Birge Hopfield Research Articles

Viking UV cameras: Calibration using model calculations and long-wavelength light leakage

In this paper we present the results of a comparison of Viking UV images of the dayglow with model calculations of the dayglow in order to assess the uncertainties in the calibration of the cameras. The procedure that was adopted was to calculate the dayglow emissions using proven photoelectron and radiative transfer codes, and convolve these results with the prelaunch camera transfer functions to estimate the emission strength for the two cameras (1304 Å (1 Å = 10−10 m) and Lyman–Birge–Hopfield (LBH)). We found quite good agreement between the model and measured intensities for the 1304 Å camera. However, the results for the LBH camera suggest that there is substantial leakage of far UV (FUV) light at 2000 Å and "white" light around 3400 Å: only about 20–30% of the emission comes from the LBH dayglow. To evaluate the contamination from the longer wavelengths, we assumed that it was due to Rayleigh scattering off air molecules and reflection off surfaces. A multilayer multiple-scattering radiative transfer code allowing for Rayleigh scattering and absorption by O2 and O3 was used. The model intensities were then convolved with "known" and extrapolated camera filter response characteristics in the region from 1800 to 7500 Å to obtain model camera signal values due to long-wavelength light leakage. A preliminary estimate of the scattering contribution to the observed intensities using the first-order guess of the camera response function (CRF) beyond 2000 Å resulted in a substantial overestimation of the LBH camera intensities. To obtain a better estimate of the CRF we used an LBH image where clouds were known to be present. This allowed us to distinguish between the 2000 and 3400 Å contributions in order to obtain a modified CRF that reproduces the data for the LBH camera. The fact that both the 1304 Å dayglow camera intensities and the model results are comparable and the LBH intensities may be modelled allowing for Rayleigh scattering suggest that the calibration of the cameras in the FUV are defined to better than about 30% and so should reproduce nighttime auroral intensities at 1304 Å and for the LBH system with comparable accuracy.

Multiple fluorescent scattering of N2 ultraviolet emissions in the atmospheres of the Earth and Titan

Absorption and reemission by N2 of the Lyman‐Birge‐Hopfield (LBH) and Birge‐Hopfield (BH) band systems in the earth aurora and Titan dayglow has been calculated by using multiple scattering in a plane‐parallel radiative transfer model and a line‐by‐line synthesis of individual bands. The effects of rotational structure are included through the use of temperature‐dependent band escape functions and band transmission functions. Iteration through successive scatterings shows that fewer than ten scatterings are important, even for the thickest bands. The excitation rate caused by multiple scattering is sensitive to the vibrational population distribution of the ambient N2, especially for the BH system in the earth aurora. For a vibrational temperature of 2000 K, the fluorescent scattering contribution at 110 km is 15 times that of a vibrational temperature of 300 K. Comparison of theoretical intensities to rocket observations of the BH (1, 10) band indicates a vibrational temperature of 1000 K, but the corresponding (1, 3) band intensity is brighter than the observation. Scattering on Titan was modeled for both a high‐ and low‐altitude source and for various vibrational temperatures. The LBH system has an enhanced sensitivity to vibrational temperature on Titan because of the rapidly changing photoabsorption cross section of CH4 around 1400 Å. It is argued that the c4′ ¹Σu+ Rydberg bands should have an intensity profile similar to the BH bands. The c4′ (0, 0) band limb brightening inferred from the full‐disk and bright limb spectra observed by Voyager indicates a source around 4000 km, but this is in conflict with the observed intensity peak at 3660 km in the limb scan data.

The extreme ultraviolet day airglow

Satellite observations of the earth's extreme ultraviolet day airglow between 350 and 1400 Å are described. The atomic spectrum shows lines of O II (538–539, 555, 601, 617, 673, 718, 834), He I 584, O I (989, 1152, 1304, 1356), N II (916, 1085), N I (1134, 1200), and H I (1025, 1216, and possibly 973). Previously unobserved weak O II lines (515, 482, 470, 442) are observed below 530 Å. The Lyman‐Birge‐Hopfield (LBH) and Birge‐Hopfield (BH) bands of N2 between 900–1100 Å are the dominant molecular lines. Large scale high latitude and equatorial enhancements and hemispheric asymmetries are evident in the near zenith O II 834, O I 989 and N II 1085‐Å line intensities.

Detection of the a 1Πg (v′=0, 1)←X 1Σ+g (v″=0) transition in N2 by laser-induced fluorescence

The a1Πg(v′=0.1)←X1Σ+g(v″=0) transitions in N2, part of the Lyman–Birge–Hopfield bands, have been seen by two-photon absorption detected by VUV fluorescence. Fluorescence was observed over a pressure range from 5 m Torr to 10 Torr. From the lifetime decrease with pressure at low pressure, quenching rate constants were determined, which are equivalent to cross sections in the range of 2.6×10−16 cm2. At higher pressures a slow decay was observed that is interpreted as being due to the a′ state which acts as an energy reservoir with a pressure dependent decay constant. We anticipate that laser-induced fluorescence will be a useful scheme for measuring ground state rotational and vibrational populations for the nitrogen molecule.

Electron-impact excitation of the singlet states of N2. I. The Birge–Hopfield system (b1πu–X1Σ+g)

The absolute emission cross sections for the excitation of the Birge–Hopfield bands (b1πu–X1Σ+g) by electron impact on N2 from threshold to 500 eV are reported. With the exception of the v′=1, 5, and 6 vibrational levels, all other b1πu levels are found to predissociate with a specific predissociation branching ratio, ≳0.99. This forbidden process is a major source of the nitrogen atoms produced by electron-impact dissociation of N2. Quite remarkably, the b1πu (v′=1) state is only slightly predissociated (<10%). The emission cross section for the v′=1 level has a maximum value of 2.3×10−18 cm2 at 35 eV. The absolute transition probabilities, emission cross sections, and the variation of the electric dipole moment with internuclear distance for 21 BH (1, v′) bands are given.

Reactions forming electronically-excited free radicals. Part 2.—Formation of N4S, N2D and N2P atoms in the H + NF2reaction, and N atom reactions

The kinetics of reactions (at 298 K) involving ground-state N 4S and excited-state N 2D, 2P atoms have been studied using resonance-fluorescence detection in a discharge-flow system. The N 4S+ NO reaction has been studied as a reference reaction: N 4S+ NO [graphic omitted] N2+ O; k1=(3.4 ± 0.3)× 10–11 cm3 molecule–1 s–1(1σ). Also, the rate constant k2 for the N 4S+ NF2 reaction has been determined: N 4S+ NF2 [graphic omitted] 2 NF; k2=(4.6 ± 0.3)× 10–12 cm3 molecule–1 s–1(1σ). The reaction H 2S+ NF2 gives NF radicals in the a1Δ state with a branching ratio 0.9: H 2S+ NF2→ NF a1Δ+ HF X1∑+. This conclusion is drawn from a study of concentration profiles of N 4S and N 2D atoms in the state-selective reactions (4a), (4b): H 2S+ NF a1Δ→ N 2D+ HF X1∑+(4a), H 2S+ NF X3∑–→ N 4S+ HF X1∑+.(4b)., Although N 2D atoms are an initial product of the H + NF reaction. N 4S atoms are formed only at longer reaction times when [N 2D] has mostly decayed. Thus, it is inferred that NF X3∑– is not a major initial product of the H + NF2 reaction.Chemiluminescence is emitted in the H + NF2 reaction from excited NF a1Δ, b1∑+ and from excited N2B3Πg and N2a1Πg in the First Positive (B–A) and Lyman–Birge–Hopfield (a–X) band systems. The precursor of NH chemiluminescence in the H + NF2 system is thought to be ground-state NH X3∑– formed in the reaction N 2D+ H2→ NH X3∑–+ H.

The absolute generalized oscillator strength for the Lyman–Birge–Hopfield band of N2 as determined by high energy electron impact spectroscopy

The absolute generalized oscillator strength for the dipole forbidden quadrupole allowed Lyman–Birge–Hopfield band [a 1Πg ← X 1Σ+g] in molecular nitrogen at an energy loss of 9.35 eV is observed by electron impact spectroscopy using 25 keV electrons over the momentum transfer range 0.04?K2?10 a.u. The results agree in the zero angle (zero momentum transfer) limit with the previous observations of Skerbele and Lassettre, but are in disagreement with previous theoretical and experimental results for K2≳0.5.

Intensity Calibration of a Spectroscopic Detecting System in the 1100–1820- Å Region

Relative intensity measurements in (υ′ = const, υ″) band progressions of the (B1Σu+ → X1Σg+) Lyman systems of H2 and HD were used to determine the relative sensitivity of a monochromator–detector combination in the wavelength region from 1100 to 1650 A. Additional measurements in three band progressions of the Lyman–Birge–Hopfield system of nitrogen yielded results that are in good agreement with the hydrogen data. The calibration range could then be extended to 1820 A.

Emission Cross Sections of N2 in the Vacuum Ultraviolet by Electron Impact

The electron beam excitation spectrum of N2 at low pressure (0.12–0.18 μ) in the wavelength range 1200–2000 Å was recorded for monoenergetic electron impact energies from 10 to 208 eV. The spectral features observed were the Lyman–Birge–Hopfield vibrational transitions (a1Πg → 1Σg and several atomic nitrogen multiplets produced by dissociative excitation of N2. The emission cross sections of the Lyman–Birge–Hopfield vibrational transitions were measured and summed to give the cross section for the entire electronic transition. The energy dependence of all vibronic transitions was the same. The atomic nitrogen transitions studied were the multiplets at 1200 Å (4P → 4S0), 1493 Å (2P → 2D0) and 1743 Å (2P → 2P0). These emissions originated from dissociative excitation of molecular nitrogen. The emission cross sections of these transitions typically have two thresholds, corresponding to dissociative excitation of N2 into two nitrogen atoms and to dissociative ionization excitation into an atom and an ion.

Transition Probabilities and Collision Broadening Cross Section of the N2 Lyman–Birge–Hopfield System

Bands of the N2 Lyman–Birge–Hopfield system (a 1Πg − X 1Σg+) have been observed in absorption in path lengths ranging from 5.6 × 10−3 to 0.5 m·atm. Comparison of theoretical and measured curves of growth and band profiles have allowed the computation of band transition probabilities, lifetimes of the a 1Πg state [τ(0) = 1.4 × 10−4sec], relative probabilities of electric quadrupole and magnetic dipole components, and a measure of the collision self-broadening cross section (σ = 1.6 × 10−14cm2).

Excitation of Nitrogen by Electrons: The Lyman–Birge–Hopfield System of N2

The intensity of vacuum ultraviolet emission produced by a beam of electrons in nitrogen has been measured to determine the probability of exciting the Lyman–Birge–Hopfield (LBH) bands of N2. Measurements included spectral scans of the region 1250–2150 Å and scans across the electron beam with a photometer sensitive in the range 1230–1800 Å. Some 60 spectral features were attributed to LBH bands. Emission in these bands persisted in a glowing region outside the electron beam. At low gas pressure (0.1 μ), relative band intensities are in rough agreement with values calculated by assuming the Franck–Condon principle applies in excitation and emission. At higher pressure, the intensity was increased by secondary processes, and bands with lower υ′ were enhanced relative to other bands. Photometer scans at low pressure yielded a profile of the glow around the beam which was fairly consistent with a profile calculated by assuming a radiative lifetime of 80 μsec for the parent state of the LBH. Based on the calculated profile and spectrometer and photometer observations, an apparent cross section of 1.1 ± 0.2 × 10−18 cm2 was obtained for excitation of the LBH system by 900-eV electrons. The dependence on energy of representative bands indicated total LBH cross sections for 100- and 2000-eV electrons of 6.7 × 10−18 and 5.2 × 10−19 cm2, respectively.

Quadrupole-Allowed Transitions in the Electron-Impact Spectrum of N2

The forbidden transitions observed in the nitrogen spectrum below 12.5 V have been studied by electron-impact methods. Since the theory indicates that electric quadrupole transitions are preferred to singlet—triplet in electron-impact spectra at intermediate energies, the observed transitions are probably electric quadrupole allowed. This has been demonstrated for the 12.25-V transition by studying its relative intensity as a function of the scattering angle. The data on the 11.86-V transition are also compatible with an electric quadrupole transition. In addition, new data on the Lyman—Birge—Hopfield bands in the 60–400-V energy range indicate that the intensity distribution among vibrational levels is independent of energy.

Intensity Distribution of the Lyman–Birge–Hopfield Band System of N2

RECENT rocket investigations of the vacuum, ultraviolet radiation emitted from the aurora and airglow have shown up the need for laboratory measurements of molecular band intensities in this spectral region. Such laboratory investigations have until now been hampered by a lack of suitable intensity standards. One prominent band system of importance in this region is the Lyman–Birge–Hopfield (LBH) (a 1IIg− X 1Σ+g) system of N2, which lies between 2000 A and 1200 A. It has recently been observed in auroral spectra taken by rocket-borne spectrometers1,2. The only laboratory measurements of intensities of bands of the LBH system in emission are estimates of photographic plate blackenings3, where no account has been taken of variation of instrumental response with wave-length.

Relative Intensities of Lyman—Birge—Hopfield Bands in Electron Impact Spectrum of Nitrogen

Relative Intensities of Lyman—Birge—Hopfield Bands in Electron Impact Spectrum of Nitrogen

Collision Cross-Section Study of Two Transitions in Nitrogen

Electron collision cross sections at three voltages have been determined for two transitions in nitrogen. One of these corresponds to the Lyman—Birge—Hopfield bands, a forbidden system, and it is shown that the collision cross sections are compatible with the term symbol assignment for the excited state.

Far-Ultraviolet Auroral Spectra

Further studies of the spectral data obtained in the February 1960 rocket experiment [ Ann. Geophys.17, 109 ( 1961)] have confirmed much of the preliminary analysis. New bands of the Lyman–Birge–Hopfield system in the 1700–2200 Å region have been identified. More precise wavelength determinations have been made, and previously indicated bands of N2 have been more positively identified. The Lyman α line of hydrogen is just detectable. The intensity distribution among the various bands during weak auroral conditions is shown to differ markedly from more intense displays. Laboratory data obtained from a microwave discharge in very pure N2 show good correlation with some features of the rocket spectra.

Interpretation of Intensity Distributions in the N2 Lyman–Birge–Hopfield and CO Fourth Positive Band Systems

IN much of the early spectroscopic literature the photographic density (measured or estimated by eye) of spectral features is reported, in addition to wavelengths, as an aid to identification. In many molecular band systems in the vacuum ultra-violet region (which are becoming of increasing interest in astrophysics and plasma physics and the intensities of which, in emission, are relatively difficult to measure) the early data on photographic blackening are still the only experimental information on band intensities available.

A NEW BAND SYSTEM OF THE P2 MOLECULE ANALOGOUS TO THE LYMAN–BIRGE–HOPFIELD BANDS OF N2

Five bands of a new band system of P2 have been photographed at high dispersion and analyzed. The upper state of the system is a 1П0 state and lies lower than any previously known excited singlet state. The lower state of the new system is the ground state of P2 and the analysis of the new bands has given improved constants for this state. The new system appears to be the analogue of the Lyman–Birge–Hopfield bands of N2. The electron configuration of the low excited states of P2 and of related molecules is discussed.

EMISSION BAND SPECTRA OF NITROGEN THE LYMAN-BIRGE-HOPFIELD SYSTEM

The near ultraviolet part of the emission spectrum of nitrogen has been photographed under high resolution. Thirteen bands of the [Formula: see text] system (Lyman–Birge–Hopfield) have been analyzed and new vibrational and rotational constants obtained. Combining the observed data with those obtained by Stoicheff from the Raman spectrum of nitrogen, refined equilibrium constants for the ground state were obtained. The predissociation in the α1Πg state was observed.

La tension de vapeur et la chaleur de sublimation du carbone IV

AbstractAll significant direct and indirect evidence on the heat of sublimation of carbon, Lc, agrees with the authors recent result Lc =141.26 kcal.Only electron collision experiments in carbon monoxyde on the one hand, in methane and its derivatives on the other, seem to lead unambiguously to the same value within the limits of experimental error (about ± 5 to 10 kcal) of this method.Spectroscopic data on CO could yield an unambiguous and much more exact result if a second dissociation (or predissociation) limit were ascertained; e. g. the convergence limit and the nature of the excited state of the Birge Hopfield bands near 1000 A.Vapour pressure measurements with the carbon are and the counter pressure method give Lc ± 140 kcal, although the interpretation of these measurements is involved and by no means free of ambiguity, except perhaps in the case of Ribaud and Bégué's experiments.Rate of evaporation measurements and earlier rate of effusion measurements, although their interpretation had been partly erroneous, lead to the same result if the low value of the evaporation (condensation) coefficient, measured by the authors, is taken into account.