Quadrupolar transitions by MCXD at L edges? Search of evidence

To explain the low-energy features of magnetic circular X-ray dichroism (MCXD) at L edges of rare earth compounds, quadrupolar electric transitions (E2) from 2p to 4f levels have been invoked. Such transitions should be distinguished from dipolar electric (E1) ones by looking at the different behaviour of the E1 and E2 MCXD cross sections as a function of the temperature and the angle between the quantization axis and the photon wave vector. However, convincing experimental proof of such transitions has not yet been given. A good candidate for these key tests is Yb, where the E2 transitions are allowed at the L3 edge and forbidden at the L2 edge. In YbFe2, we observe an important change in the shape of the spectrum with temperature at the L3 adge of Yb.