Optical activity probed with x-rays

Abstract

Optical activity (OA) was only recently discovered in the x-ray range. It is caused mainly by the electric dipole–electric quadrupole E1E2 interference terms which mix multipole moments of opposite parity and exist only in structures with odd space parity. OA related phenomena can be either even or odd with respect to time reversal: natural OA is concerned with time-reversal even effects whereas non-reciprocal OA refers to time-reversal odd contributions. Various types of x-ray dichroisms related to either natural or non-reciprocal OA have been detected and are reviewed in the present paper. Extra emphasis is put on two different non-reciprocal effects that were observed in selected magnetoelectric materials: (i) x-ray magnetochiral dichroism (XMχ D) and (ii) the non-reciprocal x-ray magnetic linear dichroism. Edge-selective OA sum rules have recently been established which show that the effective operator of XMχ D is the orbital anapole moment at the absorbing centre.