N. Djurić

Branching Fractions in Dissociative Recombination of CH{sup {plus}} {sub 2}

The absolute cross section and branching ratios for dissociative recombination of CH -->+2 with electrons have been measured by means of the heavy-ion storage ring CRYRING. Contrary to what has been previously believed, recombination of CH -->+2 is dominated by the three-body channel C + H + H (63%), whereas breakup into the CH + H and C + H2 channels occurs with branching ratios of 25% and 12%, respectively. The thermal rate coefficient for dissociative recombination at 300 K is 6.4 × 10-7 cm3 s-1, which is higher by a factor of 2.5 than the value used in modeling dark molecular clouds. The low CH production and the high production of energetic carbon atoms could be favorable factors for the turbulence model to explain the large abundance of interstellar CH+. The cross section for dissociative excitation was also measured and found to be in good agreement with results from a crossed electron-ion beam experiment.

Absolute detection efficiencies of microchannel plates for 0.1–2.3 keV electrons and 2.1–4.4 keV Mg+ ions

The absolute detection efficiencies of detectors consisting of two microchannel plates (MCP) in a chevron arrangement, were experimentally determined for 0.1–2.3‐keV electrons and 2.1–4.4‐keV Mg+ ions. Both detectors tested included a grid with 92.5% transmission in front of the first MCP. For the measurements, the observed detector count rates were compared to the corresponding particle currents collected in a Faraday cup and measured with a vibrating reed electrometer. The electron detection efficiency of the MCP detector, including the grid, decreases from 0.82 at 0.1 keV to 0.65 at 2.3 keV for electrons incident normal to the surface. The Mg+ ion detection efficiency for the same arrangement, but with 43° incidence angle, increases from 0.49 at 2.1 keV to 0.81 at 4.4 keV.

Rate constants and branching ratios for the dissociative recombination of C{sub 3}D{sub 7}{sup +} and C{sub 4}D{sub 9}{sup +}

Product branching ratios and thermal rate coefficients for the dissociative recombination of C3D(+)7 and C4D(+)9 have been measured in the ion storage ring CRYRING. The results for C3D(+)7 are believed to be slightly more accurate than those obtained earlier for C3H(+)7. Only the C-C bond breaking channels could be measured for C4D(+)9 and were found to be in excellent agreement with earlier data.

Studies of Dissociative Recombination in CRYRING

This article describes some of the most recent experiments at the ion storage ring CRYRING at the Manne Siegbahn Laboratory, Stockholm University. A comprehensive review of recent work at ion storage rings has recently been published,1 and there are several other reviews of recent date,2,3 as well as the proceedings from the previous dissociative recombination (DR) meeting at Nasslingen in Sweden.4 Finally, a brief review of three-body breakup dynamics has recently been published.5

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.

Crossed beams measurements of absolute cross sections for electron impact ionization of S

Abstract Absolute cross sections for electron impact ionization of S + have been measured in the electron energy range from 30-250 eV using the crossed-beams technique. Results are compared with sealed cross sections from other P-like ions, with a semiempirical formula and with other recently reported measurements. Expansion coefficients and formulas for generating ionization rate coefficients in the electron temperature range 10 4 K ⩽ T ⩽ 10 6 K are presented.

Merged-beam measurements of absolute cross sections for electron-impact excitation of S4+ (3s2 1S→3s3p1P) and S5+(3s2S→3p2P)

Absolute cross sections for electron-impact excitation of the dipole-allowed transitions S4+ (3s(2) S-1 -> 3s3p P-1) and S5+ (3s S-2 -> 3p P-2) 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.

Recent experiments on near-threshold electron-impact excitation of multiply charged ions

Some recent measurements of excitation of multiply charged ions by electrons studied in beam-beam experiments are highlighted. The emphasis is on absolute total cross sections measured with the merged electron-ion beams energy-loss (MEIBEL) technique, although some results obtained with the crossed-beams fluorescence method are also presented. The MEIBEL technique allows the investigation of optically-allowed and forbidden transitions with sufficient energy resolution, typically about 0.2 eV, to resolve resonance structures in the cross sections. Results from the JILA/ORNL MEIBEL experiment on dipole-allowed transitions in several ions demonstrate the success of various theoretical methods in predicting cross sections in the absence of resonances. Comparisons of R-matrix calculations and measured cross sections for spin-forbidden transitions in Mg-like Si{sup 2+} and Ar{sup 6+}, however, show that further refinements to the theory are needed in order to more accurately predict cross sections involving significant contributions from dielectric resonance`s and interactions between neighboring resonances.

Electron-impact excitation of multicharged ions: Merged beams experiments

Electron-impact excitation cross sections for several multicharged ions have been measured near threshold using the merged electron-ion beams energy loss (MEIBEL) technique. This technique allows the investigation of optically-allowed and forbidden transitions with sufficient energy resolution, typically about 0.2 eV, to resolve resonance structures in the cross sections. Results from the JILA/ORNL MEIBEL experiment on allowed transitions in several multicharged ions demonstrate the ability of various theoretical methods to predict cross sections in the absence of resonances. Comparisons of R-matrix calculations and measured cross sections for transitions in Mg-like Si2+ and Ar6+, however, indicate that theory must continue to evolve in order to more accurately predict cross sections involving significant contributions from dielectronic resonances and interactions between neighboring resonances.

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.