Channel interference in resonant Auger scattering by surface adsorbed molecules

We present theory of resonant Auger scattering in gas-phase and surface-adsorbed molecules focusing on the role of channel interference. The connections between parity selection rules, parity of the Auger electron wave functions, Bragg conditions, and channel interference are uncovered. The channel interference is found sensitively dependent on the phase factors of the Auger electron and the incident x-ray photons, something that makes the scattering cross sections strongly anisotropic and oscillatory. Parity selection rules are found that apply to a fairly broad class of molecules, but which can be violated by different dephasing mechanisms that destroy the coherence. The parity-selection rules are particularly sensitive to the degree of orientational disorder of the molecules, something that actually can be used for investigations of adsorbate orientation. Three types of coherence destroying, dephasing mechanisms are investigated: orientational disorder, vibrational and librational motion, and scattering of the emitted electron by the surrounding atoms. It is predicted that the selection rules can be obstructed by thermal vibrations and librations even at very low temperature due to the zero-point contributions. The selection rules and the dephasing mechanisms are explored for one-, two-, and three-dimensional adsorbate systems.