Separating Dipole and Quadrupole Contributions to Single-Photon Double Ionization. - Grundmann, S.; Trinter, F.; Bray, A. W.; Eckart, S.; Rist, J.; Kastirke, G.; Metz, D.; Klumpp, S.; Viefhaus, J.; Schmidt, L. Ph. H.; Williams, J. B.; Dörner, R.; et al. (Physical Review Letters, 2018)