SYMMETRY-SELECTIVE RESONANT INELASTIC X-RAY-SCATTERING OF C-60

The resonantly excited x-ray emission spectra of the C-60 molecule are presented and analyzed in terms of symmetry- and polarization-selecti ve resonant inelastic scattering processes (RIXS). The theoretical ana lysis implements recently derived formalisms for symmetry-selective x- ray scattering. Isolated properties of the RIXS spectra are simulated and their cooperative action for the buildup of various features in RI XS of C-60 are analyzed. Apart from symmetry and polarization dependen ces, the role of Stokes shifts, tail excitation, vibrational excitatio n, and interference effects are simulated in detail. Other relevant as pects are discussed in a more brief manner, such as influence of vibro nic coupling and nonresonant anomalous contributions. Several conclusi ons about the nature of RIXS from C-60 have been derived. The symmetry - and parity-selective character of the RIXS spectra is clearly visual ized by excitation in the band gap. The symmetry selectivity leads to a strong correlation between the shape of the RIXS spectrum and the sh ape of the spectral function describing the incoming excitation photon s. It also implies that the RIXS spectra become sparse in the limits o f long core hole state lifetime and narrow-band excitation. Spectra pe rtaining to higher resonant energies involving a higher density of cor e-excited states are less symmetry selective and turn progressively in to their broadband excitation and nonresonant analogues. Tail excitati on and Stokes shifts have strong influence on the appearance of the RI XS spectra, both depending crucially on frequency and form of the spec tral functions of the incoming photons and on the vibrational progress ions of the core-excited states. The band-gap generated spectra emerge as consequences of Stokes shifts when the absorption energies are det uned from the lowest unoccupied molecular orbital resonance. The polar ization and angular dependences of RIXS in C-60 are found to be compar atively weak, something which is rationalized by the highly degenerate electronic structure and the spherical shape of the molecule. The com puter simulations in this work rest on transition moments and energies obtained by ab initio Hartree-Fock calculations in the full I-h point -group symmetry.