Photoionisation of the 3p⁵4s excited states of argon

Partial cross sections for the photoionisation of the excited states 1s 2 2s 2 2p 6 3s 2 3p 5 4s 1,3 P 0 of argon between the thresholds for ionisation to the 1s 2 2s 2 2p 6 3s 2 3p 5 2 P 0 and 1s 2 2s 2 2p 6 3s3p 6 2 S e states of the positive ion have been calculated in the LS coupling approximation using the R-matrix method. For the 3p 5 kp 1 (S, P, D) e and 3p 5 kp 3 (S, P, D) e final states, these exhibit characteristic minima which occur at different energies in different final states due to the anisotropic interaction of the ejected electron with the residual ion. The total cross sections are dominated by the 1s 2 2s 2 2p 6 3s3p 6 4s 1,3 S e resonances respectively where the autoionising transition takes place in the n=3 shell and the outer 4s electron is a spectator to the transition. This is a general feature of the photoionisation of the excited states of all rare gases. The photoelectron angular asymmetry parameter is also calculated. A unitary transformation to the jls coupling scheme appropriate to argon is made and the total cross sections as well as the photoelectron angular asymmetry parameters for the photoionisation of the fine-structure levels are calculated.