THEORY OF RESONANT ELASTIC X-RAY-SCATTERING BY FREE MOLECULES

A theory of resonant elastic x-ray scattering (REXS) by free molecules is presented. Different scattering channels and their interference on the formation of the spectral profile are identified. It is shown tha t atomiclike contributions to the cross section are totally eliminated Tor orthogonal orientations of the polarization vectors of the incide nt and scattered x-ray photons. A strong deviation from a Lorentzian p rofile of the spectral bands for REXS resonances is predicted. Numeric al applications for the C K and O K spectra of carbon monoxide demonst rate the strong dependence of the spectral profile on the excitation e nergy and the spectral distribution of the incident radiation. Two qua litatively different contributions to the spectral shape are identifie d in the limit of narrow-band excitation: the first follows the excita tion energy, while the second is determined by the absorption resonanc es. The vibrational progressions are very frequency dependent, and at large detuning frequencies the REXS cross Section collapses to a singl e resonance with the width equal to the width of the photon function. This result is shown to hold for resonant as well as for nonresonant c ontributions to the REXS cross section.