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Electron impact excitation of vibrational levels in the ground electronic state and seven excited electronic states in O2 have been simulated for an International Brightness Coefficient-Category 2+ (IBC II+) night-time aurora, in order to predict O2 excited state number densities and volume emission rates (VERs). These number densities and VERs are determined as a function of altitude (in the range 80–350 km) in the present study. Recent electron impact excitation cross-sections for O2 were combined with appropriate altitude dependent IBC II+ auroral secondary electron distributions and the vibrational populations of the eight O2 electronic states were determined under conditions of statistical equilibrium. Pre-dissociation, atmospheric chemistry involving atomic and molecular oxygen, radiative decay and quenching of excited states were included in this study. This model predicts relatively high number densities for the X 3 Σ g - ( v ′ ⩽ 4 ) , a 1 Δ g and b 1 Σ g + metastable electronic states and could represent a significant source of stored energy in O2* for subsequent thermospheric chemical reactions. Particular attention is directed towards the emission intensities of the infrared (IR) atmospheric (1.27 μm), Atmospheric (0.76 μm) and the atomic oxygen 1S→1D transition (5577 A) lines and the role of electron-driven processes in their origin. Aircraft, rocket and satellite observations have shown both the IR atmospheric and Atmospheric lines are dramatically enhanced under auroral conditions and, where possible, we compare our results to these measurements. Our calculated 5577 A intensity is found to be in good agreement with values independently measured for a medium strength IBC II+ aurora.

Electron impact in the upper atmosphere produces excited states of atoms and molecules, which can radiatively decay to produce characteristic photo emission features in both aurora and dayglow. Statistical equilibrium calculations which incorporate electron cross sections, electron energy spectra, radiative transitions, predissociation and quenching, can predict both the populations of the excited states and the associated photon emission spectra. Results from such a model are used here to predict particular cases of rocket and satellite measurements of N 2 VUV auroral and dayglow emissions. Although some approximations are required due to the absence of basic atomic data, reasonable agreement is found for most predicted emissions, with some exceptions being attributable to reabsorption in the atmosphere.

Energy transfer from electrons to neutral gases and ions is one of the dominant electron cooling processes in the ionosphere, and the role of vibrationally excited N2 in this is particularly significant. We report here the results from a new calculation of electron energy transfer rates (Q) for vibrational excitation of N2, as a function of the electron temperature Te. The present study was motivated by the development of a new cross-section compilation for vibrational excitation processes in N2 which supercedes those used in the earlier calculations of the electron energy transfer rates. We show that the energy dependence and magnitude of these cross sections, particularly in the region of the well-known 2 Π g resonance in N2, significantly affect the calculated values of Q. A detailed comparison between the current and previous calculated electron energy transfer rates is made and coefficients are provided so that these rates for transitions from level 0 to levels 1–10 can be calculated for electron temperatures less than 6000 K .

We report integral cross sections (ICSs) for electron impact excitation of the sum (c 1Σ-u + A' 3Δu + A 3Σ+u) of the three states that constitute the Herzberg pseudocontinuum in O2. These ICSs were measured at seven incident electron energies in the range 9-20 eV in order to investigate for the existence of the strong resonance feature predicted by earlier R-matrix calculations. No such structure was observed in this letter.

We report integral cross sections (ICSs) for electron impact excitation of the A 3Σ+u, B 3Πg, W 3Δu, B' 3Σ-u, a' 1Σ-u, a 1Πg, ω1Δu, C 3Πu, E 3Σ+g and a'' 1Σ+g electronic states of N2. The present data, for each state, were derived at five incident electron energies in the range 15-50 eV, from the earlier crossed-beam differential cross section (DCS) measurements of our group. This was facilitated by using a molecular phase shift analysis technique to extrapolate the measured DCSs to 0° and 180°, before performing the integration. A comprehensive comparison of the present ICSs with the results of earlier experimental studies, both crossed beam and electron swarm, and theoretical calculations is provided. This comparison clearly indicates that some of the previous estimates for these excited electronic-state cross sections need to be reassessed. In addition, we have used the present ICSs in a Monte Carlo simulation for modelling the behaviour of an electron swarm in the bulk of a low current N2 discharge. The macroscopic transport parameters determined from this simulation are compared against those measured from independent swarm-based experiments and the self-consistency of our ICSs evaluated.

Integral cross sections (ICSs) for the excitation of 18 excited electronic states, and four composite excited electronic states, in nitric oxide (NO) have been determined for incident electron energies of 15, 20, 30, 40 and 50 eV. These ICSs were derived by extrapolating the respective measured differential cross sections (M J Brunger et al 2000 J. Phys. B: At. Mol. Opt. Phys. 33 783) to 0° and 180° and by performing the appropriate integration. Comparison of the present ICSs with the results of those determined in earlier optical emission measurements, and from theoretical calculations is made. At each incident energy considered, the current ICSs are also summed along with the corresponding elastic and rovibrational excitation ICSs from B Mojarrabi et al (1995 J. Phys. B: At. Mol. Opt. Phys. 28 487) and the ionization cross sections from Rapp and Englander-Golden (1965 J. Chem. Phys. 43 1464), to derive an estimate of the grand total cross sections (GTSs) for e- + NO scattering. The GTSs derived in this manner are compared with the results from independent linear transmission experiments and are found to be entirely consistent with them. The present excited electronic state ICS, and those for elastic and rovibrational excitation from Mojarrabi et al , appear to represent the first set of self-consistent cross sections for electron impact scattering from NO.

The relative flow technique, utilizing helium as a cross section standard, has been applied to measure absolute elastic differential cross sections for electron scattering from nitric oxide (NO). The present elastic measurements were conducted at nine energies in the range 1.5-40 eV, which represents an important extension to the only previous study of this system by Kubo and co-workers (1981). Furthermore, the uncertainties in the present data are significantly reduced in comparison to those quoted in the earlier work. The current elastic differential cross sections were measured at the Australian National University over an angular range 10 degrees -130 degrees . We also report the first measurements of absolute differential cross sections for rovibrational (0 to 1, 2, 3, 4) excitation of the NO ground electronic state. These were conducted at six energies in the range 7.5-40 eV and were measured at Flinders University over the angular range 10 degrees -90 degrees . The differential cross sections for both the elastic and inelastic processes are compared with the results of the recent Born-closure Schwinger variational method calculation of Mu-Tao Lee and colleagues (1992).

Differential and integral electron impact excitation cross sections are reported for the n3S, n1S and n3P (n=2,3) levels in He. Differential cross sections were measured at 30, 50 and 100 eV impact energies in the 10' to 135' angular range, extrapolated to 0 and 180', and then integrated over all angles to obtain integral cross sections at these three incident electron energies. In addition, differential cross sections were measured by excitation of the n3S (n=2,3,4,5), n1S and n3P (n=2,3,4) levels at 60 eV impact energy and 30' scattering angle. Theoretical cross sections based on first-order many-body theory (FOMBT) are also reported for these three final state symmetries for the 30-500 eV impact energy range. These new experimental and theoretical results are compared with other data.

New experimental electron-energy-loss data have been used to extract differential and integral cross sections for excitation of the 2 1P level, and for the overlapping (3 1P, 3 1D, 3 3D) levels of helium, at 30-, 50-, and 100-eV incident electron energies. First-order many-body theory (FOMBT) has been used to calculate the differential and integral cross sections for excitation of the n 1P (n = 2,...,6) levels of helium by electron impact, for incident electron energies from threshold to 500 eV. Detailed comparisons between these two new sets of data are made as well as comparisons with appropriate published experimental and theoretical results. A simple scaling relationship is derived from the FOMBT results for n = 2,...,6 that provides differential and integral cross sections for all symmetry final levels of helium with n = 6 or greater.

Integral cross sections for electron impact dissociation of F 2 as a function of incident electron energy are very important for predicting the performance of electron-beam-pumped KrF lasers. Approximate scaling relationships from similar transitions in N 2 are combined with the few available electron energy-loss spectra in F 2 to obtain an estimate of the direct dissociation cross section of F 2 by electron impact from threshold to 100 eV. A conservative estimate of the direct dissociation cross section obtained by this process is very large (1.6 A 2 at 25 ev) and appears to have a large contribution from excitation of perturbed Rydberg states at 12.87 eV. This estimated magnitude and the prediction of F + and F − fragment production below the F + ionization threshold (15.7 eV) should be checked experimentally because of the important role these processes play in rare-gas fluoride lasers.

Electron‐impact excitation plays a major role in emission from aurora and a less significant but nonetheless crucial role in the dayglow and nightglow. For some molecules, such as N2, O2 and NO, electron‐impact excitation can be followed by radiative cascade through many different sets of energy levels, producing emission with a large number of lines. We review the application of our statistical equilibrium program to predict this rich spectrum of radiation, and we compare results we have obtained against available independent measurements. In addition, we also review the calculation of energy transfer rates from electrons to N2, O2 and NO in the thermosphere. Energy transfer from electrons to neutral gases and ions is one of the dominant electron cooling processes in the ionosphere, and the role of vibrationally excited N2 and O2 in this is particularly significant. The importance of the energy dependence and magnitude of the electron‐impact vibrational cross sections in the calculation of these rates is assessed.

The development and initial results of a method for the determination of differential cross sections for electron scattering by molecular oxygen are described. The method has been incorporated into an existing package of computer programs which, given spectroscopic factors, dissociation energies and an energy-loss spectrum for electron-impact excitation, determine the differential cross sections for each electronic state relative to that of the elastic peak. Enhancements of the original code were made to deal with particular aspects of electron scattering from ${\mathrm{O}}_{2},$ such as the overlap of vibrational levels of the ground state with transitions to excited states, and transitions to levels close to and above the dissocation energy in the Herzberg and Schumann-Runge continua. The utility of the code is specifically demonstrated for the ``6-eV states'' of ${\mathrm{O}}_{2},$ where we report absolute differential cross sections for their excitation by 15-eV electrons. In addition an integral cross section, derived from the differential cross section measurements, is also reported for this excitation process and compared against available theoretical results. The present differential and integral cross sections for excitation of the ``6-eV states'' of ${\mathrm{O}}_{2}$ are the first to be reported in the literature for electron-impact energies below 20 eV.

A set of three computer programs is reported which allow for the deconvolution of overlapping molecular electronic state structure in electron energy-loss spectra, even in highly perturbed systems. This procedure enables extraction of absolute differential cross sections for electron-impact excitation of electronic states of diatomic molecules from electron energy-loss spectra. The first code in the sequence uses the Rydberg–Klein–Rees procedure to generate potential energy curves from spectroscopic constants, and the second calculates Franck–Condon factors by numerical solution of the Schrödinger equation, given the potential energy curves. The third, given these Franck–Condon factors, the previously calculated relevant energies for the vibrational levels of the respective electronic states (relative to the v″ = 0 level of the ground electronic state) and the experimental energy-loss spectra, extracts the differential cross sections for each state. Each program can be run independently, or the three can run in sequence to determine these cross sections from the spectroscopic constants and the experimental energy-loss spectra. The application of these programs to the specific case of electron scattering from nitric oxide (NO) is demonstrated.

Absolute differential cross sections (DCS's) for electron-impact excitation of the lowest forty electronic levels in atomic neon have been determined for incident electron energies of 30 and 50 eV, for the four lowest levels at 25 eV, and two levels at 100 eV. The cross sections for these forty electronic levels are grouped into fifteen features, six of which represent excitation to resolved single electronic levels and the remaining nine which contain the unresolved contributions from two or more electronic levels. These DCS's were extrapolated to 0 deg and 180 deg and integrated to yield absolute integral cross sections as a function of incident electron energy. The results are compared to other experimental and theoretical results.

Potential energy (PE) curves for the Rydberg states of F 2 , and for the ground and lowest two electronic states each of symmetry 2 Π g,u , 2 Δ g,u and 2 Σ ± g,u of F + 2 , have been obtained using modest-sized configuration-interaction calculations. These PE curves have been used to calculate spectroscopic constants for the electronic states and the results agree reasonably well with the limited experimental and theoretical results previously reported. The theoretical PE curves for the Rydberg states of F 2 are found to be strongly perturbed by valence-Rydberg-ionic interactions and these perturbations appear to be responsible for certain features in recently reported electron energy-loss spectra in F 2 . The corresponding electronic wavefunctions have been used to calculate the electronic transition moment, as a function of the internuclear distance, for dipole-allowed transitions between the lowest excited electron state of each symmetry and the appropriate ground electronic state. The radiative emission probabilities, natural lifetimes, and absorption oscillator strengths, for each band system, are also reported here. The predicted lifetimes for vibrational levels of the A 2 Π u of electronic state in F + 2 vary from 1.3–1.5 μs and agree reasonably well with the single available set of measurements. The predicted radiative lifetimes for the higher electronic states of F + 2 are substantially longer and fall into the range 5–100 ms.

The coherence and correlation parameters associated with the excitation of the n3P (n=2-8) states of helium have been calculated using first-order many-body theory (FOMBT) for the incident electron energy range 25-500 eV. The theoretical results for (Lperpendicular to ) and gamma agree well with the limited experimental data for 33P and these parameters are essentially independent of the principal quantum number for fixed incident electron energy.

For pt.I., see ibid., vol.20, p.L49 (1987). First-order many-body theory (FOMBT) is used to predict the differential cross section (DCS) and electron spin polarisation function Sp(k, k') associated with electron impact excitation of the 33P levels of the Mg atom, for incident electron energies of 10, 20 and 40 eV. The theoretical DCS agrees well with the measured DCS except at large scattering angles. By performing calculations with both single- and two-configuration representations of the Mg ground state it has been determined that configuration mixing is not very important in describing the electron scattering characteristics of the Mg atom. A comparison between comparable results for He, Mg and Hg appears to support the premise that the structure present in the electron spin polarisation function (and hence (Lperpendicular to )) is due to interference among partial waves associated with the electron scattering process.

Analysis of electron energy-loss data at incident electron energies of 40 and 60 eV has led to the determination of normalized absolute differential cross sections for electron-impact excitation of five optically-allowed singlet states, two known triplet states, and two unknown triplet-like states of N2, lying in the energy-loss range 12.5-14.2 eV. The range of scattering angles was 5 to 138 deg. The optically allowed transitions and the known triplet excitations are identified. Cross sections for excitation to two unidentified triplet-like states at 13.155 and 13.395 eV were also obtained. The relationship of the generalized oscillator strength for the dipole-allowed states obtained from the described data to known optical oscillator strengths is discussed.

The photoabsorption Schumann-Runge continuum in O2 has been decomposed into the contribution from states of both valence and Rydberg character. The decomposition of the experimental photoabsorption (or high-energy, zero-angle, electron energy-loss) data contains contributions from three valence states, four Rydberg states, and one state of unknown character. Information on the perturbed valence-state potential-energy curves in the Franck-Condon region obtained by this analysis substantiate theoretical conclusions of strong valence-Rydberg perturbations in the 8-12 eV excitation region. There is presently no unambiguous identification for the state at 8.93+or-0.04 eV, found in the optical and all the electron energy-loss data.

The photoabsorption cross section of UF6 in the wavelength region between 206.7 nm (2.5 eV) and 43.5 nm (28.5 eV) has been derived from the UF6 electron energy loss spectra at a 5° scattering angle and at incident electron energies of 75 and 100 eV. The shape of the resulting optical spectrum agrees well with available high‐resolution photoabsorption measurements in the 400 nm (3.1 eV) to 200 nm (6.3 eV) region. Below 200 nm, where no photoabsorption data are available, there is strong absorption and appreciable structure present. Absolute values of absorption cross sections have been obtained by normalizing the present relative data to the recently measured photoabsorption value at 225.5 nm.

The fine-structure effect associated with electron impact excitation of LS-coupled atomic targets from the 1S ground state to a N3L final state is discussed. The relationship between the electron impact coherence parameters (EICP) and the fine-structure effect spin-polarisation function is illustrated for the case of excitation of the N3DJ (J=1, 2, 3; N=3, 4,. . . 8) states of helium. It is shown that for L>or=3 (i.e., N3F, N3G, etc.) excitation processes, the electron spin-polarisation experiments provide different information from that obtained from electron-photon coincidence experiments. If data from spin-polarisation and EICP experiments can be combined, information about the octupole moment of the electron impact induced final atomic state can be obtained.

First-order many-body theory (FOMBT) has been used to calculate the differential cross sections (DCS) and electron impact coherence parameters (EICP) associated with excitation of the 31P, 31D and 33D states of helium for incident electron energies of 30, 40, 45 and 60 eV. No direct measurements of the DCS for excitation of the individual 31P, 31D and 33D states have been reported but the sum of the theoretical DCS for excitation of these three states is in excellent agreement with that determined from recent electron energy loss data at 30 eV. The EICP predicted by FOMBT for excitation of the 31D state are in good agreement with the recent measurements of Beijers et al (1986, 1987) but differ from FOMBT are qualitatively different from those obtained using the multichannel eikonal approximation. More accurate and extensive measurements will be needed to determine which of these two theories better describes the electron scattering process.

First-order many-body theory (FOMBT) has been used to calculate differential cross sections (DCS's) and electron-impact coherence parameters (EICP's) associated with excitation of the n/sup 1/P (n = 2,3) electronic states of helium for incident-electron energies in the range 22 eV less than or equal toEless than or equal to500 eV. The DCS's predicted by FOMBT agree reasonably well with the available experimental data and in the 50 eV less than or equal toEless than or equal to500 eV energy range, as well as, or better than, those predicted by any other theory. The EICP's predicted by R-matrix methods are found to agree better with the available experimental data in the threshold region (Eless than or equal to30 eV) than those predicted by FOMBT. However, in the intermediate-energy region, 50 eV less than or equal toEless than or equal to500 eV, FOMBT provides EICP results that are in semiquantitative agreement with the available data. Partial-wave decompositions of the parameter at various incident-electron energies show that the structure observed in this parameter is caused by interference among the free-electron partial waves and that the small-angle characteristics of this parameter are determined principally by low-l partial waves, i.e., by incident electrons with small-impactmore » parameters. Additional experimental measurements and results from more elaborate calculations at intermediate and large scattering angles and for intermediate and high incident-electron energies are needed to resolve a number of issues associated with the interpretation of the EICP's.« less

Electron-photon coherence and correlation parameters for electron-impact excitation of the n-italic /sup 1/P (n = 2--8) states of helium at 80 eV are predicted to be essentially independent of the principal quantum number (n-italic), and the available experimental data for n-italic = 2 and 3 show (semiquantitatively) the same behavior. The n-italic independence of these parameters predicted by first-order many-body theory can be explained using basic concepts associated with quantum-defect theory and, if verified by further experimental results, has important consequences for understanding the physics of the electron-atom scattering process.

Electron-impact processes with atoms and molecules play a central role in a wide variety of naturally occurring and laboratory-produced phenomena and, as a result, have been the subject of considerable research, particularly in the last ten years. The importance of electron-impact excitation processes involving ground-state molecules, and molecules in metastable excited states, has been recognized for many years in the study of ionospheric [1] and auroral [2] processes in planetary atmospheres. The discovery of high-power (electron-beam and discharge-pumped) gas lasers has resulted in an increased interest in electron collision processes because of the important role they play in creating the population inversion [3]. This renewed interest in electron scattering has resulted in the measurement of many of the electron-impact excitation cross-sections of importance during the past ten years. The need for cross-section information involving targets that are presently difficult to study experimentally (eg, molecular free radicals, etc), and recent results from electron-photon coincidence measurements [4] which probe the details of the scattering process, have further stimulated theoretical studies. In spite of this renewed activity, however, much of the electron-impact excitation cross—section information needed to model a variety of plasma and planetary processes is still not known and the absence of these basic cross-section data hinders progress toward a more thorough understanding of various phenomena.

Electron energy‐loss spectra in molecular fluorine, for energy losses from 0 to 17.0 eV, have been taken at incident electron energies of 30, 50, and 90 eV and scattering angles from 5° to 140°. Features in the spectra above 11.5 eV energy loss agree well with the assignments recently made from optical spectroscopy. Excitations of many of the eleven repulsive valence excited electronic states are observed and their location correlates reasonably well with recent theoretical results. Several of these excitations have been observed for the first time and four features for which there are no identification appear in the spectra.