DOPPLER EFFECTS IN RESONANT X-RAY RAMAN-SCATTERING

Theory for Doppler effects in resonant x-ray Raman scattering (RXS) is presented. It is shown that the ''electron'' Doppler effect is import ant in nonradiative RXS for decay transitions between continuum nuclea r states lying above the dissociation threshold, and that the averagin g of the RXS cross section over molecular orientations can lead to str ong non-lorentzian broadenings of the atomiclike resonances. The Doppl er effect is found to give a unique possibility to distinguish dissoci ating identical atoms, because different peaks correspond to atoms wit h opposite Doppler shifts. Spectral features of the atomiclike profile are predicted and analyzed. Strong oscillations of the RXS cross sect ion will occur as a consequence of the interference of the Auger elect rons. Due to the Doppler effect and the interference, the atomiclike p rofile can be associated with supernarrow spectral features, the width of which goes below the lifetime broadening and is practically indepe ndent of the spectral distribution of the incident radiation. As anoth er consequence of the oscillations and strong anisotropy caused by the interference, we predict parity selection rules for Auger decay trans itions in both bound and dissociative systems. The corresponding exper iments can be realized by measurements of resonant Auger of surface ad sorbed molecules and for molecules by the electron-ion coincidence tec hnique.