No abstract

X-ray emission from a comet was observed for the first time in 1996. One of the mechanisms believed to be contributing to this surprisingly strong emission is the interaction of highly charged solar wind ions with cometary gases. Reported herein are total absolute charge-exchange and normalized line-emission (X-ray) cross sections for collisions of high-charge state (+3 to +10) C, N, O, and Ne ions with the cometary species H2O and CO2. It is found that in several cases the double charge-exchange cross sections can be large, and in the case of C3+ they are equal to those for single charge exchange. Present results are compared to cross section values used in recent comet models. The importance of applying accurate cross sections, including double charge exchange, to obtain absolute line-emission intensities is emphasized.

In this paper we review prior measurements and report new measurements of elastic differential cross sections (DCSs) obtained in our laboratory for positron scattering by the inert gas atoms He, Ne, Ar, Kr, and Xe. The gases (Ne, Ar, Kr, and Xe) for which low-energy measurements have been made below and above the positronium (Ps)-formation thresholds show evidence that the Ps-formation channel is having an observable "absorption" effect on the elastic scattering channel when compared with calculations that only consider the elastic channel. At low energies where only the elastic channel is open, good agreement is found with available calculations. DCS measurements made at high energies (≥200 eV) for He, Ne, and Ar are also found to be consistent with theoretical calculations. At intermediate energies (above the Ps-formation thresholds and below a few hundred eV) theory has not yet been able to match the relative DCSs obtained experimentally owing to the difficulty of including the effects of the most important inelastic channels (i.e., Ps formation and direct ionization). Some initial attempts by our group to obtain absolute elastic DCSs at fixed scattering angles versus positron energy revealed unexpected structure between 55–60 eV for Ar and at 25 and 200 eV for Kr. Our most recent measurements reported here do not confirm the prior observations of these structural features.

Total cross sections (Qr's) have been measured for I -500-eV positrons and electrons scattered by 02, CH4, and SF6 using a beam-transmission technique. The positron Qr s are compared with the corresponding electron Qr's for each target gas. It is found that the positron Qr's are, in general, lower than the electron results. There are no prominent structures observed for positron scattering at low energies that would be comparable to the narrow shape resonances observed for electrons scattering from various molecules, such as SF6. The positron Qr curve for CH4 reveals a significant increase in the vicinity of its positronium formation threshold, while the 02 and SF6 curves are monotonically increasing in this vicinity. At the highest energies investigated, there are indications of a tendency toward merging of the positron and electron Qr s for these gases. Interesting similarities are found in a comparison of the present positron and electron Qr curves for CH4 with prior comparison measurements for argon.

Total cross sections (QT) for 1-500-eV positron and electron scattering by N2O are measured by a beam-transmission method. Comparisons of these results with prior electron and positron measurements of Q T for C0 2 , N 2 , and CO, made by our group, reveal several remarkable similarities in the shapes and magnitudes of the QT curves for the triatomic and also for the diatomic molecules. Small differences in the absolute Q T values for N 2 0 and CO2, and for N 2 and CO, may be due to the weak permanent dipole moments of N 2 0 and CO.