Impact of electron correlation on photoelectron angular distributions in sequential double photoionization of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow> <mml:mi>Ne</mml:mi> </mml:mrow> <mml:mo>+</mml:mo> </mml:msu

We present a combination of the R-matrix theory with time dependence (RMT) method with density matrix theory to model sequential ionization processes. Separate RMT calculations model the individual ionization stages, while a density matrix describes the coherences within the intermediate ion. As proof of concept, we quantify changes in the angular distribution of photoelectrons in the final stage of sequential ionization of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mi>Ne</a:mi> <a:msup> <a:mrow/> <a:mo>+</a:mo> </a:msup> </a:mrow> </a:math> to <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:msup> <b:mrow> <b:mi>Ne</b:mi> </b:mrow> <b:mrow> <b:mn>3</b:mn> <b:mo>+</b:mo> </b:mrow> </b:msup> </b:math> by x-ray free-electron laser radiation with photon energies of 95 eV.