Absolute emission cross sections for electron-impact excitation of the $3p-3s$ transition in ${\mathrm{Al}}^{2+}$

Crossed beams of electrons and ${\mathrm{Al}}^{2+}$ ions combined with fluorescence detection were used to measure the absolute cross section for electron-impact excitation of the ions to yield 186.3 nm and 185.5 nm photons from the $3s-3p$ transition. The measured cross section near the 6.7 eV threshold energy is about $16\times {10}^{-16}{\mathrm{cm}}^2,$ and some resonance structure near threshold is indicated. Total relative uncertainties $\left(1\mathrm{}\sigma \right)$ are typically about 6%, to be combined with an 11% absolute calibration uncertainty. Total uncertainties at high confidence level (90%) are around 20%, using a coverage factor of 1.7. Comparisons made with various theoretical calculations, specifically distorted wave and nine-state close coupling, show better agreement of experimental values with distorted wave calculations, with the measured values being higher near threshold than the most recent close-coupling calculations.