Near-Threshold X-Ray Fluorescence Spectroscopy Of Molecules

The coupling of high-energy-resolution x-ray absorption and emission spectroscopies with a high-intensity and high-resolution synchrotron-radiation beamline has opened up several new avenues of research in inner-shell molecular physics. Of particular importance is the ability to tune the incident photon energy throughout the near-threshold region for the atomic core levels of the molecules. Sub- and supra-threshold excitations have revealed a diversity of phenomena, including screening effects below threshold, satellite-free fluorescence above threshold, and fluorescence polarization in both energy regions. All of these phenomena can be used in a complementary way to uncover some of the underlying molecular dynamics which occur upon inner-shell excitation and ionization. Some of the latest results from the NBS X-Ray Spectroscopy Beamline at NSLS will be presented to highlight these effects.