Y.-S. Chung

Electron-impact dissociation of CH{sup +} ions: Measurement of C{sup +} fragment ions

Absolute cross sections for electron-impact dissociation of CH{sup +} producing C{sup +} fragment ions were measured in the 3-100 eV range using a crossed electron-ion beams technique with total uncertainties of about 10% near the cross-section peak. Although the measured energy dependence agrees well with two sets of storage ring measurements, the magnitude of the present results lies about 15-25 % below the other results at the cross-section peak near 40 eV. Below 10 eV, the present data tend to exceed the storage ring data, consistent with the presence of excited states in the CH{sup +} ion beam. For energies above 29 eV, the cross sections should also include contributions from dissociative ionization, though no clear onset of this channel is evident in any of the three sets of measured data.

Excitation of He+ to the 2 2S and 2 2P states by electron impact

The first absolute measurement of the cross section for excitation of a hydrogen-like ion is reported. The combined cross section for electron impact excitation of ground state He+ to the 2xa02S and 2xa02P excited states has been measured in the near-threshold region from 40.08 to 41.95xa0eV using the JILA/ORNL merged electron-ion beams energy-loss technique. The threshold step in the cross section is measured as (8.46±1.52)×10-18xa0cm2 at 90% confidence level of total uncertainty, which is approximately 20% larger than values calculated using close-coupling and variational methods. This contrasts with the long-standing disparity of nearly a factor of two between theory and normalized experiment for excitation to the 2xa02S state.

Experimental studies of electron impact dissociation of molecular ions

Recent progress on experimental studies of electron impact dissociation of molecular ions is presented. A technique especially configured to detect and measure ‘‘light particles’’ is described. Preliminary results of absolute cross sections for dissociation of CD2+ and Cl2+ are presented.

Merged-beam measurements of absolute cross sections for electron-impact excitation of S{sup 4+} (3s{sup 2} {sup 1}S{yields}3s3p {sup 1}P) and S{sup 5+} (3s {sup 2}S{yields}3p {sup 2}P)

Absolute cross sections for electron-impact excitation of the dipole-allowed transitions S{sup 4+} (3s{sup 2} {sup 1}S{yields}3s3p {sup 1}P) and S{sup 5+} (3s {sup 2}S{yields}3p {sup 2}P) were measured near threshold using the merged electron-ion beams energy-loss technique. Although the magnitudes of the measured cross sections are in reasonable agreement with available theoretical data, the experimental data indicate that the contributions of dielectronic resonances in the near-threshold region are underestimated by these calculations.

Crossed beams measurements of electron impact dissociative excitation of molecular ions

A crossed beams technique developed at JILA especially to detect and measure light fragment ions from electron impact dissociation is described. Preliminary results of absolute cross sections for obtaining D+ fragments from dissociation of deuterated molecules CD+, CD2+, and CD3+ are presented.