X-ray absorption spectroscopy of liquid surfaces

By combining synchrotron-based x-ray spectroscopy with liquid microjets, we have developed new techniques for the investigation of the geometric and electronic structure of volatile liquid surfaces. An endstation has been constructed to allow windowless introduction of volatile liquids to the UHV environment of synchrotron soft x-ray beamlines (Advanced Light Source, LBL). The high vacuum (10−5 torr) obtained in the photon-microjet interaction volume, allows the use of standard surface science charged particle detection to analyze the ions and electrons produced via core-level excitation. The difference in escape depths of ions and electrons from condensed phase samples yields convenient surface-bulk contrast via the total ion (TIY) (∼5 Å) versus the total electron (TEY) (∼20–50 Å) yields. This contrast has been exploited in an effort to understand the surface hydrogen bond structure of liquid water and methanol.; The temperature profile of these microjets has been characterized under the high vacuum conditions essential for measuring x-ray absorption “action” spectra. We find only moderate cooling of larger diameter (>10 microns) microjets, while smaller jets undergo rapid evaporation, yielding liquid water temperatures as low as −36°C.; Extended x-ray absorption fine structure (EXAFS) is a common technique that is an ideal probe of local “liquid structure” (near neighbor distance, coordination numbers, etc.). Moreover, the EXAFS TIY and TEY “action” spectra allow the direct comparison of surface and bulk liquid properties. In both liquid water and methanol, a sizable surface relaxation of the intermolecular O-O distance is observed. On average water molecules at the liquid surface are ∼6% further apart than their bulk counterparts.; Near edge x-ray absorption fine structure (NEXAFS) has been shown to be extremely sensitive to not only to the formation of hydrogen bonds, but more importantly to the detailed nature (acceptor vs. donor) of such bonds. TIY NEXAFS spectra combined with DFT analysis has yielded evidence for at least two different species at the liquid water surface. The sharp spectral structure in the TIY signal provides a clear fingerprint for “acceptor-only” H₂O molecules at the liquid water interface. Analysis of mass-selected NEXAFS spectra provide corroborating evidence for surface single donor molecules that have been previously identified by SFG spectroscopy. (Abstract shortened by UMI.)...