Absorption and photoionization coefficients have been measured for H2CO in the 600–2000-Å region. Integrated oscillator strengths were determined for a number of strong Rydberg transitions above 1200 Å. From the photoionization curve the first adiabatic ionization potential was found to be 10.87±0.01 eV. As an aid in interpreting the absorption spectrum, theoretical calculations were made using a single-configuration self-consistent field procedure for the Rydberg states and a model which included mixing between the Rydberg and valence states. On this basis, weak absorption features between 1340 and 1430 Å have been assigned to the B11(σ → π *) valence state. The 1A1(π → π *) valence state is deduced to be strongly autoionized just above the 2B2 ionization limit.
EUV spectra (530‐1500Å) of the day airglow in up, down and horizontal aspect orientations have been obtained with 6.5Å resolution and a limiting sensitivity of 5R from a rocket experiment. Below 834Å the spectrum is rich in previously unobserved OII transitions connecting with 4So, ²Do, and ²Po states. Recent broad‐band photometric observations of geocoronal HeI 584Å emission can be understood in terms of the newly observed OII emissions. The OI 989Å and OI 1304Å emissions exhibit similar dependence on altitude and viewing geometry with the OI 989Å brightness 1/15 that of OI 1304. Emission at 1026Å is identified as geocoronal HI Lyman‐β rather than OI multiplet emission and observed intensities agree well with model estimates. An unexpectedly high NI 1200/NI 1134Å brightness ratio is evidence of a significant contribution from photodissociative excitation of N2 to the NI 1200Å source function.
Far ultraviolet rocket spectra of N I and N2 dayglow emissions have been analyzed by using AE‐E photoelectron spectra, laboratory‐measured excitation cross sections, and photochemical models of atomic nitrogen. A self‐consistent picture of both optically thick and thin emission features is found by using a model in which the principal production mechanism for N I 1200‐Å and 1493‐Å photons is photodissociative excitation of N2. The aeronomic data require that ∼50–70% of the excited 4P atoms produced dissociatively have velocities within the Doppler core of the ambient nitrogen atoms, contrary to the expectation that those atoms are produced with large excess kinetic energy. The derived atomic nitrogen density has a maximum density of 2.7 × 107 cm−3 at 170 km, a value that is within 40% of that from recent models of odd nitrogen photochemistry.
Daytime airglow spectra between 530 and 930 ,• were obtained at --4 ,• resolution from a rocket launch at White Sands, New Mexico, June 27, 1980. Portions of the spectrum were observed in second order at --2,• resolution. The higher resolution of the present data confirms our previous identification of OII transitions and resolves the identification of OII emission at 537-539 ,• as due to both a doublet and a quartet component. For the case of viewing at 90 ø to the zenith between 196 and 242 km --1/3 of the 537-539 ,• emission originates from the OII 2s 2p 4 2P state. We infer a 538/581 • branching ratio of --3 in agreement with laboratory and calculated values of 1.6 to 3.3. Other OII branching ratios are in agreement with laboratory data. A somewhat low value for the 718/796 ratio in the flight data is interpreted as due to blended N 2 emission at 796 ,•. The observed emission rate from the 2s 2p 42p state is more adequately modeled using partial photoionization cross sections calculated using the dipole velocity rather than the dipole length approximation. This fact must be considered when computing the 834 ,• emission rate from direct photoionization. NII is seen to be a very minor source of emission below 916 ,• in the dayglow. The identification of OII 581 ,• emission confirms a prediction made in 1977 by Delaboudini•re. The OII emission at 581 ,• complicates interpretation of low resolution spectral observations near 584 ,• in the airglow and also for the case of comets and planets where O atoms and O bearing molecules are present.
Rocket observations of the far ultraviolet dayglow spectrum near solar cycle maximum are analyzed using laboratory cross sections, atmospheric composition models, and photoelectron production models. Photoelectron‐excited emissions of N2 and O are used to derive a self‐consistent description of the atmosphere at solar maximum. Spectral synthesis of the N2 Lyman‐Birge‐Hopfield bands shows a departure of a¹Πg state vibrational populations from the direct excitation theory. Observations of the N2 second positive (0, 0) 3371‐Å band and O I 1356‐Å emission indicate an exospheric temperature of 1600 K, 200 K higher than predicted by empirical models. The empirical models are also found to overestimate the O and O2 densities required to fit the data by a factor of 2 and 1.4, respectively. These results are compared to the results of an analysis of similar observations made in 1978 near solar minimum. The derived photoelectron flux at 290 km is a factor of 1.6 times larger than for the 1978 data.
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