J. Goulon et al., X-RAY NATURAL CIRCULAR-DICHROISM IN A UNIAXIAL GYROTROPIC SINGLE-CRYSTAL OF LIIO3, The Journal of chemical physics, 108(15), 1998, pp. 6394-6403
We produce the first experimental evidence of x-ray natural circular d
ichroism (XNCD) in a uniaxial gyrotropic crystal of alpha-LiIO3 that i
s known to crystallize with space group P6(3) and to exhibit a very st
rong nonlinear susceptibility. For the sake of simplicity, the optical
axis was set colinear to the direction of the exciting x-ray beam in
order to get rid of undesirable birefringence and linear dichroism eff
ects. The nicely structured XNCD spectra recorded at the L-I, L-II, an
d L-III edges of iodine are assigned to the electric dipole-electric q
uadrupole (E1.E2) interference terms which do not vanish in gyrotropic
crystals but contribute to a second order polarizability. Our interpr
etation is consistent with a band structure calculation and is fully s
upported by ab initio multiple scattering simulations. The signatures
recorded at the L-II and L-III edges are similar and have the same sig
n. This is in contrast to x-ray magnetic circular dichroism (XNCD) spe
ctra which usually exhibit opposite signs at the L-II and L-III edges:
while the exchange and spin-orbit interactions are the driving terms
in XMCD, this is not the case in XNCD. The XNCD signal detected at the
L-I edge is found to be relatively more intense and has the opposite
sign. These results leave very little space for a contribution of the
electric dipole-magnetic dipole (E1.M1) interference terms which are u
sually dominant at optical wavelengths but should be barely detectable
in the x-ray range. (C) 1998 American Institute of Physics. [S0021-96
06(98)03715-5].