Electron-impact excitation of Ar²⁺

<i>Context. <i/>Emission from Ar III is seen in planetary nebulae, in H II regions, and from laboratory plasmas. The analysis of such spectra requires accurate electron impact excitation data.<i>Aims. <i/>The aim of this work is to improve the electron impact excitation data available for Ar<sup>2+<sup/>, for application in studies of planetary nebulae and laboratory plasma spectra. The effects of the new data on diagnostic line ratios are also studied.<i>Methods. <i/>Electron-impact excitation collision strengths have been calculated using the <i>R<i/>-Matrix Intermediate-Coupling Frame-Transformation method and the <i>R<i/>-Matrix Breit-Pauli method. Excitation cross sections are calculated between all levels of the configurations 3s<sup>2<sup/>3p<sup>4<sup/>, 3s3p<sup>5<sup/>, 3p<sup>6<sup/>, 3p<sup>5<sup/>3d, and 3s<sup>2<sup/>3p<sup>3<sup/>nl (3d <i>≤<i/> <i>nl<i/> <i>≤<i/> 5s). Maxwellian effective collision strengths are generated from the collision strength data.<i>Results. <i/>Good agreement is found in the collision strengths calculated using the two <i>R<i/>-Matrix methods. The collision strengths are compared with literature values for transitions within the 3s<sup>2<sup/>3p<sup>4<sup/> configuration. The new data has a small effect on <i>T<i/><sub>e<sub/> values obtained from the <i>I<i/>(<i>λ<i/>7135 Å+ <i>λ<i/>7751 Å)/ <i>I<i/>(<i>λ<i/>5192 Å) line ratio, and a larger effect on the <i>N<i/><sub>e<sub/> values obtained from the <i>I<i/>(<i>λ<i/>7135 Å)/<i>I<i/>(<i>λ<i/>9 <i>μ<i/>m) line ratio. The final effective collision strength data is archived online.