SOFT AND HARD X-RAY RAMAN-SCATTERING BY ORIENTED SYMMETRICAL MOLECULES - SELECTION-RULES, INTERFERENCE, AND DEPHASING MECHANISMS

Resonant x-ray Raman scattering in chaotical, partial, and fixed orien ted symmetric molecules is analyzed for x-ray photon excitation freque ncies in both the soft and the hard x-ray regions. Different dephasing mechanisms and their connection with channel interference and the obs ervation of selection rules are investigated. It is predicted that for harder x-ray energies the scattering cross sections become strongly a nisotropic and oscillatory due to channel interference. The orientatio nal dephasing is predicted to be an important coherence-blocking mecha nism and can be introduced even by zero-point vibrational or libration al motions. The connections between selection rules, symmetry and phas e factors of the photon wave function, Bragg conditions and the channe l interference show that the selection rules may operate for oriented, surface adsorbed, molecules even in the hard x-ray region. The possib ility of using the interference effect for structure determination of adsorbates is discussed. (C) 1998 American Institute of Physics.