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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.}

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.

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.

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.

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.

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

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.

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.