Quenching of Symmetry Breaking in Resonant Inelastic X-Ray Scattering by Detuned Excitation

Abstract

Observations made in oxygen K emission of C${\mathrm{O}}_{2}$ show that the symmetry selective character of resonant x-ray emission is excitation energy dependent. Electronically dipole forbidden O K emission lines are observed for excitation at the O $1s\ensuremath{\rightarrow}{\ensuremath{\pi}}^{*}$ resonance, and they are gradually reduced upon detuning the excitation energy below threshold. The occurrence of forbidden transitions is explained in terms of dynamical symmetry breaking due to vibronic coupling, and the observation of quenching of the symmetry breaking upon detuning as due to an effective quenching of this vibronic coupling. Scattering theory applied to a few-level system is shown to account for the energy dependence of the symmetry selection in the limit of narrow bandpass photon excitation.