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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.

Electron impact excitation of vibrational levels in the ground electronic state and nine excited electronic states in NO has been simulated for an IBC II aurora (i.e., ∼10 kR in 3914 A radiation) in order to predict NO excited state number densities and band emission intensities. New integral electron impact excitation cross sections for NO were combined with a measured IBC II auroral secondary electron distribution, and the vibrational populations of 10 NO electronic states were determined under conditions of statistical equilibrium. This model predicts an extended vibrational distribution in the NO ground electronic state produced by radiative cascade from the seven higher-lying doublet excited electronic states populated by electron impact. In addition to significant energy storage in vibrational excitation of the ground electronic state, both the a 4Π and L2 Φ excited electronic states are predicted to have relatively high number densities because they are only weakly connected to lower electronic states by radiative decay. Fundamental mode radiative transitions involving the lowest nine excited vibrational levels in the ground electronic state are predicted to produce infrared (IR) radiation from 5.33 to 6.05 μm with greater intensity than any single NO electronic emission band. Fundamental mode radiative transitions within the a 4Π electronic state, in the 10.08–11.37 μm region, are predicted to have IR intensities comparable to individual electronic emission bands in the Heath and e band systems. Results from this model quantitatively predict the vibrational quantum number dependence of the NO IR measurements of Espy et al. [1988].

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.

Excitation of the ground vibrational levels and nine excited electronic states in NO has been simulated for an IBC II aurora. New electron impact excitation cross sections for NO were combined with a measured IBC II auroral secondary electron energy distribution and the electronic-vibrational populations were determined for conditions of statistical equilibrium. This model predicts an extended vibrational distribution in the NO ground electronic state produced by radiative cascade from the many higher lying doublet excited electronic states. In addition to energy storage in the vibrational population of the ground electronic state, both the a 4Π and L′ ²Φ excited electronic states are predicted to have relatively high number densities in aurora. This extended ground state vibrational distribution is predicted to emit infrared [IR] radiation from 1.3 to 8.2 µm and most IR bands are predicted to be stronger than band emission from any of the electonic transitions.

Low-energy ({ital E}{sub 0}=15{endash}40 eV), high-energy-resolution, electron-energy-loss spectra of nitric oxide (NO) have been measured and spectrally deconvolved. This process yielded direct absolute differential cross sections (DCS) for electron-impact excitation of the electronic states of the NO molecule. Integration of these DCS gives integral cross sections (ICS), which confirm earlier estimates that the magnitudes of the NO ICS are one to two orders smaller than the analogous transitions in N{sub 2} and O{sub 2}. In this paper we present clear evidence for the observation of the {ital L}{prime}{sup 2}{Phi} electronic state, showing a {sup 2}{Pi}{sub {ital r}}{r_arrow}{sup 2}{Phi} excitation transition in an electron-scattering experiment. {copyright} {ital 1996 The American Physical Society.}

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).

Electron impact coherence parameters and fine-structure-effect spin-asymmetry and spin-polarization functions are reported for electron impact excitation of the first few low-lying excited states of Ne. First-order many-body theory was used in these calculations. Our results are in good agreement with recent experimental results for the Stokes parameters P1, P2 and P3 in the small scattering angle region.

For the heavier rare-gas targets, Ne, Ar, Kr, there is now a reasonable amount of experimental electron impact coherence parameter data available for excitation of the lowestJ=1 states. Theoretical results for those rare-gas targets, have been restricted to distorted-wave approximation (DWA) type theories. We present a systemization of the experimental data and compare them with available theoretical results. In the case of the heavy rare gases, we compare the experimental and theoretical data available for the three species, Ne, Ar, Kr, in order to identify trends. We compare the experimental data with results from available theories (mainly DWA type) and discuss the importance of spin-orbit coupling effects and “shell” effects. We present our point-of-view as to the physical picture that is emerging from all collisional data, and conclude by recommending future experimental and theoretical activities that will, from our perspective, provide new insight into the physics of these processes.

Experimental data for the average transferred angular momentum, 〈⊥〉, have been combined with experimental differential cross section [DCS] data for electron impact excitation of the 21 P and 31 P levels of helium to obtain the individual magnetic-sublevel differential cross-sections, [DCS n ], in the natural (and atomic) coordinate system. First-order many-body theory (FOMBT) has been used to obtain corresponding theoretical predictions for DCS n which are compared to these results. This comparison shows that FOMBT generally predictsM=+1 sublevel excitation DCS better than that forM=−1.

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.

Differential and integral cross section data for electron-impact excitation of the 21P level in He have been critically reviewed. Experimental and theoretical results have been compared and a set of differential cross sections at 20° scattering angle in the 25 to 500 eV impact energy range has been deduced based on all available information. It is proposed that this set of data represents the most accurate inelastic differential cross sections available at the present time and could be used as a secondary standard for normalization of cross sections.

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.

[1] Excited N2 plays a role in a number of atmospheric processes, including auroral and dayglow emissions, chemical reactions, recombination of free electrons, and the production of nitric oxide. Electron impact excitation of N2 is followed by radiative decay through a series of excited states, contributing to auroral and dayglow emissions. These processes are intertwined with various chemical reactions and collisional quenching involving the excited and ground state vibrational levels. Statistical equilibrium and time step atmospheric models are used to predict N2 excited state densities and emissions (as a test against previous models and measurements) and to investigate the role of excited nitrogen in the production of nitric oxide. These calculations predict that inclusion of the reaction N2[A3Σu+] + O, to generate NO, produces an increase by a factor of up to three in the calculated NO density at some altitudes.

[1] Vibrationally excited N2 is important in determining the ionospheric electron density and has also been proposed to play a role in the production of NO in disturbed atmospheres. We report here predictions of the absolute vibrational distributions in the ground electronic state of N2 produced by electron impact excitation, at noon and midnight under quiet geomagnetic conditions and disturbed conditions corresponding to the aurora IBCII+ and IBCIII+ at 60°N latitude and 0° longitude, at altitudes between 130 and 350 km. These predictions were obtained from a model which includes thermal excitation and direct electron impact excitation of the vibrational levels of the N2 ground state and its excited electronic states; radiative cascade from all excited electronic states to all vibrational levels of the ground electronic state; quenching by O, O2, and N2; molecular and ambipolar diffusion; and the dominant chemical reactions. Results from this study show that for both aurora and daytime electron environments: (1) cascade from the higher electronic states of N2 determines the population of the higher vibrational levels in the N2 ground state and (2) the effective ground state vibrational temperature for levels greater than 4 in N2 is predicted to be in the range 4000–13000 K for altitudes greater than 200 km. Correspondingly, the associated enhancement factor for the O+ reaction with vibrationally excited N2 to produce NO+ is predicted to increase with increasing altitude (up to a maximum at a height which increases with auroral strength) for both aurora and daytime environments and to increase with increasing auroral strength. The contribution of the cascade from the excited electronic states was evaluated and found to be relatively minor compared to the direct excitation process.

Sunlight photoionises atoms and molecules in the Earth’s upper atmosphere, producing ions and photoelectrons. The photoelectrons then produce further ionisation by electron impact. These processes produce the ionosphere, which contains various positive ions, such as NO+, N+, and O+, and an equal density of free electrons. O+(4S) ions are long‐lived and so the electron density is determined mainly by the density of O+(4S). This density is dependent on ambipolar diffusion and on loss processes, which are principally reactions with O2 and N2. The reaction with N2 is known to be strongly dependent on the vibrational state of N2 but the rate constants are not well determined for the ionosphere. Vibrational excitation of N2 is produced by direct excitation by thermal electrons and photoelectrons and by cascade from the excited states of N2 that are produced by photoelectron impact. It can also be produced by a chemical reaction and by vibrational‐translational transitions. The vibrational excitation is lost by deexcitation by electron impact, by step‐wise quenching in collisions with O atoms, and in the reaction with O+(4S). The distribution of vibrational levels is rearranged by vibrational‐vibrational transitions, and by molecular diffusion vertically in the atmosphere. A computational model that includes these processes and predicts the electron density as a function of height in the ionosphere is described. This model is a combination of a “statistical equilibrium” calculation, which is used to predict the populations of the excited states of N2, and a time‐step calculation of the atmospheric reactions and processes. The latter includes a calculation of photoionisation down through the atmosphere as a function of time of day and solar activity, and calculations at 0.1 s intervals of the changing densities of positive ions, electrons and N2 in the different vibrational levels. The validity of the model is tested by comparison of the predicted electron densities with the International Reference Ionosphere (IRI) of electron density measurements. The contribution of various input parameters can be investigated by their effect on the accuracy of the calculated electron densities. Here the effects of two different sets of rate constants for the reaction of vibrationally excited N2 with O+(4S) are investigated. For reference, predictions using the different sets are compared with laboratory measurements. Then the effect of using the different sets in the computational model of the ionosphere is investigated. It is shown that one set gives predictions of electron densities that are in reasonable agreement with the IRI, while the other set does not. Both sets result in underestimation of the electron density at the height of the peak electron density in the atmosphere, suggesting that either the amount of vibrational excitation or the rate constants may be overestimated. Our comparison is made for two cases with different conditions, to give an indication of the limitations of the atmospheric modeling and also insight into ways in which the sets of rate constants may be deficient.

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.

We report here results from first order many body theory, distorted wave approximation, and converged close coupling calculations for polarization fractions and alignment creation parameters in the case of 21S → n1P(n = 3–5), 23S → n1P(n = 2–5), 21S → n1D(n = 3–5), 23S → n1D(n = 3–5), 23S → n3D(n = 3–5), and 21S → n3D(n = 3–5) excitations in helium for electron impact energies from threshold to several hundred 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.

First-order many-body theory predicts that the electron-impact-induced orbital angular momentum transfer to the target, (Lperpendicular to ), associated with excitation of the n1D states of helium in the energy range 45-60 eV, is negative for scattering angles less than or equal to 50', in agreement with recent measurements. This negative behaviour for the n1D states is opposite to that associated with excitation of the n1,3P and n3D states for the same range of scattering angles. The qualitatively different characteristic of (Lperpendicular to ) for excitation of n1D compared with the n3D states of helium shows that details of the excitation process itself play an important role in determining the angular dependence of (Lperpendicular to ). As is also the case for excitation of the n1,3P states, the electron impact coherence parameters for excitation of the n3,1D states are predicted to be essentially independent of the principal quantum number (for fixed incident electron energy).

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.

Normalized differential cross sections (DCS's) for the electron impact excitation of the lowest three singlet (${a}^{\ensuremath{'}}, a, \ensuremath{\omega}$), and lowest five triplet ($A, B, W, {\mathrm{B}}^{\ensuremath{'}}, C$) valence electronic states of ${\mathrm{N}}_{2}$, and of the two ($3s{\ensuremath{\sigma}}_{g}$) Rydberg states ($E, {a}^{\ensuremath{'}\ensuremath{'}}$) have been determined at seven incident electron energies ranging from 10 to 50 eV. These DCS's were obtained for the scattering-angle range 5\ifmmode^\circ\else\textdegree\fi{} to 138\ifmmode^\circ\else\textdegree\fi{} by analyzing electron energy-loss spectra in ${\mathrm{N}}_{2}$ at a number of fixed scattering angles within that range. These data, which are the first that cover such a large incident energy and scattering-angle range for such a wide variety of final target states, show that each of the DCS's for the ten final target states falls into one of four classes. Since the initial molecular target state is a singlet, and none of the final states studied here is dipole allowed, these four classes are determined according to whether the final target state is triplet, singlet, parity-unfavored, or Rydberg in character. The DCS's for all the final target states change rapidly in shape as the incident electron energy is varied. Theoretical DCS's obtained from first-order perturbation models (Born-Ochkur and Ochkur-Rudge) agree poorly with the measured DCS's.

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.

Eleven new bound electronic states within 12 eV of the ground state of N2+have been identified from detailed configuration interaction calculations. Certain of the new states offer explanations for the recent photoabsorption experiments in N2.

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 excited states of F 2 have been investigated as a function of the internuclear distance by a series of configuration interaction calculations. Numerous avoided curve-crossings between valence, Rydberg (3s and 3p), and ionic (F 2 F − ) electronic states have been found. Transition moments from the ground state to the lowest dipole allowed Rydberg states are presented.

The result of electronic structure calculations at the POL-CI level are presented for the lowest few doublet and quartet states of N2+. Dipole transition moments and radiative lifetimes of a number of states of spectroscopic interest are also given. These results are used to suggest interpretations of a number of recent experiments.

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

The quadrupole moment ( Theta zz) of the ground state of molecular nitrogen has been calculated with accurate Hartree-Fock and generalized valence bond wavefunctions. The dependence of Theta zz on internuclear separation differs markedly in the two approximations, reflecting the inability of the Hartree-Fock wavefunction to correctly describe dissociation. This difference has a substantial effect on the calculated vibrational matrix elements of the molecular quadrupole moment, especially for large off-diagonality.

First-order many-body theory has been used to calculate the differential and integral cross sections for electron-impact excitation of the argon atom to the $4^{3}P_{0}$, $4^{3}P_{2}$, $4^{3}P_{1}$, and $4^{1}P_{1}$ electronic states, for incident electron energies of 16, 20, 30, 50, and 80.4 eV. The resulting cross sections are in good agreement with recent results extracted from electron energy-loss measurements on argon. Detailed calculations show that spin-orbit coupling must be included in the $^{3}P_{1}$ wave function in order to describe the electron-impact excitation of the state properly.

Newly measured natural lifetimes for vibrational levels 1–8 and 10 of the A 2Πu state of N2+ have been analyzed by a weighted, nonlinear, least squares technique to extract the absolute transition moment and its dependence on internuclear distance. Both are considerably different from previous results and lead to different transition probabilities and oscillator strengths. Transition probabilities for both v′=0 and 9 are obtained by interpolation of the ``electronic'' portion of the band strength and result in predicted values for the lifetimes of the v′=0 and 9 levels.

Differential scattering cross sections for excitation of helium by electron impact from its ground state to its 21P excited state have been measured at four incident electron energies in the range 26-55.5 eV for scattering angles between 10° and 70°, and at 81.6 eV for scattering angles between 10° and 80°. These differential cross sections have been placed on an absolute scale by normalizing them to the experimental absolute integral cross sections of Jobe and St. John. These experimental differential and integral cross sections have been compared with the results predicted by the Born approximation, and by several other first-order approximations in which direct excitation is calculated in the Born approximation and exchange scattering by various Ochkurlike approximations. The calculations provide reliable tests of these scattering theories since they are made using the accurate generalized oscillator strengths of Kim and Inokuti. As expected, these first-order theories are poor near threshold and exchange is important at high scattering angles for all energies. Further, the absolute magnitude of the calculated integral and small-angle differential cross sections is too large and is within 50% of experiment only at energies greater than 80 eV. These first-order models are in qualitative agreement with the experimental angular dependence at 34-81.6 eV for scattering angles between 10° and 40°. At higher scattering angles (corresponding to momentum transfers greater than about 1.6 a.u.), the calculated differential cross sections fall well below the experimental ones. The phase between the direct and exchange scattering amplitudes was found to be important at all energies, and is apparently not predicted correctly by any of the first-order models examined here. Some approximations for the exchange (e.g., Ochkur approximation and the post interaction form of the Ochkur-Rudge approximation) were found to be better for integral cross sections and some (e.g., prior Ochkur-Rudge approximation) were better for differential cross sections. The use of good analytic self-consistent-field (SCF) wave functions for both the ground and excited states was tested by computing generalized oscillator strengths from them and comparing these results with the calculations using the accurate generalized oscillator strengths. The SCF functions yield differential cross sections in quantitative disagreement (20%) with the accurate results, although the energy and angle dependence of the cross sections is predicted qualitatively correctly.

New electron impact excitation cross sections for the 6 lowest triplet states (A, B, W, C, E, D) of N2 have been used and the coupled equations of statistical equilibrium solved to obtain the vibrational population of each electronic state. The results show that cascade from high levels of the A3Σu+ state and from the W3Δu state is significant in populating the lower vibrational levels of the B state and hence the character of its ‘apparent’ excitation cross section. For the B state excited under auroral conditions, the fraction of the total population due to cascade processes exceeds 25% for all levels lower than 7 and is greater than 80% for B(υ′ = 0). For the A state under similar conditions, cascade from the B state contributes 50% or more of the total vibrational population for levels lower than 7, and 80% or more for levels below 4. For levels of the A state greater than 7, the A → B transitions depopulate the levels rapidly and indicate that the Vegard-Kaplan emissions from these higher levels will be weak or totally absent in normal auroras.

A technique combining electron impact excitation and optical absorption spectroscopy was applied to UF6. The crucial features of the experiment were that: (1) the electron optics was differentially pumped relative to the scattering chamber and (2) the target UF6 beam was condensed on a liquid nitrogen cold trap placed immediately above the scattering center. Energy loss spectra are presented at an incident electron energy of 20 eV and at scattering angles between 20 and 135 degrees. It is shown that no transitions are found below the first-detected feature at 3.0 eV and an optically forbidden excitation is found at 4.2 eV. A fairly strong optical absorption at 4.8 eV is observed to 'fill-in' at a scattering angle of 20 degrees but is practically absent at higher angles.

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.