David L. Huestis

Vibrational level distribution of O2(b 1∑+ g , v = 0–15) in the mesosphere and lower thermosphere region

We have determined the vibrational distribution of emission from the O 2 (b 1 Σ + g ) state in the terrestrial nightglow. Previous nightglow studies have been limited to detecting emission from the v = 0 level, but the high sensitivity and resolution available in sky spectra taken with the Keck I telescope and the associated High Resolution Echelle Spectrometer (HIRES) have made it possible to observe O 2 (b 1 Σ + g ) levels up to v = 15. Apart from emission from v = 0, which is intense above the atmosphere due to ineffective quenching, the vibrational-level-dependent intensity distribution is bimodal, with maxima at v = 3-4 and v = 12, and a deep minimum at v = 8. Excluding the v = 0 level, the total system intensity is of the order of 150 rayleighs. The O 2 (b 1 Σ + g ) vibrational levels seem to fall into two categories: v = 0, 1 and v ≥ 2. The intensity of emission from the latter group tends to follow the intensity of the OH Meinel system, while that of the former group does not. We consider the possibility that at least a portion of the v ≥ 2 group derives from OH chemistry, and also discuss the possible role of sodium chemistry.

Optical excitation of the b 4Σ− -X Π transition in NO

Abstract The NO(b 4 Σ − ) state, in v =2−5, has been excited from the X 2 Π ground state by optical pumping, and detected by b 4 Σ − -a 4 Π Ogawa band emission. Line positions have been measured to ±0.5 cm −1 , utilizing the fact that these excitation spectra simultaneously show the b-X bands and the doublet bands that are found in the 187–205 nm tuning range used in the experiments. As a result, the relative positions of the NO quartet and doublet manifolds can be specified to a precision considerably improved over the previous estimate of ±2 cm −1 . Predissociation in b( v =5) is evident in each rotational branch commencing at J =16.5, and appears to be much stronger than in the nearby B 2 Π v =7 level. From the observed branch intensities, it is shown that the transition receives its oscillator strength through spin-orbit interaction between the b 4 Σ − state and various 2 Σ, 2 Π, and 2 Δ states. An estimate of the relative oscillator strengths for the forbidden b-X and allowed B-X transitions indicates that B-X is favored by about three orders of magnitude. As most of the NO (b) radiates to the NO(a) state, this study provides a means of directly populating the low- v levels, principally v =1, 2, of NO(a). There is strong collisional coupling between the doublet and the b 4 Σ − states, as shown by the fact that excitation spectra of the NO A 2 Σ + , B 2 Π, and D 2 Σ + states can be measured from detection of the Ogawa bands. A search was carried out for emission resulting from excitation of the b( v =6) level, the first lying above the dissociation limit. No such emission was seen, suggesting that rapid predissociation occurs through the a 4 Π continuum; earlier reports of emission from b( v =6) are shown, based on spectral simulation, to be in error. Generation of the NO(E 2 Σ + ) state was observed concomitantly with NO(D) excitation, which must involve an energy pooling mechanism.

Accurate atomic line wavelengths from astronomical sky spectra

Numerous atomic lines appearing in the terrestrial nightglow can be measured by HIRES, the echelle spectrometer on the 10 meter Keck I telescope on Mauna Kea. The observable nightglow lines include emissions from Na, K, Hg, Ne, N, O, and H. Agreement between the line positions and those from National Institute of Science and Technology (NIST) compilations is excellent for well-known lines, typically 2 mA or better. For lines which are not well-known, or cannot be measured directly in the laboratory, deviations are significant. In particular, for the optically forbidden N(2D–4S) transition the differences are substantial, 20 and 24 mA for the two components. Apart from improving the line positions for this transition, we also correct an error that has been perpetuated in the aeronomic literature for the last 30 years concerning the transition wavelengths. The potassium D1 line, recently discovered in the HIRES sky spectra, exhibits a position difference of 11 mA between the NIST and HIRES values, significa...