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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 ) and N( 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.

Earth's night sky continuously produces a faint chemiluminescence known as nightglow. Two prominent nighttime emissions around 90 km are the O 2 Atmospheric and the OH Meinel band systems. Despite a plethora of studies since their identification seven decades ago, substantial gaps persist in our understanding of the mechanisms that control them. This report shows that oxygen atoms connect these two emissions: Fast, multiquantum, vibrational-to-electronic relaxation of OH(v) by O atoms activates a pathway that generates O 2 Atmospheric band emission. This newly discovered source exhibits a strong altitude dependence and can contribute a majority of the observed O 2 Atmospheric band emission when the peaks of the OH and O-atom layers overlap. The new findings call for a reinterpretation of Earth's nightglow emissions and a revision of relevant atmospheric models.

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 cm 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.

The question of whether mesospheric OH( υ ) rotational population distributions are in equilibrium with the local kinetic temperature has been debated over several decades. Despite several indications for the existence of non-equilibrium effects, the general consensus has been that emissions originating from low rotational levels are thermalized. Sky spectra simultaneously observing several vibrational levels demonstrated reproducible trends in the extracted OH( υ ) rotational temperatures as a function of vibrational excitation. Laboratory experiments provided information on rotational energy transfer and direct evidence for fast multi-quantum OH(high- υ ) vibrational relaxation by O atoms. We examine the relationship of the new relaxation pathways with the behavior exhibited by OH( υ ) rotational population distributions. Rapid OH(high- υ ) + O multi-quantum vibrational relaxation connects high and low vibrational levels and enhances the hot tail of the OH(low- υ ) rotational distributions. The effective rotational temperatures of mesospheric OH( υ ) are found to deviate from local thermodynamic equilibrium for all observed vibrational levels., Competing Interests: Competing interests. The authors declare that they have no conflict of interest.

Energy transfer from O((1)D) to O2 is the main source of O2(b(1)Σ(+)(g) in vibrational levels υ = 0 and 1 in the Earth's thermosphere. Knowledge of the relative yields for O2(b(1)Σ(+)(g) production in υ = 0 and 1 is essential for a reliable interpretation and modeling of the O2 atmospheric band emissions (b(1)Σ(+)(g) - X (3)Σ(-)(g) from these two vibrational levels. We report laboratory measurements of the relative yields at room temperature. In the experiments, O2(b(1)Σ(+)(g), υ = 0, 1) is generated by O((1)D) + O2 collisions following partial photodissociation of O2 at 157.6 nm. O2(b(1)Σ(+)(g), υ = 0, 1) emission detection is used to monitor the temporal evolution of the vibrational level populations. The measured fractional yield for υ = 1 is 0.8 ± 0.1, in contrast with the results of previous studies that indicated dominant O2(b(1)Σ(+)(g), υ = 0) production. A revision is warranted of the values used for these relative yields in atmospheric models.