Optical activity probed with x-rays

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.