a1Πg State Research Articles

Electron impact study of the 100 eV emission cross section and lifetime of the Lyman‐Birge‐Hopfield band system of N2: Direct excitation and cascade

AbstractWe have measured the 100 eV emission cross section of the optically forbidden Lyman‐Birge‐Hopfield (LBH) band system (a1Πg → X1Σ+g) of N2 by electron‐impact‐induced fluorescence. Using a large (1.5 m diameter) vacuum chamber housing an electron gun system and the Mars Atmosphere and Volatile EvolutioN mission Imaging Ultraviolet Spectrograph optical engineering model, we have obtained calibrated spectral measurements of the LBH band system from 115 to 175 nm over a range of lines of sight to capture all of the optical emissions. These measurements represent the first experiment to directly isolate in the laboratory single‐scattering electron‐impact‐induced fluorescence from both direct excitation of the a1Πg state and cascading contributions to the a1Πg state (a′1Σ−u and w1Δu → a1Πg → X1Σ+g). The determination of the total LBH emission cross section is accomplished by measuring the entire cylindrical glow pattern of the metastable emission from electron impact by imaging lines of sight that measure the glow intensity from zero to ~400 mm radial distance and calculating the ratio of the integrated intensity from the LBH glow pattern to that of a simultaneously observed optically allowed transition with a well‐established cross section: Ni 120.0 nm. The “direct” emission cross section of the a1Πg state at 100 eV was determined to be σemdir = (6.41 ± 1.3) × 10−18 cm2. An important observation from the glow pattern behavior is that the total (direct + cascading) emission cross section is pressure dependent due to collision‐induced cascade transitions between close‐lying electronic states.

Intensities of the Martian N2 electron‐impact excited dayglow emissions

AbstractThe first N2 emissions in the Martian dayglow were detected by the SPICAM UV spectrograph on board the Mars Express spacecraft. Intensities of the (0,5) and (0,6) Vegard‐Kaplan bands were found to be about one third of those predicted more than 35 years ago. The Vegard‐Kaplan band system arises from the transition from the lowest N2 triplet state () to the electronic ground state (). It is excited in the Martian dayglow by direct electron‐impact excitation of the ground N2(X) state to the A state and by excitation to higher triplet states that populate the A state by cascading. Using revised data, we compute here updated intensities of several of the bands in the N2 triplet systems and those involving the a1Πg state, the upper state of the Lyman‐Birge‐Hopfield bands. We find that the predicted limb intensities for the (0,5) and (0,6) Vegard‐Kaplan bands are consistent with the measured values.

Kinetic study of the temperature effect on the quenching of the N2(a1Πg) and N2(a′1Σu−) states in pure nitrogen and the quenching of the N2(a1Πg) state by molecular oxygen in N2–O2 mixtures

We have investigated the effect of temperature on the collisional decay of the a1Πg and a′1Σu− states of nitrogen. A temporal study of the VUV emission of nitrogen has been performed from 1 mbar to atmospheric pressure after a selective excitation of the a1Πg state by two-photon absorption with a dye laser. In our gas temperature range, from 193 to 333 K, we observed two opposite behaviours for the decays of the a′1Σu− and a1Πg states, and we attributed these behaviours to the strong coupling between the a1Πg and w1Δu states at high pressure. We then studied the effect of molecular oxygen on the decay of the N2(a1Πg) state at 293 K and identified the product of the quenching reaction of this state by molecular oxygen.

Modeling the N2 Lyman‐Birge‐Hopfield bands in the dayglow: Including radiative and collisional cascading between the singlet states

A new model for the daytime airglow emissions from the N2 Lyman‐Birge‐Hopfield (LBH) bands (a1Πg–X1Σ+g) in the Earth's atmosphere is presented. This model gives good agreement, which other first principles models have been unable to obtain, with dayglow observations reporting increased vibrational populations, relative to direct excitation, for v ≤1 of the N2 a1Πg state. The ability of the model to match the observations is due to the inclusion of radiative and collisional cascading between the singlet states. Such cascading was not included in previous first principles dayglow calculations for the singlet states. Not only does cascading improve the fit between the observed and modeled relative vibrational populations, it also increases the total emission from the LBH bands by a factor of ∼1.6 due to the transfer of excitation from the a′ and w states to the a state, resulting in significantly greater emission than predicted by earlier models using the same excitation cross section for the a state. Such an increase in the total emission from the LBH bands is consistent with recent work by Budzien et al. [1994] and Link et al. [1994]. The coupling between the singlet states will have a significant impact on the interpretation of the LBH band emissions from the Earth's atmosphere.

Observations of the far ultraviolet airglow by the Ultraviolet Limb Imaging Experiment on STS‐39

The Ultraviolet Limb Imaging (UVLIM) experiment flew on STS‐39 in the spring of 1991 to observe the Earth's thermospheric airglow and included a far ultraviolet (1080‐1800 Å) spectrometer. We present first results from this spectrometer, including a spectroscopic analysis at 6‐Å resolution of H, O, N, and N2 day glow emissions and modeling of the observed limb‐scan profiles of dayglow emissions. The observed N2 Lyman‐Birge‐Hopfield (LBH) emission reflects a vibrational population distribution in the a¹Πg state that differs significantly from those predicted for direct electron excitation and excitation with cascade from the a′ ¹Σu− and w ¹Δu states. The Vibrational population distribution and LBH brightnesses suggest a total cascade rate 45% that of direct excitation, in contrast to laboratory measurements. For the first time, pronounced limb brightening is observed in both the N I λ1135 and N I λ1200 limb emission profiles, as expected for emissions excited by N2 dissociation which produces kinetically fast N fragments; however, optically thick components of these features are also observed. Preliminary modeling of the O I λ1356, H I λ1216, and O I λ1304 and O I λ1641 emissions agrees to within roughly 10% of the observed limb‐scan profiles, but the models underestimate the N2 LBH profiles by a factor of 1.4‐1.6, consistent with the inferred cascade effect. Other findings include: an O I λ1152/λ1356 intensity ratio that is inconsistent with the large cascade contribution to O I λ1356 from np 5P states required by laboratory and nightglow observations; nightglow observations of the tropical ultraviolet arcs exhibit a wide range of O I λ1356/λ1304 intensity ratios and illustrate the complicated observing geometry and radiative transfer effects that must be modeled; and we find a 3‐σ upper limit of 8.5 R to the total LBH vehicle glow emission.

Rocket‐borne spectroscopic measurements in the ultraviolet aurora: The Lyman‐Birge‐Hopfield bands

Ultraviolet emissions from Earth's aurora were observed at wavelengths between 1675 and 2075 Å by a sounding rocket payload launched at Churchill, Canada, on March 28, 1980. The emissions from the Lyman‐Birge‐Hopfield (LBH) and Vegard‐Kaplan (VK) bands of N2 were observed and analyzed to determine the relative populations of the υ′ = 0–6 levels of the a¹πg state and the υ′ = 4, 6, 7, and 8 levels of the A³Σg+ state respectively. The relative population of higher vibrational levels of the A³Σg+ state are consistant with direct excitation and cascade. The relative populations of the vibrational levels of the a¹πg state peak at υ′ = 2. Such a distribution has not been observed previously in the aurora.

Radiative Lifetime of the "Forbidden" a1Πg ← X1Σg+ Transition of N2

Using the second order polarization propagator method we have calculated the band oscillator strengths of the "forbidden" Lyman-Birge-Hopfield band system, a1Πg ← X1Σg+, of N2 and the radiative lifetime τ(v') of the a1Πg state for 0 ⩽ v' ⩽ 4. We find that τ(0) = 58.6 μsec and τ(4) = 68.2 μs. About 95 per cent of the intensity are magnetic dipole contributions and only about 5 per cent come from the electric quadrupole radiation.

A reexamination of important N2 cross sections by electron impact with application to the dayglow: The Lyman‐Birge‐Hopfield Band System and N I (119.99 nm)

The far ultraviolet emission spectrum (120 to 210 nm) of electron‐excited N2 has been obtained in a crossed‐beam laboratory experiment. The cross section of the Lyman‐Birge‐Hopfield (LBH) band system (a¹Πg → X¹Σg+) has been remeasured using experimental techniques we have previously developed for this metastable transition. The improved laboratory data set for the a¹Πg state allows a determination of the excitation, emission, and predissociation cross section from threshold to 200 eV for use in planetary atmosphere models of the dayglow and aurora. An analytic fit to the experimental cross section allows accurate estimates to arbitrarily high excitation energy. The close agreement in both energy dependence and absolute cross section values between the emission measurements, presented here, and published electron scattering results shows cascade is small (<5%). The total excitation cross section for the N2 a¹Πg state is estimated to be 6.22 ± 1.37×10−18 cm² at 100 eV. The absence of emission bands for υ′ ≥ 7 suggests the predissociation yield is unity. The excitation function of each vibrational level is found to have the same shape to within 5%. In the low‐energy region, ε < 20 eV, differences in excitation threshold lead to a significant departure of the relative vibrational cross sections from the Franck‐Condon distribution. Thus the relative LBH vibrational population distribution in a planetary dayglow or aurora is affected by the energy distribution of the electron flux; and we show that atmospheric models need to include this threshold effect. The N I (119.99 nm) cross section has also been remeasured and found to be 3.48 ± 0.77×10−18 cm² at 200 eV on the basis of a comparison with Lyman α emission from dissociative excitation of H2.

The ultraviolet dayglow at solar maximum: 3. Photoelectron‐excited emissions of N2 and O

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.

Analysis of nitrogen and oxygen far ultraviolet auroral emissions

A far ultraviolet rocket observation of an auroral arc has been analyzed by using laboratory measured emission cross sections and self‐consistent models of the atmospheric composition and energetic electron flux. We find that the observed arc was produced by a precipitating electron flux of 5–8 erg cm−2 s−1 energy content and 1.75‐ to 2.5‐keV characteristic energy. The N2 Lyman‐Birge‐Hopfield spectral synthesis shows that the vibrational populations for the observed emissions are in agreement with laboratory electron impact spectra and the Franck‐Condon factors to υ″ = 0. This precludes any cascade process to the a¹Πg state, which significantly changes the relative populations. Altitude profiles of the various bands can be reproduced by theory. Dissociation of N2 can account for N I lines at 1134, 1200, and 1493 Å, and N II 1085 Å. Contrary to previous observations of auroras and the dayglow, there is no evidence of multiple scattering of 1200‐Å radiation by atomic nitrogen in the present observations. The atomic oxygen density required to explain the O I 1356, 1304, and 989‐Å emissions is 4×1010 cm−3 at 120 km, a factor of 3.5 lower than the Jacchia (1971) model.

Vacuum-ultraviolet emissions from laser-excited b^′^1∑u+ state of molecular nitrogen

Selective excitation of the b′1∑u+ valence state of molecular nitrogen using a single laser source is described. The excitation process utilizes a two-photon transition to the intermediate a1Πg state followed by a single-photon transition to the target state. The excitation is confirmed by observation of the VUV emission from the b′ state to high-vibrational levels of the ground molecular state. The b′-state population of N2 is estimated to be greater than 1013 cm−3. Generation of highly excited atomic nitrogen following the two-photon excitation of the a1Πg molecular state is reported.

Excitation Process of the N2 IR (B3Πg-A3Σu+) Laser Transition

Excitation mechanism of the laser action on the first positive system (B3Πg-A3Σu+) of N2 molecule is studied with an emphasis on the time relationship between the ir and uv (C3Πu-B3Πg) laser pulses. Pressure dependence of time lag between the uv and ir pulses has been observed in the pressure range from 4 to 8 Torr using an axial-discharge tube. The observed time lag is well predicted by a model which involves direct electron-impact excitation, near-resonant collisional excitation transfer from the a1Πg state, and cascade excitation due to the uv laser action for the upper ir laser level. Relative importance of these excitation processes depends primarily on N2 pressure.

Emission Spectra of Nitrogen in the Vacuum Ultraviolet

The emission spectra of nitrogen have been photographed in the vacuum ultraviolet under high resolution. Selected bands of the b′1Σu+—X1Σg+ and the a1Πg—X1Σg+ systems have been analyzed and new rotational and vibrational constants obtained. The observed Λ-type doubling of the a1Πg state was used to determine that the Fc′ doublet component lies above the Fd′ component. Rotational perturbations were found in the v=0 level of the a1Πg state, and in the v=0 and 1 levels of the b′1Σu+ state. Vibrational perturbations were found in the v=4, 5, and 6 levels of the b′1Σu+ state and a new predissociation at v=4, J=9 was observed. The new predissociation limit lies at 13.233 ev and is equally compatible with either the 9.756 ev or the 7.373 ev ground-state dissociation energy.

PREDISSOCIATION AND DISSOCIATION OF THE N2 MOLECULE

The predissociation in the upper state of the Lyman–Birge–Hopfield bands of nitrogen has been investigated under high resolution. A clear breaking-off of the fine structure has been found above ν′ = 6, J′ = 13 both in the P and the Q branches. The predissociation limit is at 78,843 ± 50 cm.−1 which is smaller than the well known dissociation limit 97,944 cm.−1 by very nearly the excitation energy of the 2D state of the N atom. This observation eliminates the intermediate value, 8.565 ev., for the dissociation energy of the N2 molecule. A definite decision between the two remaining values, 9.756 and 7.373 ev., cannot yet be made. Acceptance of the former would involve the assumptions that the predissociation in the a1Πg state is caused by the [Formula: see text] state resulting from normal atoms and that this [Formula: see text] state is not purely repulsive. The elimination of the value 8.565 ev. for D(N2) removes the value 7.50 ev. for D(CN) which has been fairly widely accepted.