1 Non-dipole angular anisotropy parameters of photoelectrons from semi-filled shell atoms M. Ya. Amusia 1, 2 and L. V. Chernysheva 2 1 Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel 2 Ioffe Physical-Technical Institute, St.-Petersburg 194021, Russia Abstract. We present the results of calculations of outer and next to the outer shell non-dipole angular anisotropy parameters of photoelectrons for semi-filled shell atoms in the Hartree-Fock (HF) one-electron approximation and with account of inter-electron correlations in the frame of the Spin Polarized Random Phase Approximation with Exchange (SP RPAE). We demonstrate for the first time that this characteristic of photoionization process is essentially sensitive to the fact whether the photoelectron has the same or opposite spin orientation to that of the semi-filled shell. PACS 31.25.-v, 31.25.-v32.80.-t, 32.80.Fb. 1. Introductory remarks The non-dipole parameters of the photoelectron angular distribution are relatively small quantities, in general smaller than the dipole parameter by the factor 1 / << c v i , where i v is the mean speed of the ionized electron and c is the speed of light. However, as a result of construction of high intensity continuous spectrum radiation devices, namely synchrotrons and storage rings, the non-dipole parameters became measurable quantities. These parameters are of great interest including information not only on dipole transition matrix elements but also on quadrupole ones. Therefore they can demonstrate the existence of invisible in the absolute cross-section quadrupole resonances and shed light on the behavior of photoelectron scattering phases that corresponds not only to 1 ± → l l but also to 2 , ± → l l l transitions, where l is the angular momentum of the ionized electron. The expression for differential in energy cross-section is well known since long ago [1, 2, 3], as well as the concrete expressions for the dipole parameter ) ( ω β nl [3, 4] and non-dipole parameters ) (ω γ nl and ) (ω η nl [4, 5] for photoionization of an nl subshell (where ω */ is the photon frequency) in the frame of one-electron Hartree-Fock (HF) approximation and Random Phase Approximation with Exchange (RPAE), which takes into account the most important part of the inter-electron correlations. Extensive calculations of non-dipole parameters were performed mainly for closed shell ions and atoms [6, 7]. Special attention was given to noble gases [8] and negative ions [9]. There exist several papers dedicated to the results of non-dipole parameters measurements (see e.g. [10-12]). However, almost nothing was done in this direction for open shell atoms, particularly with accounting for the role of electron correlations. The latter are especially important in these atoms. * / Note, that we use the atomic system of units, 1 = = =  e m e in this paper.