Atomic structures beyond the spherical approximation along with PNC as conjectured explanations to Urbach tailing in neutral isolated ytterbium

In this paper, we try to give an explanation to the spectacular Urbach tailing associated to ytterbium. This rare earth element performs odd spectra due to its 5f-electron shell configuration and complexity of its transition dynamics within band structure. Ytterbium particular forbidden band alteration has been associated to two major inconsistencies: calculating atomic single-particle wave functions and spectra without taking into account the non-spherical effects and neglecting the high parity non conserving (PNC) amplitudes in ytterbium absorption-emission spectra. The actual band structure calculation has been achieved within the Hartree-Fock-Slater approximation. It led to a noticeable blue shift resulting in narrowing the forbidden band from upper edge. Parity non conserving (PNC) amplitudes were also proposed as an explanation to width reduction from both upper and lower band edges. The relative extent of these reductions seems to coincide with Urbach tailing dimensions recorded in recent studies