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

Absolute cross sections for electron-impact excitation of the dipole-allowed transitions ${\mathrm{S}}^{4+}$ $(3s^2{}^{1}S\to 3s3p{}^1\mathrm{P})$ and ${\mathrm{S}}^{5+}$ $(3s{}^{2}S\to 3p{}^{2}P)$ 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.

Figure 1

Schematic of the JILA/ORNL MEIBEL apparatus. See text for details.

Figure 2

Absolute cross sections for ionization of ${\mathrm{S}}^{4+}$ by electron impact as a function of center-of-mass energy. Solid circles are present results; error bars representing a 90% confidence level of relative uncertainties are smaller than the circles. The solid curve is a least-squares fit of a scaled Lotz formula for the mixture of ground-state and metastable ions. The metastable fraction determined by this fit is $0.31\pm 0.03$; see the text for an explanation. The arrows represent the thresholds for the $3s3p{}^{3}P$ metastable and the $3s^2{}^{1}S$ ground-state ions of 62.3 and $72.7\mathrm{eV}$, respectively.

Figure 3

Absolute cross sections for excitation of ${\mathrm{S}}^{4+}$ $3s^2{}^{1}S\to 3s3p{}^{1}P$ transition by electron impact as a function of center-of-mass energy. Solid circles are the present absolute results with error bars representing the 90% confidence level of relative uncertainties, with the exception of the point at $16.15\mathrm{eV}$ where the outer error bar represents the total expanded uncertainty. The open squares are the present relative measurements that have been normalized to the absolute measurements. The curves are convolutions of theories with a $0.17\mathrm{eV}$ FWHM Gaussian: solid curve, eight-state CCR results of Ref. 25; dashed curve, 31-state CCR results of Ref. 26; dot-dashed curve, 14-state CCR results of Ref. 27. Above $16.3\mathrm{eV}$ the results of Refs. 26, 27 are indistinguishable.

Figure 4

Absolute cross sections for excitation of ${\mathrm{S}}^{5+}$ $3s{}^{2}S\to 3p{}^{2}P$ transition by electron impact as a function of center-of-mass energy. Solid circles are present results with error bars representing a 90% confidence level of relative uncertainties, with the exception of the point at $13.89\mathrm{eV}$ where the outer error bar represents the total expanded uncertainty. The solid curve is a convolution of five-state CCR results of Ref. 28 with a $0.17\mathrm{eV}$ FWHM Gaussian representing the experimental energy resolution.