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We have measured the differential and integral cross sections for electron impact excitation of the a 3 , a 3 + , d 3 and A 1 states of CO in the 10-15 eV impact energy region. These measurements fill the gap which existed between the low-energy (threshold to about 12 eV) data of Zobel et al and the intermediate energy (20-50 eV) results of Middleton et al. Recent electron time-of-flight measurements are utilized to place the relative cross sections (deduced from energy-loss spectra) on the absolute scale.The cross sections are in excellent agreement with the results obtained by Zobel et al at the overlapping impact energies and always smoothly extrapolate to their low-energy results. Extrapolation to the intermediate energy domain and comparison with the results of Middleton et al is not so clear.Comparison with theoretical differential and integral cross sections of Sun et al and the integral cross sections of Morgan and Tennyson reveals angular and energy dependences which resemble those found for the experimental results but there are serious discrepances concerning the absolute magnitudes.

We have measured and calculated differential cross sections for electron-impact excitation of the . . . 6s6p 1 P 1 level in ytterbium at collision energies of 5, 10, 20, 40 and 80 eV. Calculations were made in the unitarized distorted-wave approximation. Measurements of the relative differential cross section, taking account of volume correction effects, were placed on an absolute scale by normalization to the theoretical integral cross section at 80 eV with extrapolation to lower energies based on the measured excitation function of Shimon et al (1981 Opt. Spektrosk. 50 571). Excellent agreement between theory and experiment is obtained in the regime of low momentum transfer.

Superelastic electron scattering from laser-excited '"Ba(. . .6s6p 'P) is investigated both theoretically and experimentally. The theoretical framework is developed and incorporated into a model of a realistic scattering geometry of finite spatial extent. Model calculations predict that the dependence of the superelastic scattering intensity on the linear polarization direction of the laser beam can be severely distorted due to the existence of a finite scattering volume. Our measurements confirm the presence of this distortion and are well described by the model calculations. Conclusions are drawn concerning the inAuence of this effect on the extraction of electron-impact coherence parameters.

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