A symmetry analysis of circular dichroism in photoabsorption of fixed-in-space molecules: Chiral and nondipole effects

An effect of circular dichroism in photoabsorption of fixed-in-space molecules or, equivalently, the circular dichroism in the molecular ion orientation distribution (CDM), is studied. This effect causes forward-backward asymmetry of specific ionic fragments, which is easily observable with today's experimental techniques. We specify a general statistical-tensor approach developed for the ionization of molecules in the gas phase for the description of the CDM and provide the corresponding expressions in terms of partial wave amplitudes. The formalism includes a full multipole expansion of the radiation field. The symmetry of the target molecule puts restrictions on the form of the CDM. We discuss these restrictions for the dipole and the first-order nondipole approximations. The resulting patterns of the dichroism as a function of the orientation of the molecule with respect to the incident circularly polarized photon beam are similar to those, which are known from recent solid state studies. Effects of chirality and possibilities of their experimental observation are discussed.