Violation of electronic optical selection rules in x-ray emission by nuclear dynamics: Time-dependent formulation - art. no. 042706

The resonant photon emission following the excitation of a highly symmetric system to core-excited electronic states is discussed within a time-depend ent formulation. Two types of vibrational modes-localizing modes and symmet ry breaking but nonlocalizing modes-are considered, named according to thei r impact on dynamical symmetry breaking and localization accompanying the p rocess. The decay rates are proportional to the population of a coherent su perposition df the relevant core states vibronically coupled via the approp riate vibrational modes. This population is essentially a product of partia l contributions of the two types of vibrational modes mentioned above. The general arguments are illustrated on the CO2 molecule. Here, the bending mo de is the symmetry breaking but nonlocalizing mode and the asymmetric stret ching mode is the localizing mode. The decay rates and total resonant photo n emission intensities are calculated in the leading terms approximation of the potential matrix Hamiltonian. The impact of the asymmetric mode on loc alization and hence on the optical selection rules as a function of time is discussed in detail. It is shown that the vibronic coupling via the bendin g mode changes the polarization of the emitted light and exerts an impact o n the effect of vibronic coupling via the asymmetric stretching mode.