QUENCHING OF SYMMETRY-BREAKING IN RESONANT INELASTIC X-RAY-SCATTERINGBY DETUNED EXCITATION

Observations made in oxygen K emission of CO2 show that the symmetry s elective character of resonant x-ray emission is excitation energy dep endent. Electronically dipole forbidden O K emission lines are observe d for excitation at the O 1s --> pi resonance, and they are gradually reduced upon detuning the excitation energy below threshold. The occu rrence of forbidden transitions is explained in terms of dynamical sym metry breaking due to vibronic coupling, and the observation of quench ing of the symmetry breaking upon detuning as due to an effective quen ching of this vibronic coupling. Scattering theory applied to a few-le vel system is shown to account for the energy dependence of the symmet ry selection in the limit of narrow bandpass photon excitation.