Airglow Derived Measurements of Q-Branch Transition Probabilities for Several Hydroxyl Meinel Bands

Abstract

Spectroscopic measurements of the hydroxyl (OH) airglow emissions are often used to infer neutral temperatures near the mesopause. Correct Einstein coefficients for the various transitions in the OH airglow are needed to calculate accurate temperatures. However, studies from some studys showed experimentally and theoretically that the most commonly used Einstein spontaneous emission transition probabilities for the Q-branch of the OH Meinel (6,2) transition are overestimated. Extending their work to several Δv = 2 and 3 transitions from v′ = 3 to 9, we have determined Einstein coefficients for the first four Q-branch rotational lines. These have been derived from high resolution, high signal to noise spectroscopic observations of the OH airglow in the night sky from the Nordic Optical Telescope. The Q-branch Einstein coefficients calculated from these spectra show that values currently tabulated in the HITRAN database overestimate many of the Q-branch transition probabilities. The implications for atmospheric temperatures derived from OH Q-branch measurements are discussed.

OH level populations and accuracies of Einstein-A coefficients from hundreds of measured lines

“…According to the theoretical considerations of Pendleton and Taylor (2002) triggered by the OH(6-2) line intensity ratios measured by 305 French et al (2000), this can be explained by the general neglection of orbital angular momentum uncoupling, which is related to rotational-electronic mixing of the electronic ground state X 2 Π and the first excited state A 2 Σ + , for the calculation of the available Einstein-A coefficients. OH line measurements in near-infrared spectroscopic data from the Nordic Optical Telescope at La Palma (Spain) by Franzen et al (2019) indicate that too low Q-branch populations or too high Einstein-A coefficients (based on HITRAN) are also an issue for OH bands with ∆v = 2 and 3 not covered by our study.…”

OH level populations and accuracies of Einstein-<i>A</i> coefficients from hundreds of measured lines

“…However, the range is relatively wide with values between −0.51 for TL89 and −0.02 for LWR86. The latter is the only satisfactory set for this comparison at low E as it was already found by French et al (2000) for OH(6-2) low- N lines. For levels with high rotational energy, y tends to be positive.…”

“…Except for B+16, our selection of sets agrees with Liu et al (2015) and Hart (2019b), who studied the impact of Einstein-A coefficients on the populations of low rotational levels. Other comparisons used a smaller number of sets (French et al, 2000;Cosby and Slanger, 2007;Noll et al, 2015;Parihar et al, 2017;Noll et al, 2018b). Note that the three oldest sets lack a significant number of the measured 723 doublets.…”

“…the different sets fail in a similar way. According to the theoretical considerations of Pendleton and Taylor (2002) triggered by the OH(6-2) line intensity ratios measured by French et al (2000), this can be explained by the general negligence of orbital angular momentum uncoupling, which is related to rotational-electronic mixing of the electronic ground state X 2 and the first excited state A 2 + , for the calculation of the available Einstein-A coefficients. OH line measurements in near-infrared spectroscopic data from the Nordic Optical Telescope at La Palma (Spain) by Franzen et al (2019) indicate that too low Q-branch populations or too high Einstein-A coefficients (based on HITRAN) are also an issue for OH bands with v = 2 and 3 not covered by our study.…”

“…Mies, 1974;Langhoff et al, 1986;Turnbull and Lowe, 1989;Nelson et al, 1990;Goldman et al, 1998;van der Loo and Groenenboom, 2007;Brooke et al, 2016) and evaluation (e.g. French et al, 2000;Pendleton and Taylor, 2002;Cosby and Slanger, 2007;Liu et al, 2015;Hart, 2019b). Apart from the derivation of absolute OH level populations or densities (Noll et al, 2018b;Hart, 2019b), the quality of these transition probabilities especially affects OH-based temperature estimates (Liu et al, 2015;Noll et al, 2015;Parihar et al, 2017;Hart, 2019b) and abundance retrievals for species like atomic oxygen (Mlynczak et al, 2013;Noll et al, 2018b).…”