mesosphere and indicates that the deactivation of O(2)(X (3)Sigma(g) (-),upsilon=1) by O((3)P) atoms is significantly faster than the nominal values recently used in atmospheric models.

Vibrational relaxation of O2(X 3sigma(g)-, upsilon=2,3) by O2 molecules is studied via a two-laser approach. Laser radiation at 266 nm photodissociates ozone in a mixture of molecular oxygen and ozone. The photolysis step produces vibrationally excited O2(a 1delta(g)) that is rapidly converted to O2(X 3sigma(g)-, upsilon=2,3) in a near-resonant adiabatic electronic energy-transfer process involving collisions with ground-state O2. The output of a tunable 193-nm ArF laser monitors the temporal evolution of the O2(X 3sigma(g)-, upsilon=2,3) population via laser-induced fluorescence detected near 360 nm. The rate coefficients for the vibrational relaxation of O2(X 3sigma(g)-, upsilon=2,3) in collision with O2 are 2.0(-0.4)(+0.6) x 10(-13) cm3 s(-1) and (2.6+/-0.4) x 10(-13) cm3 s(-1), respectively. These rate coefficients agree well with other experimental work but are significantly larger than those produced by various semiclassical theoretical calculations.